UPTEC F 18051

Examensarbete 30 hpSeptember 2018

Probability of Positive Identification with an IFF E-scan System

Tora Lutnaes

Teknisk- naturvetenskaplig fakultet UTH-enheten Besöksadress: Ångströmlaboratoriet Lägerhyddsvägen 1 Hus 4, Plan 0 Postadress: Box 536 751 21 Uppsala Telefon: 018 – 471 30 03 Telefax: 018 – 471 30 00 Hemsida: http://www.teknat.uu.se/student

Abstract

Probability of Positive Identification with an E-scan IFFSystem

Tora Lutnaes

Both military and civil aircrafts depend on a built-in identification system called SSR ona daily basis. IFF is the military adaption of the system for use by fighter jets in orderto separate friendly aircrafts from foes but also to coexist with commercial aircrafts.The system is built around two devices, an interrogator on the own aircraft and atransponder on the target. The aim is to be able to perform an interrogation with ashigh probability of correct identification as possible. There are several factors that cancompromise or prohibit a friendly aircraft from answering to an interrogation and itshould therefore not be classified as a foe.

In this master thesis work, the ability of identification for Saab AB’s new fighter jetGripen E has been investigated. A system model has been created using Simulink andMatlab, mirroring a full link connection between the IFF E-scan system on Gripen Eand a target aircraft. The model includes six subsystems, covering the fighter jet’santenna array system in both transmitter and receiver mode, the transponderequipment, link budgets and signal propagation. Different data sets with main andcontrol beam antenna patterns for the Gripen E IFF system are loaded into the modelvia user selected beam steering..

It has been found that the model simulates the environmental effects on thetransmitted patetrns at different distances well. It can be used to investigate how theantenna coverage changes when applying beam steering to the E-scan system and atwhich distances identification is possible for a chosen steering angle.

ISSN: 1401-5757, UPTEC F 18051Examinator: Tomas NybergÄmnesgranskare: Dragos DancilaHandledare: Mikael Håkansson Borg

Popularvetenskaplig sammanfattning

Bade militara och civila flygplan forlitar sig dagligen pa ett inbyggt identi-fieringssystem kallat SSR. IFF ar den militara applikationen av systemet ochskapades ursprungligen for jaktflygplan under andra varldskriget. Dess syftevar att forhindra eldgivning mot egna flygplan genom att kunna skilja dem franfientliga. Nufortiden ar systemets huvudsyfte fortfarande att forsoka skilja vanfran fiende men ocksa att samexistera med kommersiella flygplan. Systemetar uppbyggt kring tva apparater, en interrogator pa det egna flygplanet ochen transponder pa malflygplanet. Onskvart ar att kunna utfora en interroga-tionsfraga och fa ett svar med sa hog sannolikhet i korrekt identifiering sommojligt. Det finns flera faktorer som kan forhindra ett allierat flygplan fran attsvara pa en fraga och ska i detta fall inte klassificeras som en fiende.

I detta examensarbete har malet varit att ge Saab AB en uppfattning om hurbra identifieringsformagan ar hos foretagets nya stridsflyplan Gripen E, somar under utveckling. En systemmodell over detta har skapats genom program-varan Simulink och Matlab. Modellen innehaller block som representerar deolika delarna i systemet och simulerar hur tva signaler, en huvud och en kon-trollsignal paverkas nar de tar sig igenom detta. Modellens block representerarinterrogatorsystemet pa Gripen E i bade sandar- och mottagarlage, transponder-systemet, lankbudget for upp- och nerlank samt propagering. Antenndata franIFF-systemet pa Gripen E laddas in i modellen via ett vred med fordefinieradeutsyrningsvinklar, vilket regleras av anvandaren.

Det visar sig att modellen beskriver identifieringssystemet val och hur antenner-nas utsanda huvud- och kontrollmonster alterneras nar de fardas genom detta.Simulering med modellen resulterar i tre plottar, tva som visar detektionsvinklarvid interrogator respektive transponder samt en som visar for vilka vinklar i az-imuth och elevation som full lankforbindelse sker mellan det egna flygplanet ochmalflyplanet. Den visar alltsa tackningsdiagrammen for anennerna nar styrningappliceras pa IFF-systemet. Modellen skulle kunna utvecklas genom att imple-mentera fasstyrningen att galla for andra vinklar an de fordefinierade inom detintressanta utstyrningsintervallet.

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Contents

1 Introduction 51.1 Background and purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.2 Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.3 Structure of thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.3.1 Coordinate system specifications . . . . . . . . . . . . . . . . . . . . . . . . 6

2 Theoretical background 82.1 Identification Friend or Foe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.1.1 Identification system on Gripen E . . . . . . . . . . . . . . . . . . . . . . . 82.2 Secondary Surveillance Radar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2.1 Specifications and standards . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.2.2 Interrogation modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.2.3 Interrogation signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102.2.4 Side lobe suppression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.2.5 Transponder replies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.2.6 System problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.3 Modulation techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.3.1 Amplitude modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.3.2 Phase Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.4 Antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.4.1 Field regions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182.4.2 Radiation pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.4.3 Antenna reciprocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.4.4 E↵ective aperture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.4.5 Directivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.4.6 Radiation e�ciency and antenna gain . . . . . . . . . . . . . . . . . . . . . 202.4.7 Active Electronically Scanned Array . . . . . . . . . . . . . . . . . . . . . . 212.4.8 Antenna arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.5 Link budget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232.5.1 TX output power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232.5.2 System losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242.5.3 TX antenna gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252.5.4 Equivalent Isotropically Radiated Power . . . . . . . . . . . . . . . . . . . . 252.5.5 Path loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252.5.6 Friis transmission formula . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252.5.7 Two-Ray Propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252.5.8 RX antenna gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272.5.9 Fading margin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3 System Model 293.1 Total system model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.1.1 User alterable blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293.1.2 Data transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.2 Interrogator antenna array subsystem (TX) . . . . . . . . . . . . . . . . . . . . . . 313.2.1 Beam steering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323.2.2 Pattern generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323.2.3 Antenna routing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343.2.4 Beam pattern plotting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.3 Propagation subsystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353.3.1 Propagation models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

3.4 Uplink linkbudget subsystem (Interrogation) . . . . . . . . . . . . . . . . . . . . . 373.4.1 EIRP interrogator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393.4.2 Losses XPDR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

3.5 Transponder subsystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

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3.5.1 Technical specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413.5.2 Beam pattern plotting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423.5.3 XPDR functions subsystem . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

3.6 Downlink linkbudget sunsystem (XPDR reply) . . . . . . . . . . . . . . . . . . . . 433.6.1 EIRP XPDR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 443.6.2 Losses Interrogator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

3.7 Interrogator antenna array subsystem (RX) . . . . . . . . . . . . . . . . . . . . . . 453.7.1 Beam steering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473.7.2 Pattern generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473.7.3 Detection level plotting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

4 Results 494.1 Front array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

4.1.1 y2o beam steering, Friis propagation . . . . . . . . . . . . . . . . . . . . . . 494.2 Side array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

4.2.1 x4o beam steering, Two-ray propagarion . . . . . . . . . . . . . . . . . . . . 504.2.2 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

5 Discussion 535.1 Goals met . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 535.2 Further work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

6 Appendix A: Simulink blocks 55

7 Appendix B: MATLAB Code 57

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Acknowledgements

I would like to give my biggest thanks to my supervisor Mikael Hakansson Borg at Saab for all thehelp, support and for facing all my questions. Thanks to Torgny Lindquist for reading this tomeand for the feedback. I would also like to thank all the coworkers at the IFF and Radar sectionsat Saab for their welcoming, sharing their technical skills and making my time spent at Saab andin Linkoping truly enjoyable.

I would also like to thank my supervisor Dragos Dancila at Uppsala University for monitoringmy progress. Thanks to everybody who have found the time to proof read my report and eyingmy figures. And finally, I would like to send my sincerest gratitude to my brother Carl Lutnaes,the best support there is.

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Abbreviations

AESA Active Electronically Scanned Array

AM Amplitude Modulation

BPPK Binary Pulse Position Keying

DPSK Di↵erential Phase Shift Keying

EIRP Equivalent Isotropically Radiated Power

FRUIT False Replies Unsynchronized In Time

IC Interrogation Code

ICAO International Civil Aviation Organization

IFF Identification Friend or Foe

HPBW Half Power Beamwidth

LM Link Margin

LOS Line of Sight

ML Mismatch Loss

MTL Minimum Trigger Level

PM Phase Modulation

RF Radio Frequency

RX Receiver

RF Radio Frequency

SLL Side Lobe Level

SLS Side Lobe Suppression

SNR Signal-to-Noise Ratio

SSR Secondary Surveillance Radar

STANAG Standard NATO Agreement

TRU Transmit Receive Unit

TX Transmitter

UHF Ultra High Frequency

VSWR Voltage Standing Wave Ratio

XPDR Transponder

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1 Introduction

1.1 Background and purpose

Being able to identify your friends from your enemies is a key element to prevent friendly-fire. Theneed for this feature grew during World War II, as the use of aircraft in combat became morewidespread than before. The old system relied on visual confirmation and when that could beachieved, it was often too late to prevent a situation from arising. A new system for identificationwas implemented, which locks on to a target after being located by a primary radar. The systemhas evolved since then, but still works on the same principles [1]. Today, the identification systemis used for both commercial and military purposes. Civil Air Tra�c Control (ATC) services relieson it for daily management as do military authorities for developing secure defense systems.

In this thesis the probability of positive identification for Saab’s next generation fighter jet GripenE has been investigated. Positive identification means that identification actually is being achieved,which can not be guaranteed when using the system. Gripen E is currently being developed bySaab and is based on the Gripen C/D platform which is in production today. Gripen E strives tocomplete its predecessor by incorporating newer and more advanced technology.

This thesis has been an evaluation of the performance of the fighter jet’s E-Scan IdentificationFriend or Foe (IFF) interrogator system. E-scan is short for electronically scanned, which meansthat the antenna elements in the system are steered electronically rather than mechanically. Theantennas can be pointed in a certain direction without being physically moved by using computercontrol.

The work has been carried out at Saab AB, business unit Aeronautics, at the section for Radar andIFF in Linkoping which works with integration of present and future radar and IFF systems. Thesection is a part of the tactical functions division which develops functions within target acqui-sition, reconnaissance, electronic warfare, communication and data links, navigation and decisionsupport. The content of this thesis has been modified as it would otherwise contain defense clas-sified content regarding data and structure of the fighter jet’s systems. The report also exist in acomplete version only for use at Saab.

Figure 1: The Gripen E fighter jet [2]

1.2 Goals

The goal of this thesis is to give Saab an idea of how good the ability of identification Gripen Ehas. This against both military and civil platforms. This thesis aims to answer following questions:

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• How much does the positive identification vary when the coverage diagram of the antennasare generated and investigated?

• Which parameters should be taken into account regarding the link budget for uplink anddownlink?

• How does the performance look like on civil transponders and how does it a↵ect the abilityto identify?

• How do di↵erent modulation modes, amplitude versus phase, a↵ect the probability?

These questions and their e↵ect on the ability should be highlighted through the developmentof a mathematical model. The model is to be fully or partially implemented in a mathematicalcalculation program.

1.3 Structure of thesis

The thesis report begins by going through theory regarding the system of identification and howthe it is implemented in Gripen E, antennas and wireless communication. Key elements that willhave an e↵ect on the ability for Gripen E’s identification are pinpointed. The second part describesthe developed model and its function in detail. The third part contains the results obtained andthe conclusion. Lastly the discussion and recommendation for future work is included. In theappendices, blocks used in the mathematical model and the code implemented are shown.

1.3.1 Coordinate system specifications

Throughout the thesis, a coordinate system generally used when treating antennas and their ra-diation patterns is used. In this coordinate system, the xyz-axes are the principal axes and spanstwo planes of interest.

Figure 2: Azimuth angles

The xy-pair spans the azimuth plane in which a circle covering 360� lies around the fighter jet.When drawing the plots later on, azimuth angles in the interval -180� to 0� corresponds to angles 0�

to 180� in the counter-clockwise direction and azimuth angles in the interval 0� to 180� to angles 0�

to 180�in the clockwise direction in figure 2. The xz-pair forms the elevation plane, which describesthe height relative to the plane. Elevation angles < 0� covers the lower hemisphere and elevation

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angles ¿0� the upper hemisphere. Together, the azimuth and elevation planes spans a sphericalsurface around the fighter jet.

Figure 3: Sphere spanned by azimuth and elevation planes

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2 Theoretical background

2.1 Identification Friend or Foe

IFF was originally a system created for military authorities to be able to identify friendly aircraftand vehicles. It was developed during the wartime of World War II under highest national secrecyby rival countries respective forces. The first attempt was to use resonant dipoles to measurethe target’s reflection of radar waves. It showed that the dipole was too dependent on directionand polarization and was too narrow in bandwith to be reliable. An investigation if it would bepossible to use detection of pulse transmissions instead started [3]. This led to the development ofthe system used today. It is a two-channel system using a device called a transponder located on thetarget, which transmits a coded reply signal when questioned by another device called interrogator.Not receiving an answer from an interrogation could imply that the object is a hostile part. Thereexist a level of uncertainty due to the fact that this can not be determined for sure. There arefactors that could prevent or decrease a friendly aircraft’s ability to respond to the question andit should therefore not be classified as a foe.

2.1.1 Identification system on Gripen E

This section gives a short description of the basic function of the E-scan IFF interrogator systemon Gripen E.

The IFF system’s interrogator is responsible for transmitting the main and control signals de-scribed in section ?? used during the identification operations. They are sent through a TransmitReceive Unit (TRU) out to the system’s antenna arrays. The signals are fitted for later phasealignment, since their phases must add up correctly at the target transponder in order to initializesignal decoding.

Figure 4: System overview

Processing and alternations of the signals are done inside the TRU. The TRU contains electri-cal components used for amplification and attenuation to set the right magnitude levels. Phaseshifters are used for beam steering since this process is performed inside the TRU as well. The an-tenna arrays are designed to cover a respective area of interest. All arrays are steered electronically.

The interrogator system also has a receive mode for processing the reply signal from the transpon-der upon reception, backwards through the TRU. It is therefore acting as both a transmitter (TX)and a receiver (RX).

2.2 Secondary Surveillance Radar

Secondary Surveillance Radar (SSR) is the universal name of the radar system used for identi-fication. SSR can be used by both ground radar and airborne systems. While a primary radaris passive, measuring the reflection of radiation from an object, SSR is dependent on an activeresponse from its designated target. After locating the target by the primary radar, a signal istransmitted via pulses from the interrogator in the target’s direction, containing the question ofinterest. The frequency of the emitted signal is 1030 MHz and it is detected by the transponder

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on the corresponding aircraft being questioned. The transponder decodes the message and sendsback an answer at a frequency of 1090 MHz. The polarization angle is always vertical [4]. IFF isthe military application of SSR so it is therefore referred to as the IFF-system on Gripen E.

The transmission of information in the SSR-system is carried out by electromagnetic waves. Boththe interrogation and transponder reply frequencies at 1030 MHz and 1090 MHz respectively lieswithin the Radio Frequency (RF) area. The RF window in the electromagnetic spectrum rangesfrom 3 kHz to 3000 GHz, with frequencies ranging from 300 MHz-3 GHz are defined as Ultra HighFrequency (UHF) [5].

Figure 5: SSR system exchange

2.2.1 Specifications and standards

The IFF system operates by common standards controlled by the International Civil AviationOrganization (ICAO) together with the Standard NATO Agreement (STANAG) for additionaldetails regarding military units. Interrogation and reply pulse specifications and transponderperformance recommendations such as sensitivity, gain and receiving characteristics are coveredby ICAO Annex 10. Additions for military encrypted Modes are covered by STANAG 4193 [1].

2.2.2 Interrogation modes

The interrogation signal sent out can be chosen from di↵erent preset and regulated modes. Themode used depends on the type information about the target aircraft the answer is required tohave as can be seen in table 1. The modes can either be used for both civilian and military purposeor strictly military.

Table 1: Interrogation modes

Mode User Purpose1 Military Identification, mission2 Military Identification3/A Military/Civil IdentificationC Civil Flight level4 Military IFF5 Military IFFS Military/Civil Multipurpose

Only one mode can be transmitted at each time. Multiple modes can be transmitted by using apredefined interlace program which alternates between the modes, for example A,C,2,A,C,2... andso on. The modes are activated by a mode generator, which controls the timing functions insidethe interrogator [1].

Mode S is an additional mode with the S standing for Select. It is a selective mode consid-ered as the next generation ATC system and is implemented worldwide. It enables increased ATCcapacity by limiting the usage of channels and is designed to cope with the growth of the ATC

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system. Mode S relieves the shortage of Mode 3/A codes, a problem arising with increased tra�c[6]. By selective addressing of aircrafts, it is possible to send all data in one transmission insteadof two. It is compatible with existing Mode 3/A and Mode C equipment as it uses the samefrequency bands as the other modes. Mode S is also modulated with respect to the old IFF systemto avoid interference. Interrogation signals uses Di↵erential Phase Shift Keying (DPSK) and thetransponder replies uses Binary Pulse Position Keying (BPSK) which is resistant to IFF randompulses[7].

Mode 4 and Mode 5 are secure modes relying on encryption to avoid being copied by hostileaircrafts. They are used for military purposes only.

2.2.3 Interrogation signals

The interrogation signal system is built upon the principle of timing. From the equipment on boardthe interrogating aircraft, pulses are sent out with a predefined spacing between them. The timebetween the pulses determine which kind of interrogation mode is being used, as can be seen infigure 8. The pulses are sent out via the interrogator’s antenna system. The first and the last pulsesnamed P1 and P3 are sent out by using the main, also known as sum beam antenna pattern. Thesecond pulse P2 is transmitted using the control, also known as di↵erence beam antenna pattern.The overall shape of the beam patterns are determined by the type of antennas used in the systembut the main and control relation must always be achieved [1]. The di↵erence beam is larger inmagnitude than the main beam for all angles except in the direction of its peak value, illustratedin figure 6. The two patterns are used to control the interrogation replies, as described in section2.2.4.

Figure 6: Sum and di↵erence beam patterns

The signals producing the sum and di↵erence patterns are processed and generated inside the TRUusing a four-port network component, the 180� hybrid shown in figure 7. If the signal is loaded intoinput 1 with input 2 isolated, the signals at outputs 3 and 4 will be equal and in-phase. If the signal

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is loaded into input 2 with input 1 isolated, they will be divided into two equal components 180�

out of phase at outputs 3 and 4. This will give P1 and P3 their sum shape and P2 its di↵erenceshape [5].

Figure 7: 180o hybrid junction

All three pulses have a duration of 0.8µs. Pulse P2 always occurs 2µs after the rise of P1. Theinterrogation pulse spacing with tolerance levels included and the pulse rise and decay times forthe normal modes are regulated by ICAO Annex 10, leaving out the military ones. For Mode3/AP1-P3 spacing is 8µs and for Mode C 21µs.

Figure 8: Interrogation pulses

The mode generator mentioned in section 2.2.2 produces a trigger for a plot extractor in the in-terrogator. It is responsible for detecting and processing the received replies after interrogationwhich needs to be synchronized with the transmitter. The plot extractor measures the P1-P3 pulsespacings to determine the active mode and to synchronize internal clocks [1].

In Mode S, P1 and P2 are spaced by 2µs. A transponder designed for only A and C modeswill interpret this as sidelobe suppression, implying that no reply is required. Mode S does notinclude the P3 pulse and uses an extended interrogation signal pulse called P6 instead. P6 is16.25µs or 30.25µs long and includes a number of phase reversals that carries the transmitted dataas shown in figure 9. Mode S interrogation therefore uses a data pulse instead of time separationbetween pulses. In P6, the first phase reversal occurs 1.25µs after the P6 pulse begins. The purposeof this pulse is to synchronize the internal clock in the transponder so it will start decoding theupcoming data. This so the decoding of the data part is completed within the suppression time ofthe transponder, the time it can spare to be busy.

The data is decoded by using DPSK, each phase reversal will indicate a data bit value. A phasereversal of 180� gives a binary 1 and a 0� phase change a binary 0. The value of the first bitsdetermines the interrogation type, the Interrogation Code (IC). The rest of the data bits supportsseveral forms of interrogation based on the type used. The last 24 bits contains an address, if thetarget’s own address is not in the field the transponder will not reply to the interrogation [1].

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Figure 9: Mode S interrogation

The interrogation formats for Mode S can be classified into two groups, All-Call and Selecitveinterrogations. For an All-Call interrogation, all transponders in the beam area will respond withits own address in the reply. As soon as a target has been tracked, it will be locked out to prevent itto answer to more than one All-Call from the same interrogator. To reach the same target again,the interrogator must send out a Selective interrogation equipped with its specific transponderaddress directed in close azimuth angle to it [? ].

2.2.4 Side lobe suppression

Side Lobe Suppression (SLS) is used in order to prevent one of the problems arising when usingthe SSR-system. There exists a risk that aircrafts may or may not answer to an interrogation whenthey are supposed, or not supposed to. In figure 10 aircraft A1 is hit by a main beam that has alarger magnitude than the control beam and should therefore answer to the interrogation. A2 onthe other hand receives pulse P2>P1 and should remain silent as it is not in the intended beamdirection. The side lobes in the main beam pattern emerging from the interrogator antenna canpossibly be larger in magnitude than the control beam. The case when P1>P2 in another lobethan the main lobe is called sidelobe punchthrough. The transponder on board A2 could thereforebe triggered to respond even if the interrogation is not intended for it.

Figure 10: Side lobe levels

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The transponder on the target aircraft is measuring the relative amplitude level between pulses P1and P2. A reply should only be transmitted when the di↵erence is greater or equal to a specifiedvalue in dB. In ICAO Annex 10 this value is set to 9 dB [1]. This give rise to the following cases:

C1: The target should be located in the main beam of the interrogation direction if P2 has amaximum amplitude less than 9 dB of P1, which tells the transponder that it must send aresponse.

C2: P2 has a maximum amplitude that is below P1 but is larger than the threshold value indi↵erence, P2>|P1| - 9 dB. The transponder must not reply to the interrogation but mightdo so, creating an uncertainty about the outcome.

C3: The maximum amplitude of P2 is larger than P1 and the transponder might not reply to aninterrogation, which it is actually not supposed to do.

[8] By implementation of these conditions, unwanted replies from side lobes can be suppressed aslong as the levels are within the given requirements [9].

Figure 11: Uncertainty area between main and di↵erence beams

In figure 11 the area of uncertainty between the sum and di↵erence beams is highlighted in blueas described in C2. At the red crosses, the magnitude of both beams are equal. Above this levellies the risk of triggering unwanted responses in an interval of 9dB. This area is of interest as itdetermines where the transponders that might or might not be triggered by the IFF system arelocated.

In Mode S, SLS is performed by using a new pulse P5 transmitted via the antenna di↵erencebeam as can be seen in figure9. The timing of pulse P5 is calculated so that it overlaps with thesynchronizing phase reversal in P6. If the received signal comes from a sidelobe, P5 will be strongerthan P6 except in the main beam where decoding is possible. If P5 should happen to be strongerthan P6 in the main beam, the transponder will fail to detect the synchronization, preventing itstrigger to start decoding the data or reply to the interrogation [1].

2.2.5 Transponder replies

The transponder is the receiving device located on the target aircraft. On modern aircrafts, thetransponder usually relies on a dual-antenna installation, particularly if they are compatible withMode S interrogations This is a two antenna system and the antenna receiving the strongest inter-rogation signal will be transmitting the reply as it indicates in which direction it was sent from.The orientation of the antennas, with one top and one bottom antenna, will generate coverage ofboth upper and lower hemisphere [1].

The transponder has a RX sensitivity, the Minimum Trigger Level (MTL), which is the thresholdlevel for the lowest magnitude the received signal can have to be able to trigger the transponder.

13

If MTL is not met, the transponder will not start decoding the interrogator message. As the mainlobe has a larger gain than the sidelobes in the main antenna pattern, it can reach much fartherdistances before being attenuated below the MTL level. It is therefore a lower risk to trigger nearbytransponders that do not lie directly in the main direction.

Figure 12: Format for reply signals

The structure of a transponder reply for normal modes 3/A and C that is sent back in response toan interrogation is shown in Fig. 12. Pulses F1 and F2 are named framing pulses and are alwaystransmitted. Their purpose is to tell when the reply begins and ends. The pulses representing thereply data being transmitted are called A, B, C and D followed by su�xes 1, 2 or 4. Combining thepulses with the su�xes gives a total of 12 bits data which gives 4096 combinations to send the replydata by based on the interrogation mode used. The SPI pulse is an optional pulse, transmittedafter F2 only by request for further identification from ATC [1].

Table 2: Interrogation modes

Mode Codes Annotation1 32 B4, all C and D pulses not used1 4096 Some national authorities uses all codes2 4096 -3/A 4096 -C 2048 D1 pulse not usedS 16,777,216 Transponder addresses

The transponder replies for encrypted military modes are strictly confidential to the authorityusing it. The Modes utilizes cryptos and specific crypto keys, which are needed in order to be ableto decode the transmission.

The reply format for Mode S, All-Call format, has a di↵erent appearance from the other modesas illustrated in figure 13. Four preamble pulses form a pattern in order to trigger interrogatordecoding. Their spacing prevents overlapping with two Mode A or Mode C replies since any oftheir leading edges can be used for initiation if the others are obscured by another pulse. Thelong pulse following the four pulses contains the reply data in bits, generating a message. Thenumber of bits can be either 56 or 112, the same as for interrogation. Each bit lasts for 1µs dividedinto one pulse period represented by binary 1 and one no-pulse period represented by binary 0 at0.5µs each. The first bits identifies the reply type and the last 24 bits always contains a combinedtransponder address field and parity. Parity is a form of error correction used in the reply toavoid retransmission and reduce e↵ect of interference. A reply containing errors would give theinaccurate transponder address.

14

Figure 13: Format for Mode S reply signals

2.2.6 System problems

The SSR-system su↵ers from system problems at both the interrogator and the transponder. Theseproblems can roughly be classified into two categories: aircraft detection errors and reply datadecoding errors [1]. Some of the problems than can arise and possibly limit or compromise thesystem functions are summarized in this section. For optimizing the SSR-system and improve thereliability, design choices for minimizing these e↵ects should be considered.

• Synchronous interference arises when an interrogator happens to be in another’s area ofcoverage. The interrogator can catch answers to intended for the other one, creating falsetargets as the aircraft can hear replies from a target not actually being in the own area. Theproblem with extra replies is named False Replies Unsynchronized In Time (FRUIT). Sinceall aircrafts replies at 1090 MHz, a reply intended for another aircraft can be picked up bythe interrogator. The responding aircraft can happen to be located the main beam of two ormore interrogating aircrafts. FRUIT can also be asynchronous, if two replies overlap in theRX antenna the reply of interest might be lost [10].

• The framing pulses F1 and F2 from two or more aircrafts can possibly be separated in timeequal to the F1-F2 pulse separation for one reply when arriving at the interrogator. If F1from one reply is paired with F2 from another reply, framing brackets for a phantom replyfrom aircrafts that simply does not exist can be formed [1].

• Garbling occurs when simultaneous overlapping of multiple signals are received. Separationof the individual pulses of two replies can not be done, causing decoding errors. Aircraftsare often at di↵erent elevation angles but in the same azimuth angle space. If the azimuthseparation is narrow, the overlapping replies can happen to be received by all interrogatorsin the beam area [11].

• Transponder hijack arises when the transponder is prevented from answering an interrogation.Hijack can stem from di↵erent situations:

– The transponder is busy answering a question from another interrogator. It is allowedto have a maximum suppression time up to 125µs after an interrogation but should beready to answer to a new interrogation as soon as possible .

– The transponder is silenced by SLS from another interrogator’s beams or by pulse P2being stronger than P1 from an intended interrogation. The suppression time of thetransponder is between 25µ and 35µ.

– The transponder answer rate is decimated by a built-in protection against overload.The transponder only have a limited transmission ability, restrictive actions takes placewhen the rate of replies is at 1200-2000 replies per second. Unlucky interrogators willbe denied answers when this limit is exceeded.

– The transponder is busy sending out answers spontaneously, called squitting. This canbe a response to noise spikes or interference from other equipment on board the aircraft.No more than 30 squitted replies per second are allowed [1].

15

• The accessibility to reach a specific transponder will be lowered with many interrogators inthe area since the probability that it will be busy is higher.

• A Mode S IC prevents the transponder to reply more than once to an All-Call-question fromthe same interrogator. An All-Call is an interrogation sent to to include targets equippedwith either Mode A/C or Mode S equipment [12]. Re-use of IC:s in areas covering each othercan block an interrogator from detecting a target that already received that code.

• Unrecognized transponder answers are generated if a system uses a transponder that createsa reply that is not recognized by a civilian SSR.

16

2.3 Modulation techniques

Modulation is used in wireless communication in order to raise the strength of the transmittedinformation signal without alternating the original appearance of it. This signal is often referredto as the modulating signal [13]. As the signal travels a distance over the wireless channel, externalinterference and noise additions will weaken it. It is strengthened by modulation by a carrier signalwhich will carry the signal to the RX. The carrier signal has no information in itself but is setwith a specific amplitude, frequency and phase. Modulation can be done by tweaking either ofthese three parameters. Two modulation types considered in this thesis are amplitude modulationand phase modulation. Both are analog types of modulation [14]. At the RX, demodulation isperformed to restore the original state [15]. Noise a↵ects the sensitivity of the RX, and if thequality of the signal incident upon it can be improved then the probability of detection can beraised.

2.3.1 Amplitude modulation

In Amplitude Modulation (AM), the amplitude of the carrier is varied with respect to the mod-ulating signal. AM is inclined to be sensitive to noise levels since noise is amplitude based. Thelarger amplitude the higher noise level [16].

2.3.2 Phase Modulation

In Phase Modulation (PM), variations in the phase of the carrier wave is introduced. For signalswith low amplitudes PM shows similar behavior as AM by being poor in e�ciency. By beingconstant in amplitude, PM provides for a better resistance to interference from noise but at thecost of requiring a larger bandwidth. This creates a trade-o↵ between these two properties toconsider when choosing modulation technique.

17

2.4 Antennas

An antenna is constructed for receiving and radiating RF waves by being the transition betweena device and free-space [17]. It connects the TX and the RX in a wireless communication system,enabling the possibility to use propagation of electromagnetic waves [14]. Antennas can be con-structed in a variety of di↵erent shapes and sizes in order to tailor its radiation characteristics fora desired application. In this section important parameters used for designing an antenna will bedescribed and an introduction to the concept of antenna arrays will be given.

2.4.1 Field regions

The distance in which the antenna radiates relative to its aperture can be divided into three di↵erentregions. The radiated energy sent out by the antenna is influenced di↵erently within these regions.The region farthest away from the antenna is called the far-field, or the Fraunhofer, radiationregion where an energy field point P is located far from the source. Evaluation of radiation fieldsis typically done in this region as the antenna radiation pattern becomes more well formed andcharacteristic here than in the nearer regions [17]. This is also the region of operation for mostantennas since they usually work at long distances. The condition for this region is

r � 2D2

�

[m] (1)

where r is the radial distance from the antenna, D is the largest linear dimension of the antennaand � is the wavelength.

Figure 14: Field regions relative to antenna aperture

The angular field distribution is independent of the distance to the antenna aperture in the far-field region [18]. The radiated fields will still grow weaker with but the pattern will not changeshape with the distance [19]. Therefore, the spherical wavefront radiated from the antenna can beapproximated to be represented by a plane wave with an ideal planar phase front for a RX at afar distance [14]. This will later be useful when dealing with antenna arrays and the adaption ofbeam steering.

18

2.4.2 Radiation pattern

The directional strength in which an antenna radiates is often shown in a radiation pattern. Thepattern is a plot of the strength of the radiated field at a given distance, commonly in the far-fieldregion, around the antenna. The pattern can be plotted in several ways but often shows anglearound the aperture versus the amplitude power, the magnitude of the electric field in linear ordecibel (dB) scale [14]. An example is shown in figure 15.

From the radiation pattern, di↵erent parts called lobes can be distinguished. An antenna withuniform radiation distribution, forming a sphere around the aperture, is called isotropic antenna,with no lobes visible. A lobe is a part of the pattern surrounded by weaker radiation intensityrelative to that particular part [17]. The lobe showing the direction of the maximum radiated valueis called the main lobe, or the main beam. The lobe located 180� from the main lobe is calledthe back lobe. The ratio between the peak amplitudes of the main and back lobe is the front-backratio. The other lobes are named side lobes and consists generally of unwanted radiation andshould be kept as small as possible. The side lobes are separated by nulls, where no radiation ispresent. The ratio of the biggest side lobe and the main lobe is called the Side Lobe Level (SLL)[20].

Figure 15: Antenna pattern plotted in (a) polar form and (b) rectangular form. E is the magni-tude of the electric field [21]

The beamwidth of an antenna can also be found by studying its radiation pattern. The area ofinterest is the maximum radiation, given by the shape of the main beam. An antenna with anarrow main beam will cover a smaller angular region than one with a broader main beam. Theradiation pattern is usually symmetrical around the antenna boresight axis, a straight line drawnacross the center of the main lobe. The Half-Power Beamwidth, also known as the 3 dB beamwidthsince 10 log(0.5) ⇡ �3 dB, is a measurement of the angular separation of the interception pointson each side of the boresight. They are located 3 dB below maximum power intensity, half of thepeak power. The HPBW is normally referred to when the term beamwidth is used, if no otherspecifications are given [22].

19

2.4.3 Antenna reciprocity

One useful property regarding antennas is the antenna reciprocity. It states that an antenna hasidentical receive and transmit properties. If the radiation pattern is known for one mode, it willbe the same in the other if the frequency remains constant [23].

2.4.4 E↵ective aperture

When an antenna is receiving, the captured power from the incident plane wave is represented bythe antenna’s e↵ective area. The receiving antenna should be located in the main beam which isthe direction of maximum radiation. The expression for e↵ective aperture is

A

e

=�

2

4⇡G (2)

where G is peak antenna gain and � is the wavelength. It is a useful property to keep in mindwhen looking at propagation for receiving the maximum power possible [24].

2.4.5 Directivity

The antenna directivity is an important design parameter, since the ability to focus the radiatedpower in a specific direction is a fundamental property of an antenna. If the radiation patterncan be directed in a favorable way, the power available at the receiving antenna can be increased.For the IFF-system, this is useful when performing an interrogation as being able to focus thepower increases the probability that the signal will have enough power to reach the target insteadof wasting it in non-interesting directions.

The directivity is dimensionless, defined as the ratio between the maximum radiation, Umax

, inthe main lobe and the average intensity over the total space, U

avg

:

D =U

max

U

avg

=4⇡U

max

P

max

=4⇡U

maxR⇡

✓=0

R 2⇡�=0 U (✓,�) sin ✓d✓d�

(3)

A high directivity implies a sharper main beam and a low directivity a flatter. The directivity istherefore a measure of the focusing ability of the antenna used. It should not be considered asdirectly related to the beamwidth since both quantities use radiation pattern di↵erently.

Directivity and e↵ective area are related to each other. For an antenna with the aperture area A,The maximum directivity that can be achieved is

D

max

=4⇡

�

2A (4)

where � is the wavelength [14].

A higher directivity enables a better resolution for the interrogating aircraft. Two targets canhappen to lie in the same interrogation beam if it is too broad, which can generate inconclusiveresults.

2.4.6 Radiation e�ciency and antenna gain

No antenna is perfect, there will always be losses in the system due to material limitations. Anantenna will not radiate all of its given input power and the relation is defined as the radiatione�ciency

e

rad

= 1� P

loss

P

in

(5)

where P

loss

is the loss in the antenna and P

in

is the supplied power. The e�ciency can be appliedto both TX and RX antennas.

20

The Gain of an antenna is related to both the radiation e�ciency and the directivity:

G = e

rad

D (6)

The gain can not be larger than the magnitude of the directivity since e

rad

1 [14].

2.4.7 Active Electronically Scanned Array

An Active Electronically Scanned Array (AESA), shorted E-scan, enables the possibility to steerthe antenna without moving the antenna aperture. AESAs are steered by electronically inducedphase shifts which enables faster steering with higher precision. In Gripen E, the antenna systemsis designed as an AESA

2.4.8 Antenna arrays

An antenna array is composed of two to several antenna elements positioned separated from eachother. The elements are placed in a geometrical configuration, typically a row or a grid. Thegeometry of a linear array is shown in figure 16. By constructing an antenna array, radiationcharacteristics that cannot be obtained by using just a single antenna element be achieved [22].Increasing the aperture area by including more antenna elements a↵ects the antenna gain anddirectivity. As can be seen in equation 4, a larger area gives higher D

max

and a higher directivitygives a larger gain according to equation6.

Figure 16: Linear antenna array relative to an incoming planar wave

As a result of the elements being spaced at a distance from each other, the incoming planar wavewill not hit them simultaneously. The direction is given by the angle ✓ between the array normaland the incoming rays. In figure 16 the elements are at distance d from each other. To reachelement K � 1, the wave will have to travel an extra distance than for element K. This distancecan be represented by x = d sin(✓). The distance to reach element K � 2 is given by x = 2d sin(✓).This repeats up to the last element, giving x = (K � i) d sin(✓). If the phase at element K in figure16 is set to be 0, the relative phase di↵erence � at the other elements for the plane wave is writtenas:

�� =d sin ✓

�

2⇡ (7)

where � is the free space wavelength.

21

In complex form, the signal Si

(✓) received by each element in the array is given by

S

i

(✓) = S

e

a

i

e

jk0(K�i)d sin(✓) for i = 1, 2, ...., K (8)

where S

e

(✓) is the complex radiation pattern of one element, k0 = 2⇡�0

representing the wave num-ber and a

i

the amplitude. When discussing phased arrays the amplitude is often set to a

i

= 1,assumed to be a normalized distribution for the array. Attenuation of the signal will be higher forrays having longer way to travel but the di↵erence can be neglected here [22].

The total radiated pattern from an antenna array is composed of two parts the array factor andthe pattern of one single element. If the elements in the array are non-isotropic, the total radiatedfield will be achieved by multiplying the array factor S

a

(✓) with the field of one single elementS

e

(✓) for the specific type of antenna used in the array. The total number of elements are thenadded together. This is valid only if all elements in the array are identical [17].

One useful adaption of antenna arrays is beam steering. By inducing a phase shift, the mainbeam can be steered in a desired direction. The radiation pattern can be shaped by the excitationof the phase of each element individually. An uniform excitation of each element is achieved byadding a progressive phase at each antenna element. For a linear phased array antenna, the arrayfactor with the added phase taper is given by

S

a

(✓) =KX

i=1

e

jk0(K�i)d[sin(✓)+ i] (9)

with = ✓0 is the phase shift.The total radiation pattern for the array is therefore given by

S(✓) = S

e

(✓)Sa

(✓) = S

e

(✓)KX

i=1

e

jk0(K�i)d[sin(✓)+ i] (10)

where S

e

(✓) is the element pattern.

Due to antenna reciprocity as stated in 2.4.3, the transmit mode of an antenna array will fol-low the same theory.

Figure 17: Beam steering in positive and negative directions

Beam steering will have e↵ects on the radiated patterns. When steering the antenna, the overallshape will change from the 0o degree state. Unpredictable pattern changes can occur due to inducederror e↵ects in the components inside the array system for each antenna element. Therefore it is ofinterest to investigate how the relation between the main and control beam patterns changes whensteering. This will a↵ect the area of uncertainty in figure 11, which will change the interrogationSLS and reply image.

22

2.5 Link budget

When designing and implementing an end-to-end system, the signal level received will di↵er fromthe signal level transmitted. By setting up a link budget, the factors that determine this level areidentified and estimated. All gains and losses in the complete system are taken into account andthe result is the remaining received power after all factors have been summarized. The link budgetis important for monitoring the performance of the system so successful operation of the desiredapplication can be guaranteed.

Figure 18: Components in a link budget system

The remaining power after the link budget calculations are done is the power available for theapplication to use. The link budget equation for the system in 18 would be given by

P

r

= P

TX

� L

TX

+G

TX

� L

pl

+G

RX

� L

RX

(11)

Figure 19: Gains and losses in a link budget system: (1) TX output power, (2) System losses, (3)TX antenna gain, (4) EIRP, (5) Path loss, (6) RX antenna gain, (7) System losses, (8) RX inputpower, (9) Fading margin and RX sensitivity level

The e↵ects and sources of the factors in Fig. 19 are described more closely in the followingsubsections.

2.5.1 TX output power

The TX output power is simply the amount of RF power in Watts supplied to the transmittingequipment. It should be designed to be optimized at a high enough level with respect to thetrade-o↵ with the system’s capability and noise image.

23

2.5.2 System losses

The total amount of power supplied to the system will not be available for transmission at theantenna output. One degrading source is due to impedance mismatch between the feed line andthe load, which gives a reflection of the transmitted signal. An antenna is a type of load and willgenerate mismatch loss in a system. Ideally, maximum power is delivered to the antenna when itis properly matched to the characteristic impedance of the feed line. In reality there will alwaysbe some degree of mismatch loss and waste of power [14].

By using the Voltage Standing Wave Ratio (VSWR), the impedance matching between the feedline and the load can be described. VSWR is a ratio expressed as a function of the reflectioncoe�cient, 0 � 1, of the line:

VSWR =V

max

V

min

=1� |�|1 + |�| (12)

where V

max

and V

min

are voltage levels of the line. If VSWR = 1, the load is perfectly matchedwith no reflection [5].

With VSWR, the mismatch loss in the feed line is expressed by

MLdB = �10 · log10✓1�

✓VSRW� 1

VSRW+ 1

◆2◆[dB] (13)

Mismatch loss is valid for both TX and RX systems and should be calculated for both cases.

Another source for loss in the system is cable loss. Attenuation of the signal strength is inducedas it propagates along the length of the cable. The magnitude of the loss is also dependent on theresistive and dielectric properties of the type of cable used.

Also adding to the system losses are di↵erent kinds of noise. Noise is induced interference inan electronic communication system and can corrupt the information in it. It also determines thethreshold for the minimum signal level that can be reliably detected by a receiver, as noise canconceal signals [14]. Noise contributions comes from both the external environment around theantenna and from the components inside the TX and RX systems.

Noise is present in all communication systems and the most limiting noise source in a radar isinternal noise [25]. The antenna noise contribution comes with temperature generated internallywithin the system known as thermal noise. Thermal noise is the largest type of noise in mostRF systems. Noise is critical in the microwave region because noise increases with bandwidthand therefore a↵ects high-frequency signals more than low frequency signals. As the IFF-systemoperates in the UHF region, it gives a large contribution. Thermal noise is calculated by

N = kTB (14)

where k = 1.38 · 10�23J/K is Boltzmann’s constant, T is the temperature in Kelvin and B is thebandwidth. The noise is classified as white noise, which means that the power spectral density isflat. It is independent of frequency in the bandwidth range and can be considered equivalent overit [5].

One important measure when it comes to noise is the Signal-to-Noise ratio (SNR). The ratiorelates the power of the received signal with the noise power present at the RX according to

SNR =S

N

=P

r

N

(15)

with S = P

r

as the signal power and N as the noise [20]. If the noise is high the ratio will be low.If it is too high it will drown the signal power and the information in it will be corrupted or lost.

24

2.5.3 TX antenna gain

As stated in 2.4.6, the antenna gain is a combination of the antenna directivity and e�ciency. Itis a measure of how well the antenna can transmit its given power in a direction.

2.5.4 Equivalent Isotropically Radiated Power

The Equivalent Isotropically Radiated Power (EIRP) is a measure of how much power is radiatedin a specific direction. EIRP is used as a comparison between the achieved measured value and theamount of power needed to produce equivalent power density, if an isotropically radiating antennawas used instead. When having a system that uses beam steering, isotropic radiation is not anoption. EIRP considers TX parameters by the following equation:

EIRP = P

T

+G

T

� L

T

[dB] (16)

where P

T

is transmitted power, GT

is antenna gain and L

T

are losses in the transmitter system[26].

2.5.5 Path loss

The power radiated out from the antennas will propagate through space before being received.Propagation is calculated in the path loss aspect of the link budget. As the radar waves movesthrough the surrounding medium, they will be a↵ected by current conditions. The signal willbe weakened or altered before reaching its target. There are several ways and models created todisplay what happens to a signal as it propagates, used in di↵erent applications. A model createdfor telecommunications in a city will not have the same mathematical structure as one for satellitecommunication. In this thesis two types of propagation have been investigated and compared whencreating the total model, Friis transmission formula and Two-ray propagation.

2.5.6 Friis transmission formula

One very important equation regarding antennas and propagation is the Friis transmission formula.It assumes free-space conditions, where no obstacles are along the transmission path between theTX and RX antennas. It states that the received power P

R

is

P

R

=P

T

G

T

G

R

�

2

(4⇡R)2[W] (17)

where P

T

is TX power, GT

and G

R

TX and RX gain respectively, � is the wavelength and R isthe distance between the antennas. Friis transmission formula can also be written as

P

R

=P

T

G

T

G

R

c

2

(4⇡Rf)2[W] (18)

since � = c

f

where c is the speed of light. From Eq. 18 it can be seen that using higher frequenciesgives a higher path loss for a system with specified gains. The SSR-system working in the 1 GHzspectrum enables a decent quality reception for transmission at long distances [27].

2.5.7 Two-Ray Propagation

The other propagation model that has been investigated in this thesis is the Two-ray model. Thisis used when more than one signal path is incident on the RX antenna at the same time, whichcan e↵ect the performance of the system. It could be a more accurate model than Frii’s for thisapplication and is therefore included as an option.

Two-ray propagation is also called Flat-Earth model. Instead of one, the RX antenna mightpick up two waves. One coming from the direct Line of Sight (LOS) and one reflected from theground, or another surface lower than the TX antenna. This is referred to as multipath propaga-tion since there exists more than one path between TX and RX systems. The reflected wave has a

25

longer way to travel, giving a time di↵erence between it and the LOS signal. The time delay willtranslate into a phase shift when the signals add up at the RX antenna. They can do so eitherconstructively or destructively, the RX signal is strengthened if the rays are in phase and weakenedif they are not. If the rays are 180� out of phase they will cancel each other out completely if theyare of the same magnitude.

Figure 20: Two-ray propagation

The simplified equation for the received power from Two-ray propagation is given by

P

r

⇡ P

t

G

t

G

r

h

t

h

r

d

4[W] (19)

assuming that d >>

4⇡hthr�

. P

t

is the transmit power, Gt

and G

t

transmitted and received gain,h

t

and h

r

is the height of the TX antenna and RX antenna respectively above the reflected surfaceand d is the distance between them.

A plot of the the non simplified Two-ray path loss as a function of distance between TX andRX systems is shown in figure 21. The free-space path loss is plotted for reference.

26

101

102

103

104

105

!180

!160

!140

!120

!100

!80

!60

!40Two!ray propagation

Distance, m

Pa

th lo

ss,

dB

blue line: two!ray

orange line: free space

Figure 21: Two-ray propagation versus free space propagation

The e↵ects Two-ray propagation can have on the IFF-system include timing and pulse strengthalternations at the transponder. The reflected ray can interfere with the transponder performanceas the range of the IFF-system is reduced if LOS and reflected signals largely cancel each otherout. As can be seen in figure 21, there are dips at certain distances between the interrogationand receiving system. This indicates destructive interference if the distance to the target aircraft’scoincide with one of them.

The ratio of P1 and P2 can be compromised, since both of the interrogation pulses can be af-fected di↵erently. At some elevation or azimuth angles, P2 might be reduced in amplitude leadingto ”sidelobe punchthrough” and SLS might not be initialized by the transponder. At other angles,P1 can be too narrow in the main beam to detect transponders in the destined region, referred toas ”main beam killing” [1].

If the reflected wave happen to hit another airplane, extra replies can be triggered from the wrongtarget.Considering these aspects, Two-ray can therefore impose worse conditions than free-spacemodel for the IFF-system.

2.5.8 RX antenna gain

The RX antenna gain is a measure of how well the receiving antenna system is converting theincident waves into power for later use.

2.5.9 Fading margin

The attenuation of a signal in a system is a varying process. The many factors can all changerandomly, the noise image can be larger or smaller than expected for example. To account forall unexpected turns and provide for reliable transmissions, high enough TX power is deliveredto ensure that the received power is high enough even at its minimum [20]. The probability thatthe signal level can stay above this threshold should be calculated for the used system to keepit as robust as possible. Fading margin is the adaption of the Link Margin (LM) which is thedi↵erence between the design value of the received power value and the minimum threshold valueof it according to

LM = P

r

� P

r(min) [dB] (20)

27

as it mirrors fading e↵ects. The LM can be increased by either increase transmit power or reducethe receiver sensitivity, the latter is often a costly and complex process [5].

28

3 System Model

The software used for developing the mathematical model has beenMatlab together with Simulink.Simulink is an environment used for simulation and model-based design of dynamic systems. Dif-ferent blocks can be drag-and-dropped and paired together to create a system model. Simulationsand analyses can be performed to examine the functions of the design. Simulink is integrated inMatlab which enables data transfer between them. The functions of the Simulink-blocks usedin the model created for this thesis are described in Appendix A. The Matlab code written for thefunction blocks in the model can be found in Appendix B.

The model has been developed iteratively by creating the parts separately and then combiningthem together to form the total system. In this section, the system model will be showcased andthe functions of the di↵erent parts will be described. The real model made at Saab is done ac-cording to Gripen E specifications but in this report, a model with arbitrary values and modifiedcontent will be showed. This model has the same overall appearance as the one made for thefighter jet. In the model, the transponder is shortened to XPDR.

3.1 Total system model

The total system model consists of six larger subsystems transferring the radiated antenna patternsthrough it. Each subsystem covers a specific part in the process. The order in which the subsystemsare activated is as follows:

1. Interrogator antenna array subsystem (TX)

2. Propagation subsystem (once for uplink)

3. Uplink linkbudget subsystem (Interrogation)

4. Transponder subsystem

5. Propagation subsystem (once for downlink)

6. Downlink linkbudget subsystem (XPDR reply)

7. Interrogator antenna array system (RX)

The total system model can be seen on the next page in figure 22.

3.1.1 User alterable blocks

Eight blocks can be altered by the user in order to simulate di↵erent scenarios. The generatedtemperature inside the interrogator and the transponder can be set in Celsius, since thermal noiseis the most critical type of noise. The coordinates for the interrogating aircraft and the targetaircraft can be set giving height di↵erence in the z-dimension and distance in the x-dimensionbetween them. These are altered in order to investigate how the antenna patterns will changewith distance and how this a↵ects the probability for signal detection. Four switches are used forchanging between Friis transmission formula and the Two-ray model for both main and controlbeams so that the two propagation models can be compared.

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Figure

22:ThetotalSim

ulinksystem

mod

el

30

3.1.2 Data transfer

The Data Store Memory, Read and Write blocks enables data transfer between subsystems andblocks without having them connected by a line. They are used for blocks using the same databut are spaced far apart. Each pairing of Read, Write and Memory blocks sharing the same nameuses the same data stored. In the model this is according to:

• AM - Determines which array transmitting the main beam is active. Contains a constantvalue of 1, 2 or 3.

• AC - Determines which array transmitting the control beam is active. Contains a constantvalue of 1, 2 or 3.

• FBS - The beam steering value for the front array. Constant value set by the user.

• SBS - The beam steering value for the side array. Constant value set by the user.

• GM - Is the received gain in the main channel in the interrogator antenna arrays. Stores amatrix.

• GC - Is the received gain in the control channel in the interrogator antenna arrays. Storesa matrix.

• PAF - Contains data with punchthrough, suppressed and MTL denied angles for the frontarray. Stores a matrix.

• PAS - Contains data with punchthrough, suppressed and MTL denied angles for the sidearray. Stores a matrix.

3.2 Interrogator antenna array subsystem (TX)

This subsystem simulates the structure and properties of the interrogator TRU antenna arraysystem in the transmitting mode.

Figure 23: Interrogator antenna array subsystem (TX)

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Table 3: Subsystem block parameters

Port Function

Input - -

Output

Front array beam steer Steering angle front array

Side array beam steer Steering angle side array

Active array Main Shows transmitting array

Active array Control Shows transmitting array

G TX Control beam Gain in main beam pattern

G TX Control beam Gain in control beam pattern

P TX INT Array system output power

The number of antenna elements and the fed power in Watts for each element is multiplied togetherto achieve the total output power. It is scaled by a constant to account for the array gain whichwill otherwise add to it. Before being transferred out through the outport, it is converted to dBfor use in later calculations.

3.2.1 Beam steering

Beam steering is activated by the use of two knobs, one for the forward looking array and onefor the side looking array. The user can choose between di↵erent, preset beam steering angles foranalysis by tweaking them. The value of the knob will then be loaded into the pattern generationsubsystems. For the front array, angles between y1� < ✓ < y7� can be chosen with y4� as thearray boresight axis. For the side array, values between x1� < ✓ < x6� can be chosen with x3� asthe array boresight axis. Selecting a steering angle to the left of the boresight will move the sideantenna array elements in the counter-clockwise direction.

3.2.2 Pattern generation

Four subsystem blocks are used for loading the antenna array patterns, two for the main beamand two for the control beam. The subsystem consists of a function block named after the arrayit is steering and the pattern it loads. Figure 28 shows the function block radPat side Mainwhich will load the main pattern for the side looking array.

Figure 25: Function block for pattern generation

The pattern for each antenna array is generated by loading sheet data for the array into the Matlabworkspace via stored .mat-files. The data consists of radiation patterns for the antenna arrays onGripen E. Since the antenna data is strictly confidential it will not be shown in this thesis. The.mat-file loaded is matched to the value from the inport. The data is arranged into intervals andstored into a matrix with each row representing an elevation angle and each column the transmittedmagnitude for each azimuth angle. The elevation angle vector consists 15 values ranging from �

= [-35:5:35] degrees and the azimuth angel vector of 1801 values from ✓ = [-180:0.2:180] degrees,creating a [15x1801] matrix. The loaded pattern is then transferred from the subsystem via theoutport.

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Figure

24:System

insidetheInterrogator

antennaarraysubsystem

(TX)

33

3.2.3 Antenna routing

Four switches are used for setting which antenna array is transmitting.

Figure 26: Two of the routing switches. The remaining two shares the same appearance

The first switch pair takes the pattern for the side antenna and loads it into the function blockgenerateSide. The block flips the pattern vector around its vertical axis in order to cover theother side of the fighter jet. This gives a beam steering of x1� < ✓ < x6� with x3� as the arrayboresight axis, but selecting a steer angle to the left will rotate the antenna elements in a clockwisedirection instead.

Figure 27: Function block for side pattern generation

The other switch pair sets which array is currently transmitting and the pattern data for thisarray will be sent out to the total model. One switch sets the main beam and the other the controlbeam from the chosen array. In order to be able to do evaluations and plot functions later in theTransponder subsystem and the Interrogator antenna array system (RX) these blocks must knowwhich antenna is active. In the function block activeArray a constant value associated witheach array is set and routed via the outport.

Since all the subsystems generates beam data for every model run, the function block comparesthe array value to a reference which is the chosen transmitting array. The values set for the arraysare:

1 = Side array active2 = Switched side array active3 = Front array active

Figure 28: Function block for active array verification

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3.2.4 Beam pattern plotting

The function block plotPatternINT will generate two surface 3D-plots for both the main andthe control beam of the chosen antenna array. The plots shows the antenna gain coverage in theelevation and azimuth planes as the beams exit the interrogator in transmit mode at 1030 MHz.

Figure 29: Pattern plotting for interrogator antenna system

3.3 Propagation subsystem

The Propagation subsystems simulates the attenuation e↵ects induced on the signals as they travelbetween the TX and RX antennas. This in order to see how the signal is altered at di↵erentdistances and if it can a↵ect the probability of detection. The subsystem represents the wirelesspart of the system.

Figure 30: The Propagation subsystem

The subsystem takes inputs from the Interrogator antenna array (TX) subsystem and the Transpon-der subsystem. The gain in the antenna channels are routed via Data Store blocks GM and GCfrom the Interrogator antenna array (RX) subsystem. The coordinates for the two aircrafts whichare set by the user in km are also loaded into the block.

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Table 4: Subsystem block parameters

Port Function

Input

G TX Main Gain main beam pattern

G TX Control Gain control beam pattern

P TX INT Array system output power

Coord. interrogator Interrogator coordinates

Coord. target Target coordinates

G RX XPDR Transponder receive gain

G TX XPDR Transponder transmit gain

P TX XPDR Transponder output power

G RX Main Received gain main beam at INT

G RX Control Received gain control beam at INT

Output

Friis Main 1030 Friis on main beam uplink

Friis Control 1030 Friis on control beam uplink

Two-ray Main 1030 Two-ray on main beam uplink

Two-ray Control 1030 Two-ray on control beam uplink

Friis Main 1090 Friis on main beam downlink

Friis Control 1090 Friis on control beam downlink

Two-ray Main 1090 Two-ray on main beam downlink

Two-ray Control 1090 Two-ray on control beam downlink

Within the subsystem, two function blocks handles the two di↵erent propagation models. Theuplink frequency of 1030 MHz and the downlink frequency of 1090 MHz are given via two constantblocks. The user specified coordinates are transformed into meters and loaded into the functionblocks together with the other parameters. The Friis transmission function block will only usethe x-values while the Two-ray propagation function block will use both x and z-values since themodel is dependent on height.

Figure 31: System inside propagation subsystem

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Calculations on both main and control beam patterns are done with both propagation models.This in order to be able to compare their relative radiation pattern strengths when arriving at thetarget’s transponder and back at the interrogator.

Figure 32: Close-up on the Two-ray propagation function bock

3.3.1 Propagation models

The two propagation models considered in this thesis are modeled by two function blocks FriisTransmissionand twoRayProp, which generates eight outputs. All outputs labeled with 1030 are used for uplinkand all labeled with 1090 are used for downlink calculations.

3.4 Uplink linkbudget subsystem (Interrogation)

Uplink refers to the channel connection made between the interrogator and the transponder forthe interrogation at 1030 MHz. The linkbudget calculations for the system’s uplink is performedwithin this block.

Figure 33: Uplink linkbudget subsystem (Interrogation)

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Table 5: Subsystem block parameters

Port Function

Input

G TX Main Gain main beam pattern

G TX Control Gain control beam pattern

P TX INT Array system output power

Friis Main 1030 Friis on main beam uplinklink

Friis Control 1030 Friis on control beam uplinklink

Two-ray Main 1030 Two-ray on main beam uplink

Two-ray Control 1030 Two-ray control beam uplink

G TX XPDR Transponder receive gain

Temperature INNT Generated temperature interrogator

Temperature XPDR Generated temperature transponder

Output

P RX Control Friis Received power control beam

P RX Control Two-ray Received power control beam

P RX Main Friis Received power main control beam

P RX Main Two-ray Received power main beam

The subsystem takes input from the Propagation subsystem, the Transponder subsystem, theInterrogator antenna array subsystem and the temperature blocks. The four outports gives thegain for every angle in the interval in the transponder.

Figure 34: Inside the Uplink linkbudget subsystem

Inside the subsystem, two other subsystem handles the linkbudget calculations for the main andthe control beam respectively so that their relative strengths can be compared. An illustration ofthe linkbudget is showed for the control beam in figure 35.

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Figure 35: Linkbudget for the control beam

Both Friis and Two-ray propagation with their respective values are added separately to the linkbudget, generating two di↵erent outputs from the block.

3.4.1 EIRP interrogator

The EIRP subsystem block summarizes the gains and losses in the interrogator system and willgive the highest output power possible for each beam angle.

Figure 36: EIRP block for the interrogator

The system losses considered in the model are cable loss and impedance mismatch. Defined levelsis the VSWR for the system and the cable losses. In the thesis model, ideal conditions witha VSWR=1 and cable losses of 0 dB are considered to avoid using the real values. Since thearray consists of several antenna elements, the VSWR must be accounted for each of the antennasince they are all considered as loads. The system noise contribution comes from the temperaturegenerated inside the interrogator system.

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Figure 37: Function block for the interrogator noise

3.4.2 Losses XPDR

When arriving at the target, the transponder contributions to the link budget is the gain in the theantenna and the system losses. These are included in subsystem named Losses XPDR accordingto figure 38. Ideal system values are also used for this case. The system noise is calculated in thesame manner as in figure 37 with the temperature generated i the transponder instead.

Figure 38: System losses in the transponder

3.5 Transponder subsystem

The Transponder subsystem block simulates the functions of the transponder.

Figure 39: The Transponder subsystem

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Table 6: Transponder subsystem block parameters

Port Function

Input

P1 and P3 pulse Main beam pattern

P2 pulse Control beam pattern

Active array Main Transmitting array main beam

Active array Control Transmitting array control beam

Output

P TX XPDR Transponder output power

G TX XPDR Transponder transmit gain

Punch angles Front array Punchthrough data front array

Punch angles Side array Punchthrough data side array

The block takes input regarding the pulses from the Uplink link budget system which is theremaining power available in the main and control beams. The active array inputs are routed viaData Store-blocks AM and AC from the Interrogator antenna subsystem (TX). The block givesoutputs for transponder gain and power for later use in the system. The punchthrough anglesand detection levels for the front and side arrays are loaded into Data Store blocks PAF and PASrespectively.

Figure 40: System inside the Transponder subsystem

3.5.1 Technical specifications

Within the subsystem, technical characteristics as specified in ICAO Annex 10 will set the limitsfor the transponder functionality. The antenna gain for the target transponder is set to 0 dB,which is valid for both G TX and G RX. P TX has the peak power of 120 W, converted into dBbefore output. MTL is specified to -76 dBm which is also converted to dB.

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3.5.2 Beam pattern plotting

The function block plotPatternXPDR will generate two surface 3D-plots for the remaining powerfor both main and control beams of the antenna gain in the elevation and azimuth planes. Theplots have the same appearance as the plots for the patterns exiting the transmitting interrogatorsystem for comparison in order to show the magnitude of attenuation during uplink.

Figure 41: Function block plotting radiation patterns at transponder

3.5.3 XPDR functions subsystem

The block labeled XPDR functions Modes 1, 2 3/A and C controls the transponder functions forthe given modes.

Figure 42: System inside XPDR functions subsystem

Inside the subsystem, the SLS limit of 9 dB taken from ICAO Annex 10 determines the condi-tions between the pulse magnitudes for the transponder according to 2.2.4. The function blocktrigSLS measures the ratios between the P1 and P2 pulses as they have been a↵ected throughthe system up to this point. P1 represents the main beam pattern and P2 the control beam pattern.

The SLS-levels are controlled for all azimuth angles for all elevation angles in the specified in-terval. Manipulation of the row lengths of the matrices containing the azimuth main and controlbeam data is performed based on which antenna array is active. This is determined inside theblock by evaluating the values of inputs active main and active control. If the front arrayis active, the full range from �180� < ✓ < 180� are evaluated. If the side array is active then thesecond half of the row vector values will be used, giving the range from 0� < ✓ < 180�. If theother side array is active the first half of the row vector values will be used, giving the range from-180� < ✓ < 0�. The vector manipulation will generate row vectors of 901 data points instead of1801. This is done in order to isolate the respective areas of interest for each array. Inside thefunction block, di↵erent values are assigned a specific value for each angle depending on the ratiobetween the P1 and P2 pulses. The values are assigned according to:

1 = The value of the beam at the angle is too weak to trigger XPDR.2 = SLS is guaranteed to be in action, XPDR will not be triggered.3 = The ratio is within the SLS uncertainty area, XPDR might be triggered.4 = Punch through, XPDR will be triggered.

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Two matrices, one for front array and one for side array, for storing these values are preparedusing commands:

punch angles front=zeros(15,1801);punch angles side=zeros(15,900);

These matrices are the outputs from the function block and will be routed from it out to thetotal system.

The assignment of values 1-4 are done by evaluating the di↵erent logical situations that can arise.The matrices will store a value according to the outcome of the situation. The code for this blockcan be studied in detail in trigSLS.m. Later these matrices will be mapped into images to showthe detection possibilities for a given distance. Since the transmitted gain in the side lobes inthe main beam pattern is much lower than in the main lobe, they will not be able to trigger atransponder at the same distance if it is too far.

3.6 Downlink linkbudget sunsystem (XPDR reply)

Downlink refers to the channel connection made between the transponder and interrogator systemsat the transponder reply at 1090 MHz.

Figure 43: The Downlink linkbudget subsystem

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Table 7: Downlink linkbudget subsystem block parameters

Port Function

Input

Temperature INT Generated temperature inside interrogator

Temperature XPDR Generated temperature inside transponder

Friis Main 1090 Friis on main beam downlink

Friis Control 1090 Friis on control beam downlink

Two-ray Main 1090 Two-ray on main beam downlink

Two-ray Control 1090 Two-ray control beam downlink

G RX Control INT Gain in control channel

G RX Main INT Gain in main channel

P TX XPDR Transponder output power

G TX XPDR Transponder gain

Output

P RX Friis Main Received power main beam

P RX Two-ray Main Received power main beam

P RX Friis Control Received main control beam

P RX Two-ray Control Received power control beam

The subsystem takes input from the Propagation subsystem, the Transponder subsystem and thetemperature blocks. The four outports gives the gain for every angle in the interval back at theinterrogator. The linkbudget calculations are done in the same way as in the Uplink subsystemsbut with reverse positions for the interrogator and the transponder. The calculations are done formain and the control beam respectively as well.

Figure 44: System inside the downlink subsystem

3.6.1 EIRP XPDR

The EIRP for the transponder is calculated inside the subsystem which gives the maximum poweravailable for the reply signals leaving the transponder system. The subsystem includes systemlosses and noise image generated from thermal noise inside the transponder.

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Figure 45: System inside the EIRP block

3.6.2 Losses Interrogator

When arriving at the interrogator, the contributions to the link budget from the array system isthe gain in the the antennas and the system losses. These are included in subsystem named LossesINT according to Fig. 38. Again, ideal system values are used. The system’s thermal noise iscalculated in the in 37 with the temperature generated i the interrogator.

Figure 46: Losses inside the transponder

3.7 Interrogator antenna array subsystem (RX)

This subsystem simulates the structure and properties of the interrogator TRU antenna arraysystem in the receiving mode. It will treat the reply signal from the transponder after beingattenuated through the Downlink subsystem.

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Figure 47: Interrogator antenna array (RX) subsystem

Table 8: Subsystem block parameters

Port Function

Input

Punch angles front array Steering angle front array

Punch angles side array Steering angle side array

G RX XPDR reply Main Received gain main channel

G RX XPDR reply Control Received gain control channel

Active array Main Shows active array

Active array Control Shows active array

Front array beam steer Steering angle front array

Side array beam steer Steering angle side array

OutputG RX Main Gain main channel

G RX Control Gain control channel

The subsystem takes inputs from the Downlink linkbudget subsystem. The active array inputs arerouted via Data Store blocks AM and AC and the beam steering angles via Data Store blocks FBSand SBS from the Interrogator antenna subsystem (TX). The data containing the punchtroughangles and detection image for the transponder is routed via Data Store blocks PAF and PASfrom the Transponder subsystem. The gain in the main and control channels in the interrogatorantenna arrays are loaded into Data Store blocks GM and GS respectively for later use. Inside thesubsystem beam steering, triggered angles for the interrogator and plotting for detection levels isperformed. The MTL for the interrogator on Gripen E is included, even though it is masked here,and converted into dB. This sets the minimum magnitude the received beams can have in orderto trigger the interrogator functions.

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Figure 48: System inside the Interrogator antenna array (RX) subsystem

3.7.1 Beam steering

The beam steering angle set by the user back in the interrogator TX subsystem is loaded viainputs 7 an 8 into two new subsystems, one for the main and one for the control beam patternsrespectively. The same angle as for the transmit mode is used to verify that the transponder replycomes back from the same direction as the interrogation was sent.

3.7.2 Pattern generation

The pattern generation function block must also know which array is active automatically so thatthe user doesn’t have to set it manually in both interrogation antenna blocks. Based on the valueof the input active array beam data for the transmitting array data for the 1090 MHz casewill be loaded. The data has also been arranged in .mat-files and based on. Based on the angledi↵erent data sets will be loaded and modified to be stored into a matrix. If active array =3, the beam steering value of rec front will determine the data set loaded for the front arrayfor example while rec front will remain unused.

Figure 49: System inside the Interrogator antenna array (RX) subsystem

3.7.3 Detection level plotting

Plots showing the di↵erent detection levels for the signal beams as they pass through the systemis generated in the function block plotDetection. Two plots showing the detection image atthe transponder and at the interrogator in receive mode are displayed in the same figure using thecommand subplot which allows multiple plots in the same window.

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Figure 50: Function block for plotting detection levels

To conclude at which angles a complete link is formed a second plot is generated in the functionblock by comparing the stored values in matrices beam angles front with punch angles frontand beam angles side with punch angles side. A complete link is achieved when detec-tion is made at both the transponder and the interrogator. This shows at which distances anglesthe interrogator actually registers an answer to its question. The full link image is achieved bylogically comparing the data in these matrices and store the results in an new matrix according to:

0 = No full link connection for the angle.1 = Full link connection made but with uncertainty at the transponder.2 = Full link connection is achieved for the angle.

To achieve the desired appearances of the plots, the loaded matrices are plotted as images usingMatlab commands imagesc and colormap. With imagesc, pixelated images can be displayedwith scaled colors. Since every cell in the matrices is represented by a specific index, they canbe mapped to corresponding pixel in the plot. Every index being represented by a color. Withcolormap a specific color scheme can be set. A higher index will generate a brighter color andlower indexes a darker color. Since every situation described in 3.5.3 might not occur at the givendistance, the transponder can be triggered for all angles for example leaving out index 1, the limitsof the color scheme must be set beforehand. Otherwise, if there is no index 4 in the matrix, index 3will be plotted with the color index 4 is supposed to have since it will be the highest index instead.The limits are set with

lowerlimit XPDR = 1;upperlimit XPDR = 4;

�for the transponder plot

lowerlimit INT = 1;upperlimit INT = 3;

�for the interrogator plot

lowerlimit full = 0;upperlimit full = 2;

�for the full link plot.

Before plotting, the function block must determine which antenna array is active in order toset the right axes intervals when displaying the plots. When loaded into the block, the y-axisindexes will range from 1 to 15 set to -35 to 35 by the yaxis command. The x-axis ranges from 1to 1801 and are set to -180 for the front array, -180 to 0 for side array and 0 to 180 for other sidearray by xaxis.

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4 Results

In this section, examples of the resulting plots generated from the model will be displayed. Forthis report, mock-up plots have been created in order to illustrate how the interrogator-target-interrogator image looks like. Three plots at di↵erent distances between the fighter jet and thetarget are generated for each chosen beam steering. These distances are named shortest, middleand longest distance to illustrate how the attenuation e↵ects varies with distance. The plots willshow two cases, one for the front and one for the side array.

4.1 Front array

The boresight of the front antenna array is moved with y2o beam steering, causing the main lobeto point in the same direction. The antenna coverage diagrams are shown in Fig.51, 52 and 53for the three di↵erent distances. In the mode, Friis propagation is activated for the linkbudgetcalculations.

4.1.1 y2o beam steering, Friis propagation

(a) Detection angles (b) Full link connection angles

Figure 51: y2o beam steering at the closest distance

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(a) Detection angles (b) Full link connection angles

Figure 52: y2o beam steering at the middle distance

(a) Detection angles (b) Full link connection angles

Figure 53: y2o beam steering at the longest distance

4.2 Side array

4.2.1 x4o beam steering, Two-ray propagarion

The boresight of the front antenna array is moved with x4o beam steering, causing the main lobeto point in the same direction. The antenna coverage diagrams are shown in Fig.51, 52 and 53for the three di↵erent distances In the model, Two-ray propagation is activated for the linkbudgetcalculations.

50

(a) Detection angles

(b) Full link connection angles

Figure 54: x4o beam steering at the closest distance

(a) Detection angles

(b) Full link connection angles

Figure 55: x4o beam steering at the middle distance

51

(a) Detection angles

(b) Full link connection angles

Figure 56: x4o beam steering at the longest distance

4.2.2 Conclusions

For all beam steering angles, reply contributions can unfortunately be received and detected forother angles than the intended by the interrogator in the receiver mode. The full link connectionplots shows from which angles the extra, unwanted replies from other aircrafts than the target canbe received. The amount of extra replies are reduced with increased distance. So is the width ofthe main beam. At far distances the MTL-level might not be attained for some angles and areaswith no connection risks to creep in on the main beam’s detection interval. This increases the riskof obscuring target aircrafts and fail to identify them.

The simulations also showed that the two propagation models gave di↵erent detection imagesfor the same distance. Both models gave roughly the same detection span in the main lobe de-tection intervals at the transponder and the interrogator in receive mode. Two-ray propagationhowever was more inclined to generate sidelobe punchthroughs and larger areas of uncertainty atthe transponder at the transponder. Friis transmission in the other hand had a shorter reach withsignals dropping under below MTL earlier than Two-ray.

The Two-ray model was not sensitive to the height contribution. It gave roughly the same cover-age image for 2 km height distance as 3 km height distance between the two aircrafts for example.The transponder is modeled with constant gain of 0 dB for all angles, the target can be seen asmoving in a sphere around the interrogator with a constant radius. Then it is only the gain in theinterrogator antenna arrays that determines the performance of the system.

Increasing the signal strength without compromising the design of the antenna system is thereforeof interest This can be done through modulation. Considering the two modulation techniquesdiscussed in 2.3, the most e↵ective technique might be phase modulation since the thermal noiseis the largest contributor in the linkbudget giving a lower SNR than amplitude modulation.

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5 Discussion

This section includes comments about the specified goals set in 1.2 and suggestions for furtherwork to develop and improve the model. Before moving to the discussion about the goals, somepros and cons regarding the software of choice are made.

Pros: Simulink is very useful when building and connecting large system since it is rather straight-forward. It is a visual tool and simple to familiarize with without having to understand and read alot of code. It is also easy to make changes in the model if a new function, system or other aspectis introduced due to the drag-and-drop design of the software. Being an integrated part of Matlabis also a strength, since it makes it possible to tailor the functions in the model.

Cons: The plots are updated at every time-step when the simulation is running. This causesthem to blink on the screen which is a bit tiresome. This is valid for all things printed out onscreen, for an iteration of 10 time-steps, 10 message displays are printed out when only one iswanted for example. The toolboxes included in Simulink are licensed. This means that you haveto buy the license for the specific toolbox you want to use. Since no licenses were available forthe most useful toolboxes, another solution to the modeling of the system had to be figured out.It would save a lot of time having access to these toolboxes. When the system grows larger andmore complex the simulation time increases. The time consumption could be significantly loweredhaving the model written in code instead of having to wait a few minutes for every simulation.

5.1 Goals met

• The system model can be used to simulate the outcome and the behavior for the IFF systemon Gripen E. It can be used to study the coverage images of the antenna arrays at di↵erentdistance and di↵erent beam steering angles between the fighter jet and the target. An imageof the full link conditions can be achieved which can be of assistance when monitoring theperformance of the system,

• Key elements of the link budget for both uplink and downlink have been identified andmodeled.

• The transponder in the system has been modeled according to ICAO Annex 10 standards.This means that the civil transponders will behave in roughly the same way since they followthe same standard.

• Phase and amplitude modulation have not been included due to time limitations in the modelbut reasoning with focus on their impact on the possibility of detection has been made.

5.2 Further work

Some aspects for further work and implementation in the model are:

• Beam steering can only be made at specific angles. This is due to the data available forthe antenna arrays on Gripen E, which is only specified for some angles. Initially, beamsteering was supposed be specified by the user for any angle in the interval. To achieve fullfunctionality, this should be developed further.

• The theory behind beam steering and its formulas have been studied in the reading phaseof the thesis work in section 2.4.8. In the first attempts when creating the Simulink model,these formulas were implemented. However when trying to induce phase shifts in the antennaelements the result did not behave in an expected way. The idea was to take the data valuesgenerating the 0o beam steering antenna pattern and then multiplying with a given phaseto achieve the total beam pattern. When comparing with the data for the steered anglesthe patterns did not match. This could be due to that the given data was not compatiblewith the equations. If the proper mathematical formulas for the antenna elements used inthe arrays could be obtained then true beam steering might be possible to achieve,

53

• Another idea in order to obtain a beam steering for all angels could be to try to use scientificcomputing on available data. This by applying linear regression or interpolation on the datavalues and see if a mathematical representation of the data can be written and implementedin the model.

• No measures has been taken to account for attenuation by the aircraft’s body on the antennapatterns. This could be modeled for the angles radiating in direction to the body.

• The link budget could be developed further to include more aspects. There are more com-ponents in the electrical system that could possibly have an impact on the linkbudget cal-culations. Even if their respective contributions are small, their total value could a↵ect theoutcome.

• The model could be used to investigate some other propagation models. The fighter jetoperates in the sky so selecting a model treating the atmospheric attenuation on di↵erentaltitudes and/or the curvature of the Earth could be of interest.

• At the moment the modes included and investigated in the transponder block are Mode 1 2,3A and C. Further work could be to include the other encrypted Modes as well to achievethe system’s performance even for the military modes.

• Since the SSR-system is based on timing of the pulses, the model could be developed totreating the aspect if the propagation model used causes some kind of time delay.

• Phase and amplitude detectors could be modeled in order to see if the conclusion regradingthe suitability of using these methods in the model is correct or if other e↵ects are achieved.

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6 Appendix A: Simulink blocks

This appendix contains descriptions of the Simulink blocks used in the model:

Line: The line is used to connect blocks together. The value is the same atthe output of the previous block and the input of the next block.

Constant: With the Constant block, a real or complex value can be generated.The dimension of the value can either be a scalar, vector or matrix based onthe output settings.

Sum: The sum block adds or subtracts the value of two or more inputs inscalar, vector or matrix form.

Inport: Inports are connections between subsystems to other outside systems.The external input fed into the system is unchanged until altered in thesubsystem.

Outport: Outports are connections between subsystems to other outsidesystems. The outport value can be used in the external systems.

Data Store Memory: The Switch block allows the user to choose which of twoinputs to pass through the output by double-clicking on the Switch block icon.The Swith can be toggled while Simulink is executing the model, in this waythe signal flow can be changed both before and during simulation. It acceptsboth scalar and vector values.

Data Store Memory: Data Store blocks are used for signal routing. TheMemory block initializes a shared data store with the blocks Data Store Readand Data Store if they share the same data store name. The functionality ofthis system is dependent of hierarchy. If the Data Store Memory block is in atop-level system, Data Store Read and Data Store Write blocks at any levelcan acess the storage. If it on the other hand is in a subsystem, only DataStore Read and Data Store Write blocks at the same level or below can acessthe data store. The value of the data can be in scalar, vector, matrix and N-Darray form.

Data Store Read: The Data Store Read block copies stored data to its output.Several Data Store Read blocks in the model can read from the same datastore if they share the same name. The data is retrieved from the location ofthe Data Store Memory block which defines the data store.

Data Store Write: The Data Store Write block copies stored data to its input.When a write operation is performed the previous content of the blocks datastore is replaced. The amount of the data must the same as in the Data StoreMemory block. Multiple Data Store Write Blocks can write from the samedata Store, but unpredictable results can arise if two blocks attempt to do soduring the same simulation step.

Subsystem: Multiple subsystems can be linked together to form a largersystem. A subsystem is created by grouping together multiple blocks.

55

Embedded Matlab Function block: Within this block, the user can write anown custom function using Matlab’s built-in editor for use in Simulink. Theblock accepts inputs from the Simulink model and and gives outputs back toit. Both are defined in the function header as arguments and return values.The block can also be modeled without either input or output.

Knob: The knob block can be connected to another block parameter of choicein order to tune its value for the simulation. The range and values of the Knobblock can be modified to fit the data.

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7 Appendix B: MATLAB Code

Matlab code used in the system model. Displayed in alphabetical order.

activeArray.m:

1 function active_array_main = activeArray(front_array, side_array, ref)23 % Setting a constant depending on which antenna array is active.4 if ref == front_array5 active_array_main = 3;6 elseif ref == side_array7 active_array_main = 2;8 else9 active_array_main = 1;10 end11 end

checkMTL.m:

1 function [beam_angles_front,beam_angles_side] = checkMTL(MTL,G_RX_INT_main, G_RX_INT_control, active_main, active_control)

23 % Prepearing matrices for storing detection levels:4 beam_angles_front = zeros(15,1801);5 beam_angles_side = zeros(15,900);67 % Parameter name change:8 main = G_RX_INT_main;9 control = G_RX_INT_control;1011 % Index values describes different situations as follows:12 % 1 = The value of the beam at the angle is too weak to trigger

interrogator.13 % 2 = Control level is higher than Main level. Interrogator will not be

triggered.14 % 3 = Main level is higher than Control level. Interrogator will be

triggered.1516 % For front array:17 if active_main == 2 && active_control == 21819 beam_levels = main > control;20 below_MTL = main < MTL;2122 for i = 1:1523 for j = 1:length(beam_levels(i,:))24 if beam_levels(i,j) == 125 beam_angles_front(i,j) = 3;26 else27 beam_angles_front(i,j) = 2;28 end29 for j = 1:length(below_MTL(i,:))30 if below_MTL(i,j) == 131 beam_angles_front(i,j) = 1;32 end33 end

57

34 end35 end3637 % For side array:38 elseif active_main == 1 && active_control == 13940 beam_levels = main(1:15,length(main)/2+1:end) >41 control(1:15,length(control)/2+1:end);42 below_MTL = main(1:15,length(main)/2+1:end) < MTL;4344 for i = 1:1545 for j = 1:length(beam_levels(i,:))46 if beam_levels(i,j) == 147 beam_angles_side(i,j) = 3;48 else49 beam_angles_side(i,j) = 2;50 end51 end52 end53 for i = 1:1554 for j = 1:length(below_MTL(i,:))55 if below_MTL(i,j) == 156 beam_angles_side(i,j) = 1;57 end58 end59 end6061 % For other side array:62 else6364 beam_levels = main(1:15,1:length(main)/2) >65 control(1:15,1:length(control)/2);66 below_MTL = main(1:15,1:length(main)/2) < MTL;6768 for i = 1:1569 for j = 1:length(beam_levels(i,:))70 if beam_levels(i,j) == 171 beam_angles_side(i,j) = 3;72 else73 beam_angles_side(i,j) = 2;74 end75 end76 end77 for i = 1:1578 for j = 1:length(below_MTL(i,:))79 if below_MTL(i,j) == 180 beam_angles_side(i,j) = 1;81 end82 end83 end84 end85 end

dBconverter.m:

1 function dB = dBconverter(W)

58

23 % Converting Watts into dB:4 dB = 10*log10(W/1);56 end

FriisTransmission.m:

1 function [Friis_Main1030, Friis_Control1030, Friis_Main1090,Friis_Control1090] = FriisTransmission(f_1090, f_1030,coor_TX,coor_RX, G_TX_Main, G_TX_Control, P_TX_INT, G_RX_XPDR, G_TX_XPDR,P_TX_XPDR, G_RX_main,G_RX_control)

23 c = physconst('LightSpeed');45 % Wavelength calculations for different frequencies:6 lambda_inter = 20*log10(c/f_1030);7 lambda_transp = 20*log10(c/f_1090);89 % Distance to target:10 d = coor_RX(1)-coor_TX(1);111213 % Frii's transmission formula calculated in dB for uplink:14 Friis_Main1030 = (P_TX_INT + G_TX_Main + G_RX_XPDR + lambda_inter)15 -(10*log10((4*pi*d)ˆ2));16 Friis_Control1030 = (P_TX_INT + G_TX_Control + G_RX_XPDR17 +lambda_inter)-(10*log10((4*pi*d)ˆ2));1819 % Frii's transmission formula calculated in dB for downlink:20 Friis_Main1090 = (P_TX_XPDR + G_TX_XPDR + G_RX_main + lambda_transp)

-(10*log10((4*pi*d)ˆ2));21 Friis_Control1090 = (P_TX_XPDR + G_TX_XPDR + G_RX_control +

lambda_transp)-(10*log10((4*pi*d)ˆ2));2223 end

generateSide.m:

1 function radPat_side = generateSide(pat_val)23 % Flips loaded vector horizontally:4 radPat_side = fliplr(pat_val);56 end

misMatch.m

1 function ML= misMatch(VSWR);23 % Mismatch loss in dB:4 ML = -10*log10(1-((VSWR-1)/(VSWR+1))ˆ2);56 end

plotDetection.m:

59

1 function plotDetection(beam_angles_front, beam_angles_side,active_main,active_control, punch_angles_front, punch_angles_side)

23 coder.extrinsic('imagesc');4 coder.extrinsic('colorbar');56 % Setting limits for colormap plot, XPDR:7 lowerlimit_XPDR = 1;8 upperlimit_XPDR = 4;910 % Setting limits for colormap plot, INT:11 lowerlimit_INT = 1;12 upperlimit_INT = 3;1314 % Setting limits for colormap plot, full link connection:15 lowerlimit_full = 0;16 upperlimit_full = 2;1718 % For front array:19 if active_main == 2 && active_control == 22021 % Setting plot axes:22 xtick = -180:0.2:180;23 ytick = -35:5:35;2425 % Plotting punchthrough and SLS for front array at XPDR:26 figure(3)27 subplot(2,1,1)28 imagesc(xtick,ytick,punch_angles_front);29 set(gca, 'YDir', 'normal');30 colormap(hot);31 caxis('manual');32 caxis([lowerlimit_XPDR upperlimit_XPDR]);33 colorbar34 title('Punchthrough and SLS angles front array, XPDR')35 xlabel('Azimuth angle')36 ylabel('Eleveation angle')3738 % Plotting detection levels for front array at interrogator:39 subplot(2,1,2)40 imagesc(xtick,ytick,beam_angles_front);41 set(gca, 'YDir', 'normal');42 colormap(hot);43 caxis('manual');44 caxis([lowerlimit_INT upperlimit_INT]);45 colorbar46 title('Detection angles front array, INT')47 xlabel('Azimuth angle')48 ylabel('Eleveation angle')4950 % Storing all punchthrough angles in a matrix. Multiplying by51 % factor 2 for later colormap plot.52 full_punch = 2*(punch_angles_front == 4 & beam_angles_front ==3);5354 % Storing all grey-area angles in a matrix.

60

55 full_grey = punch_angles_front == 3 & beam_angles_front ==3;5657 % Adding the matrices together to achieve full link image:58 full_link = full_punch + full_grey;5960 % Plotting full link angles for front array:61 figure(4);62 imagesc(xtick,ytick,full_link);63 set(gca, 'YDir', 'normal');64 colormap(hot);65 caxis('manual');66 caxis([lowerlimit_full upperlimit_full]);67 colorbar68 title('Full link connection, front array')69 xlabel('Azimuth angle')70 ylabel('Eleveation angle')7172 % For side array:73 elseif active_main == 1 && active_control == 174 xtick = 0:0.2:180;75 ytick = -35:5:35;7677 figure(3)78 subplot(2,1,1);79 imagesc(xtick, ytick, punch_angles_side)80 set(gca, 'YDir', 'normal');81 colormap(hot);82 caxis('manual');83 caxis([lowerlimit_XPDR upperlimit_XPDR]);84 colorbar85 title('Punchthrough and SLS angles side array, XPDR');86 xlabel('Azimuth angle');87 ylabel('Eleveation angle');8889 subplot(2,1,2);90 imagesc(xtick, ytick, beam_angles_side)91 set(gca, 'YDir', 'normal');92 colormap(hot);93 caxis('manual');94 caxis([lowerlimit_INT upperlimit_INT]);95 colorbar96 title('Detection angles side array, INT');97 xlabel('Azimuth angle');98 ylabel('Eleveation angle');99100 % Storing all punchthrough angles in a matrix. Multiplying by101 % factor 2 for later colormap plot.102 full_punch = 2*(punch_angles_side == 4 & beam_angles_side ==3);103104 % Storing all grey-area angles in a matrix.105 full_grey = punch_angles_side == 3 & beam_angles_side ==3;106107 % Adding the matrices together to achieve full link image:108 full_link = full_punch + full_grey;109110 % Plotting full link angles for front array:

61

111 figure(4);112 imagesc(xtick,ytick,full_link);113 set(gca, 'YDir', 'normal');114 colormap(hot);115 caxis('manual');116 caxis([lowerlimit_full upperlimit_full]);117 colorbar118 title('Full link connection, side array')119 xlabel('Azimuth angle')120 ylabel('Eleveation angle')121122 % For switched side array:123 else124 % Setting plot axes:125 xtick = -180:0.2:0;126 ytick = -35:5:35;127128 figure(3)129 subplot(2,1,1);130 imagesc(xtick, ytick, punch_angles_side)131 set(gca, 'YDir', 'normal');132 colormap(hot);133 caxis([lowerlimit_XPDR upperlimit_XPDR]);134 caxis('manual');135 colorbar136 title('Punchthrough and SLS angles side array, XPDR');137 xlabel('Azimuth angle');138 ylabel('Eleveation angle');139140 subplot(2,1,2);141 imagesc(xtick, ytick, beam_angles_side)142 set(gca, 'YDir', 'normal');143 colormap(hot);144 caxis('manual')145 caxis([lowerlimit_INT upperlimit_INT]);146 colorbar147 title('Detection angles side array, INT');148 xlabel('Azimuth angle');149 ylabel('Eleveation angle');150151 % Storing all punchthrough angles in a matrix. Multiplying by152 % factor 2 for later colormap plot.153 full_punch = 2*(punch_anglesfront == 4 & beam_angles_front ==3);154155 % Storing all grey-area angles in a matrix.156 full_grey = punch_anglesfront == 3 & beam_angles_front ==3;157158 % Adding the matrices together to achieve full link image:159 full_link = full_punch + full_grey;160161 % Plotting full link angles for front array:162 imagesc(xtick,ytick,full_link);163 set(gca, 'YDir', 'normal');164 colormap(hot);165 caxis('manual');166 caxis([lowerlimit_full upperlimit_full]);

62

167 colorbar168 title('Full link connection, other side array')169 xlabel('Azimuth angle')170 ylabel('Eleveation angle')171 end172 end

plotPatternINT.m:

1 function plotPatternINT(INT_tx_pattern_main, INT_tx_pattern_control)23 % Azimuth angles vector:4 az_ang = (-180:0.2:180);56 % Elevation angles vector:7 ele_ang = -35:5:35;89 % Plotting beam patterns leaving interrogator in 3D:10 figure(1);11 colormap parula;12 subplot(2,1,1)13 surf(az_ang, ele_ang, INT_tx_pattern_main, 'LineStyle', 'none')14 title('Main beam from TRU')15 xlabel('Azimuth angle, degrees')16 ylabel('Elevation angle, degrees')17 zlabel('Magnitude, dB')1819 subplot(2,1,2)20 surf(az_ang, ele_ang, INT_tx_pattern_control, 'LineStyle', 'none')21 title('Control beam from TRU')22 xlabel('Azimuth angle, degrees')23 ylabel('Elevation angle, degrees')24 zlabel('Magnitude, dB')2526 end

plotPatternXPDR.m:

1 function plotPatternXPDR(XPDR_rx_pattern_main, XPDR_rx_pattern_control)23 % Azimuth angles, degrees:4 az_ang = (-180:0.2:180);56 % Elevation angles, degrees:7 ele_ang = -35:5:35;89 % Plotting beam patterns arriving at transponder in 3D:10 figure(2)11 subplot(2,1,1)12 surf(az_ang, ele_ang, XPDR_rx_pattern_main, 'LineStyle', 'none')13 title('Main beam at target')14 xlabel('Azimuth angle')15 ylabel('Elevation angle')16 zlabel('Magnitude')17 colormap('parula');18

63

19 subplot(2,1,2)20 surf(az_ang, ele_ang, XPDR_rx_pattern_control, 'LineStyle', 'none')21 title('Control beam at target')22 xlabel('Azimuth angle')23 ylabel('Elevation angle')24 zlabel('Magnitude')25 end

radpat Control1090.m:

1 function radPat_out = radPat_Control1090(rec_front, rec_side,active_array)

23 % Intervals with beam data:4 % int1 contains data for the biggest elevation angle from the sheet.5 % int2 contains data for the middle elevation angle from the sheet.6 % int3 contains data for the smallest elevation angle from the sheet.7 int1 = 1:1801;8 int2 = 2001:3801;9 int3 = 4001:5801;1011 % Side array acrive12 if active_array == 11314 % % Loading the right .mat file according to beam steering: x15 if rec_side == 016 cont = load('side1090_cont_0.mat');17 elseif rec_side == -1018 cont = load('side1090_cont_neg10.mat');19 elseif rec_side == -2020 cont = load('side1090_cont_neg20.mat');21 elseif rec_side == -3022 cont = load('side1090_cont_neg30.mat');23 elseif rec_side == -5024 cont = load('side1090_cont_neg50.mat');25 else26 cont = load('side1090_cont_10.mat');27 end2829 radPat_out = [cont.side_data_cont2(int1)';30 cont.side_data_cont3(int3)'; cont.side_data_cont3(int2)

'; cont.side_data_cont3(int1)';31 cont.side_data_cont4(int3)'; cont.side_data_cont4(int2)

'; cont.side_data_cont4(int1)';32 cont.side_data_cont5(int3)'; cont.side_data_cont5(int2)

'; cont.side_data_cont5(int1)';33 cont.side_data_cont6(int3)'; cont.side_data_cont6(int2)

'; cont.side_data_cont6(int1)';34 cont.side_data_cont7(int3)'; cont.side_data_cont7(int2)

'];3536 % Front array active37 elseif active_array == 33839 % % Loading the right .mat file according to beam steering:40 if rec_front == 0

64

41 cont = load('front1090_cont_0.mat');42 elseif rec_front == 1543 cont = load('front1090_cont_15.mat');44 elseif rec_front == 3545 cont = load('front1090_cont_35.mat');46 elseif rec_front == 5547 cont = load('front1090_cont_55.mat');48 elseif rec_front == -1549 cont = load('front1090_cont_neg15.mat');50 elseif rec_front == -3551 cont = load('front1090_cont_neg35.mat');52 else53 cont = load('front1090_cont_neg55.mat');54 end5556 radPat_out = [cont.front_data_cont2(int1)';57 cont.front_data_cont3(int3)'; cont.front_data_cont3(int2)

'; cont.front_data_cont3(int1)';58 cont.front_data_cont4(int3)'; cont.front_data_cont4(int2)

'; cont.front_data_cont4(int1)';59 cont.front_data_cont5(int3)'; cont.front_data_cont5(int2)

'; cont.front_data_cont5(int1)';60 cont.front_data_cont6(int3)'; cont.front_data_cont6(int2)

'; cont.front_data_cont6(int1)';61 cont.front_data_cont7(int3)'; cont.front_data_cont7(int2)

'];6263 % Other side array active:64 else6566 % Loading the right .mat file according to beam steering and67 % flips it to achoeve other side pattern:68 if rec_side == 069 cont = load('side1090_cont_0.mat');70 elseif rec_side == -1071 cont = load('side1090_cont_neg10.mat');72 elseif rec_side == -2073 cont = load('side1090_cont_neg20.mat');74 elseif rec_side == -3075 cont = load('side1090_cont_neg30.mat');76 elseif rec_side == -5077 cont = load('side1090_cont_neg50.mat');78 else79 cont = load('side1090_cont_10.mat');80 end81 radPat = [cont.side_data_cont2(int1)';82 cont.side_data_cont3(int3)'; cont.side_data_cont3(int2)';

cont.side_data_cont3(int1)';83 cont.side_data_cont4(int3)'; cont.side_data_cont4(int2)';

cont.side_data_cont4(int1)';84 cont.side_data_cont5(int3)'; cont.side_data_cont5(int2)';

cont.side_data_cont5(int1)';85 cont.side_data_cont6(int3)'; cont.side_data_cont6(int2)';

cont.side_data_cont6(int1)';86 cont.side_data_cont7(int3)'; cont.side_data_cont7(int2)'];87 radPat_out = fliplr(radPat);

65

88 end89 end

radpat Main1090.m:

1 function radPat_out = radpat_Main1090(rec_front, rec_side, active_array)

23 % Intervals with elevation angles beam data:4 int1 = 1:1801;5 int2 = 2001:3801;6 int3 = 4001:5801;78 % Front array active:9 if active_array == 31011 % Loading the right .mat file according to beam steering:12 if rec_front == 013 main = load('front1090_main_0.mat');14 elseif rec_front == 1515 main = load('front1090_main_15.mat');16 elseif rec_front == 4517 main = load('front1090_main_35.mat');18 elseif rec_front == 5519 main = load('front1090_main_55.mat');20 elseif rec_front == -1021 main = load('front1090_main_neg10.mat');22 elseif rec_front == -3523 main = load('front1090_main_neg35.mat');24 else25 main = load('front1090_main_neg55.mat');26 end27 radPat_out = [main.front_data_main2(int1)';28 main.front_data_main3(int3)'; main.front_data_main3(

int2)'; main.front_data_main3(int1)';29 main.front_data_main4(int3)'; main.front_data_main4(

int2)'; main.front_data_main4(int1)';30 main.front_data_main5(int3)'; main.front_data_main5(

int2)'; main.front_data_main5(int1)';31 main.front_data_main6(int3)'; main.front_data_main6(

int2)'; main.front_data_main6(int1)';32 main.front_data_main7(int3)'; main.front_data_main7(

int2)'];3334 % Side array active:35 elseif active_array == 13637 % Loading the right .mat file according to beam steering:38 if rec_side == 039 main = load('side1090_main_0.mat');40 elseif rec_side == -1041 main = load('side1090_main_neg10.mat');42 elseif rec_side == -2043 main = load('side1090_main_neg20.mat');44 elseif rec_side == -3045 main = load('side1090_main_neg30.mat');

66

46 elseif rec_side == -5047 main = load('side1090_main_neg50.mat');48 else49 main = load('side1090_main_10.mat');50 end5152 radPat_out = [main.side_data_main2(int1)';53 main.side_data_main3(int3)'; main.side_data_main3(int2)

'; main.side_data_main3(int1)';54 main.side_data_main4(int3)'; main.side_data_main4(int2)

'; main.side_data_main4(int1)';55 main.side_data_main5(int3)'; main.side_data_main5(int2)

'; main.side_data_main5(int1)';56 main.side_data_main6(int3)'; main.side_data_main6(int2)

'; main.side_data_main6(int1)';57 main.side_data_main7(int3)'; main.side_data_main7(int2)

'];5859 % Other side array active:60 else6162 % Loading the right .mat file according to beam steering and63 % flips it to achieve other side pattern:64 if rec_side == 065 main = load('side1090_main_0.mat');66 elseif rec_side == -1067 main = load('side1090_main_neg10.mat');68 elseif rec_side == -2069 main = load('side1090_main_neg20.mat');70 elseif rec_side == -3071 main = load('side1090_main_neg30.mat');72 elseif rec_side == -5073 main = load('side1090_main_neg50.mat');74 else75 main = load('side1090_main_10.mat');76 end7778 radPat = [main.side_data_main2(int1)';79 main.side_data_main3(int3)'; main.side_data_main3(int2)';

main.side_data_main3(int1)';80 main.side_data_main4(int3)'; main.side_data_main4(int2)';

main.side_data_main4(int1)';81 main.side_data_main5(int3)'; main.side_data_main5(int2)';

main.side_data_main5(int1)';82 main.side_data_main6(int3)'; main.side_data_main6(int2)';

main.side_data_main6(int1)';83 main.side_data_main7(int3)'; main.side_data_main7(int2)'];84 radPat_out = fliplr(radPat);85 end86 end

radpat front Control.m:

1 function radPat_Front = radpat_front_Control(steerAngle)23 % Elevation angle intervals with beam data:

67

4 int1 = 1:1801;5 int2 = 2001:3801;6 int3 = 4001:5801;78 % Loading the right.mat file according to beam steering:9 if steerAngle == 010 cont = load('front1030_cont_0.mat');11 elseif steerAngle == 1512 cont = load('front030_cont_15.mat');13 elseif steerAngle == 3514 cont = load('front1030_cont_35.mat');15 elseif steerAngle == 5516 cont = load('front1030_cont_55.mat');17 elseif steerAngle == -1518 cont = load('front1030_cont_neg15.mat');19 elseif steerAngle == -3520 cont = load('front1030_cont_neg35.mat');21 else22 cont = load('front1030_cont_neg50.mat');23 end24 radPat_Front = [cont.front_data_cont2(int1)';25 cont.front_data_cont3(int3)'; cont.

front_data_cont3(int2)'; cont.front_data_cont3(int1)';

26 cont.front_data_cont4(int3)'; cont.front_data_cont4(int2)'; cont.front_data_cont4(int1)';

27 cont.front_data_cont5(int3)'; cont.front_data_cont5(int2)'; cont.front_data_cont5(int1)';

28 cont.front_data_cont6(int3)'; cont.front_data_cont6(int2)'; cont.front_data_cont6(int1)';

29 cont.front_data_cont7(int3)'; cont.front_data_cont7(int2)'];

30 end

radpat side Main.m:

1 function radPat_Front = radpat_front_Main(steerAngle)23 % Elevation angle intervals with beam data:4 int1 = 1:1801;5 int2 = 2001:3801;6 int3 = 4001:5801;78 % Loading the right.mat file according to beam steering:9 if steerAngle == 010 main = load('front1030_main_0.mat');11 elseif steerAngle == 1512 main = load('front030_main_15.mat');13 elseif steerAngle == 33.7514 main = load('front1030_main_35.mat');15 elseif steerAngle == 50.6216 main = load('front1030_main_55.mat');17 elseif steerAngle == -11.25

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18 main = load('front1030_main_neg15.mat');19 elseif steerAngle == -33.7520 main = load('front1030_main_neg35.mat');21 else22 main = load('front1030_main_neg50.mat');23 end24 radPat_Front = [main.front_data_main2(int1)';25 main.front_data_main3(int3)'; main.

front_data_main3(int2)'; main.front_data_main3(int1)';

26 main.front_data_main4(int3)'; main.front_data_main4(int2)'; main.front_data_main4(int1)';

27 main.front_data_main5(int3)'; main.front_data_main5(int2)'; main.front_data_main5(int1)';

28 main.front_data_main6(int3)'; main.front_data_main6(int2)'; main.front_data_main6(int1)';

29 main.front_data_main7(int3)'; main.fr30 ont_data_main7(int2)'];31 end

radpat side Control.m:

1 function radPat_Side = radpat_side_Control(steerAngle)23 % Elevation angle intervals with beam data:4 int1 = 1:1801;5 int2 = 2001:3801;6 int3 = 4001:5801;78 % Loading the right.mat file according to beam steering:9 if steerAngle == 010 cont = load('side1030_cont_0.mat');11 elseif steerAngle == -1012 cont = load('side1030_cont_neg10.mat');13 elseif steerAngle == -2014 cont = load('side1030_cont_neg20.mat');15 elseif steerAngle == -5016 cont = load('side1030_cont_neg50.mat');17 else18 cont = load('side1030_cont_10.mat');19 end20 radPat_Side = [cont.side_data_cont2(int1)';21 cont.side_data_cont3(int3)'; cont.side_data_cont3(

int2)'; cont.side_data_cont3(int1)';22 cont.side_data_cont4(int3)'; cont.side_data_cont4(

int2)'; cont.side_data_cont4(int1)';23 cont.side_data_cont5(int3)'; cont.side_data_cont5(

int2)'; cont.side_data_cont5(int1)';24 cont.side_data_cont6(int3)'; cont.side_data_cont6(

int2)'; cont.side_data_cont6(int1)';25 cont.side_data_cont7(int3)'; cont.side_data_cont7(

int2)'];26 end

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radpat side Main.m:

1 function radPat_Side = radPat_side_Main(steerAngle)23 % Elevation angle intervals with azimuth beam datapoints:4 int1 = 1:1801;5 int2 = 2001:3801;6 int3 = 4001:5801;789 % Loading the right.mat file according to beam steering:10 if steerAngle == 011 main = load('side1030_main_0.mat');12 elseif steerAngle == -1013 main = load('side1030_main_neg10.mat');14 elseif steerAngle == -2015 main = load('side1030_main_neg20.mat');16 elseif steerAngle == -5017 main = load('side1030_main_neg50.mat');18 else19 main = load('side1030_main_10.mat');20 end2122 % Creating beam data matrix:23 radPat_Side = [main.side_data_main2(int1)';24 main.side_data_main3(int3)'; main.side_data_main3(int2)

'; main.side_data_main3(int1)';25 main.side_data_main4(int3)'; main.side_data_main4(int2)

'; main.side_data_main4(int1)';26 main.side_data_main5(int3)'; main.side_data_main5(int2)

'; main.side_data_main5(int1)';27 main.side_data_main7(int3)'; main.side_data_main7(int2)

'];28 end

thermalNoise INT.m:

1 function Thermal_noise = thermalNoise_INT(Bandwidth,T_INT)23 k = physconst('Boltzmann');45 % Celsius to Kelvin:6 T_Kelvin = T_INT + 273.15;78 % Thermal noise in dB:9 Thermal_noise = 10*log10(k*T_Kelvin)+10*log10(Bandwidth);1011 end

thermalNoise XPDR.m:

1 function Thermal_noise = thermalNoise_XPDR(Bandwidth,T_XPDR)23 k = physconst('Boltzmann');

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45 % Celsius to Kelvin:6 T_Kelvin = T_XPDR + 273.15;78 % Thermal noise in dB:9 Thermal_noise = 10*log10(k*T_Kelvin)+10*log10(Bandwidth);1011 end

trigSLS.m:

1 function [punch_angles_front, punch_angles_side] = trigSLS(lim, MTL,active_main, active_control, P1, P2)

23 coder.extrinsic('imagesc');45 % Preparing matrices for storing punchthrough and detection angles:6 punch_angles_front = zeros(15,1801);7 punch_angles_side = zeros(15,900);89 % Checking sidelobe levels and MTL for main and control beam at10 % XPDR:1112 % Index values describes different situations as follows:13 % 1 = The value of the beam at the angle is too weak to trigger XPDR.14 % 2 = SLS is guaranteed to be in action, XPDR will not be triggered.15 % 3 = SLS uncertainty area, XPDR might be triggered.16 % 4 = Punch through, XPDR will be triggered.1718 % For front array:19 if active_main == 3 && active_control == 320 lobe_punch = P1 > P2+lim;21 lobe_grey = P1 > P2 & P1 < P2+lim;22 below_MTL = P1 < MTL;23 for i = 1:1524 for j = 1:length(lobe_punch(i,:))25 if lobe_punch(i,j) == 126 punch_angles_front(i,j) = 4;27 else28 punch_angles_front(i,j) = 2;29 end30 end31 for j = 1:length(lobe_grey(i,:))32 if lobe_grey(i,j) == 133 punch_angles_front(i,j) = 3;34 end35 end36 for j = 1:length(below_MTL(i,:))37 if below_MTL(i,j) == 138 punch_angles_front(i,j) = 1;39 end40 end41 end4243 % For side array:44 elseif active_main == 1 && active_control == 1

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454647 % For side array:48 else49 lobe_punch = P1(1:15,length(P1)/2+1:end) > P2(1:15,length(P1)

/2+1:end)+lim;50 lobe_grey = P1(1:15,length(P1)/2+1:end) > P2(1:15,length(P1)/2+1:

end) &51 P1(1:15,length(P1)/2+1:end) < P2(1:15,length(P1)/2+1:end)+lim

;52 below_MTL = P1(1:15,length(P1)/2+1:end) < MTL;53 for i = 1:1554 for j = 1:length(lobe_punch(i,:))55 if lobe_punch(i,j) == 156 punch_angles_side(i,j) = 4;57 else58 punch_angles_side(i,j) = 2;59 end60 end61 for j = 1:length(lobe_grey(i,:))62 if lobe_grey(i,j) == 163 punch_angles_side(i,j) = 3;64 end65 end66 for j = 1:length(below_MTL(i,:))67 if below_MTL(i,j) == 168 punch_angles_front(i,j) = 1;69 end70 end71 end72 end73 end

twoRayProp.m:

1 function [Two_ray_main1030, Two_ray_control1030,Two_ray_main1090,Two_ray_control1090] = twoRayProp(coor_TX, coor_RX, G_TX_main,G_TX_control, P_TX_INT, G_RX_XPDR, G_TX_XPDR, P_TX_XPDR, G_RX_main,G_RX_control)

23 % Height of transmitter/interrogator over reflective surface with

coordinates [0,0]:4 h_tx = coor_TX(2);56 % Height of receiver/target over reflective surface with coordinates

[0,0]:7 h_rx = coor_RX(2);89 % Distance between interrogating and target aircraft:10 dist = coor_RX(1)-coor_TX(1);1112 % Two-ray propagation calculations in dB-scale for uplink:13 Two_ray_main1030 = P_TX_INT + G_TX_main + G_TX_control + G_RX_XPDR +

10*log10(h_tx*h_rx) - 40*log10(dist);14 Two_ray_control1030 = P_TX_INT + G_TX_main + G_TX_control + G_RX_XPDR +

10*log10(h_tx*h_rx) - 40*log10(dist);

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1516 % Two-ray propagation calculations in dB-scale for dwpnlink:17 Two_ray_main1090 = P_TX_XPDR + G_RX_main + G_TX_XPDR + 10*log10(h_tx*

h_rx) - 40*log10(dist);18 Two_ray_control1090 = P_TX_XPDR + G_RX_control + G_TX_XPDR + 10*log10(

h_tx*h_rx) - 40*log10(dist);1920 end

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References

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