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MARSIS: Mars Advanced Radar for Subsurface and Ionosphere Sounding

MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) is a lowfrequency, nadir-looking pulse limited radar sounder and altimeter with groundpenetration capabilities, which uses synthetic aperture techniques and a secondary

receiving antenna to isolate subsurface reflections.

MARSIS will send low frequency radio waves (1.3-5.5 MHz) towards the planet from a40 m long antenna which will be unfurled after the spacecraft goes into orbit. The entireinstrument including antenna and data processing unit weighs about 12 kg.

MARSIS functions by transmitting a linear frequency modulated chirp using a nadir-looking dipole antenna. The return signal is received on both the dipole antenna and asecondary monopole antenna oriented along the nadir axis. The secondary antennahas a null in the nadir direction and receives primarily the off-nadir surface reflections.This signal can be subtracted from the main received signal during ground processingto reduce surface clutter. Both received signals are down converted to range offset

video signals before being passed to an analogue to digital converter. The resultantdata are formatted by the MARSIS on-board digital processor and passed to thespacecraft for transmission to Earth.

Sounding the subsurface

The radio waves will be reflected from any surface they encounter. For most, this willbe the surface of Mars. But because of the low frequency, a significant fraction willtravel through the crust to encounter further interfaces between layers of differentmaterial. Consequently, a layer containing liquid water should generate a radar echo.The presence of weaker signals after the first strong surface return will enable to detect

subsurface interfaces, while the time delay between the two signals will enable tomeasure the depth of the interfaces.

By sending two different frequencies at the same time and analysing the echoesgenerated, MARSIS will be able to extract information on the electrical properties of thereflecting surface and hence its composition. The radio waves will be reflected at anyinterface, not only that between rock and water, so MARSIS should reveal much aboutthe composition of the top 5 km of crust in general. It should, for example, pick outlayers of rock interspersed with ice, which are more likely to exist close to the Martiansurface than liquid water.

Ilustracin 1. Principle of MARSIS operation

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The best ground penetrating studies are made during night when the Martianionosphere is least active and when the spacecraft is less than 800 km from theMartian surface.

Sounding the ionosphere

MARSIS ionospheric measurements employ both passive and active techniques.The passive technique uses the thermal emission line at the local electronplasma frequency to make highly accurate measurements of the local electron density.The active technique uses radar signals (soundings) to measure the vertical range tothe ionospheric reflection point as a function of frequency.

During the day, sunlight ionises the upper atmosphere and long wavelength radiowaves bounce off it. Those that are reflected from the ionosphere can reveal muchabout its structure. MARSIS will measure the electron density in the ionosphere andhence quantify the effect of charged particles streaming out from the Sun on the upperatmosphere. Such measurements will help to find out whether the unremitting

depredations of the solar wind over billions of years have stripped Mars of much of itsatmosphere.

Operation Modes

MARSIS operates in the following modes:

Subsurface Sounding

Active Ionospheric Sounding

Receive Only

Calibration

MARSIS will perform Subsurface Sounding when the spacecraft is less than 800 kmabove the Martian surface. Over the nominal mission lifetime, extensive coverage at alllatitudes will be possible. To achieve this global coverage MARSIS supports bothdayside and nightside operations, although performance is maximised during the nightwhen the ionosphere plasma frequency drops significantly and the lower frequencybands, which have greater ground penetration capabilities, can be used.

Active Ionospheric Sounding will be carried out during certain orbital passes whenthe orbiter is less than 1200 km above the surface, in order to gather scientific data onthe Martian ionosphere.

Receive Only mode will mainly be used to characterise, from an electromagnetic pointof view, the environment in which MARSIS is working.

MARSIS will be operated in Calibration mode periodically throughout the operationalphase of the mission. The purpose of this mode is to acquire a limited amount of datain an unprocessed format. The unprocessed data is used to determine thecharacteristics of the adaptive matched filter computation that is used by the MARSISprocessor to compress the dispersed echo signals from the planet surface andsubsurface boundaries.

MARSIS Subsystems

MARSIS is composed of three subsystems:

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the Antenna Subsystem (AS), including the primary dipole antenna for

transmission and reception of the sounder pulses, and the secondarymonopole antenna for surface-clutter echo reception only;

the Radio Frequency Subsystem (RFS), including both the transmit channel

and the two receive channels for the dipole and monopole antennas,

respectively; the Digital Electronics Subsystem (DES), including the signal generator,

timing and control unit and the processing unit.

Spac

ecraft

Power and

control

Processor

TransmitterSignal

Generator

Analog to

Digital

Conversor

Receiver

Analog to

Digital

Conversor

Receiver

Simple sounder

Surface Cancellation Channel

Dipole

Antenna

Monopole

Antenna

Ilustracin 2. MARSIS functional block diagram

The receivers and digital electronics are housed together within the spacecraft. Thetransmitter electronics is housed in a separate box, also within the spacecraft.

The main transmit and receive antenna is a deployable dipole with two 20 metreelements, arranged so that its peak gain is in the spacecraft nadir direction. The cluttercancellation antenna is a 7 metre long deployable monopole, arranged so that its gainnull is in the spacecraft nadir direction. The clutter cancellation antenna is equippedwith a low-noise preamplifier. Due to severe limitations on the available mass, theantennas are of a novel design, each consisting of a folding composite tube thatsupports a pair of wires constituting the conductive element of the antenna. Theantennas are deployed by pyrotechnic release mechanisms.

The transmitter is connected to the primary antenna through an impedance matchingnetwork. The nominal operating frequency of the transmitter in the subsurface soundermodes is 1.3 to 5.5 MHz, with an instantaneous bandwidth of 1 MHz. For ionosphericsounding, the operating frequency varies between 0.1 and 5.4 MHz. The transmittertakes the chirp generated by the receiver/local oscillator electronics and amplifies it,delivering 5 W of RF power to the antenna.

The receiver electronics consists of the chirp generator/local oscillator and a dualchannel receiver that down converts the received echoes. Each receiver channel has aselectable bandpass filter, a mixer, an amplifier chain, low-pass filtering and ananalogue to digital converter. The output of the analogue to digital converters is passedto the digital electronics for processing prior to being sent to the ground station via the

spacecraft's on-board data handling system.

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The digital electronics is responsible for:

Synthesis of the transmit chirp and local oscillator signals

Control of the transmitter and receivers

Processing of the digital data from the receivers

Receipt and execution of telecommands from the spacecraft Transmission of formatted science, event and housekeeping data to the

spacecraft.

MARSIS subsurface sounding mode characteristics

Centre frequency (MHz) 1.8 3.0 4.0 5.0

Bandwidth (MHz) 1.0 1.0 1.0 1.0

Radiated power (W) 1.5 5.0 5.0 5.0

Transmit pulse width (s) 250 or 30 in mode SS5

Pulse repetition rate (s-1

) 130Minimum science data rate (kbps) 18

Maximum science data rate (kbps) 75

MARSIS ionosphere sounding mode characteristics

Start frequency (kHz) 100

End frequency (MHz) 5.4

Number of frequencies 160

Transmit pulse length (S) 91.43

Frequency step (kHz) 10.937

Pulse repetition rate (s-1) 130

Sweep duration (s) 7.38

Other parameters

Receive window size per channel (baseline)(s) 350

Analog to digital conversion rate (MHz) 2.8

Analog to digital conversion (bit) 8

Max. no. simultaneous frequencies 2

Radiation Gain (dB) 2.1

Max data volume daily (Mbit) 285

Mass (Kg) 17

Max Power (includiong margins) (W) 64.5Tabla 1. Principal parameters of MARSIS