The Seeker Experimental System at MIT Lincoln Laboratory Alexander G. Hayes, George Downs, Anthony Gabrielson, David C. Harrison,

Eric L. Hines, Leaf A. Jiang, Jonathan M. Richardson, Jonathan Swenson Lincoln Laboratory, Massachusetts Institute of Technology, 244 Wood St., Lexington MA 02420

ABSTRACT The Seeker Experimental System (SES) is the passive range within MIT Lincoln Laboratory’s Optical System Test Facility (OSTF). The SES laboratory focuses on the characterization of passive infrared sensors. Capable of projecting static and dynamic scenes in both cryogenic and room temperature environments, SES supports sensors that range from tactical ground based systems through strategic space-based architectures. Optical infrared sensors are a major component of military systems, having been used to acquire, track, and discriminate between potential targets and improve our understanding of the physics and phenomenology of objects. This paper delineates the capabilities of the SES laboratory and describes how they are used to characterize infrared sensors and develop new algorithms and hardware in the support of future sensor technology. The SES Cryogenic Scene Projection System vacuum chamber has recently been upgraded to allow dynamic projection of radiometrically accurate two-color infrared imagery. Additional capabilities include the ability to combine imagery from multiple sources, NIST traceable radiometric calibration, and dynamic scene projection in an ambient environment using a combination of high speed mirrors, point source blackbodies, and resistive array based dynamic infrared scene projectors. Keywords: Infrared, Cryogenic, Radiometric, NODDS, Seeker

1. OVERVIEW The Seeker Experimental System (SES), located at MIT Lincoln Laboratory (MIT LL), is a reconfigurable passive infrared sensor testing and emulation laboratory. Passive infrared sensors are crucial to ballistic missile and air defense technologies, providing detection, track, and phenomenology information from launch through reentry. SES was developed in the mid-1990s to provide midwave infrared (MWIR) focal plane array (FPA) and algorithm testing for Army and Navy interceptor programs. The goal of the research was to evaluate possible FPA technologies and assess their operation in a simulated environment. Today SES supports short-wave infrared (SWIR), MWIR, and long-wave infrared (LWIR) sensor characterization capabilities utilizing an array of cryogenic and ambient environment test assets. In the fall of 2002 SES became part of MIT LL’s Optical System Test Facility (OSTF), which incorporates passive, active, aerosol, and materials test facilities located throughout the Laboratory [1]. The purpose of the OSTF is to establish a coordinated approach to optical testing and maximize research capabilities by opening an array of separate facilities to a broad range of programs at MIT LL. SES is staffed by the Seeker and Interceptor Technology group and plays a vital role in MIT LL’s end-to-end seeker testing methodology (see Figure 1-1). MIT LL is involved in all phases of ballistic missile defense (BMD), from software simulation to live-fire testing. In additional to seeker technology characterization, SES supports BMD by providing a test bed for mission simulation and algorithm development. Algorithms developed and tested in a software environment can be validated with seekers by using the infrared projection technologies in SES. In this way, SES becomes part of the feedback loop that develops and proves discrimination architectures prior to live-fire tests (see Figure 1-1). Similarly, SES is used as a test bed for technological advancements such as advanced signal processors, large format FPAs, and new read-out integrated circuit (ROIC) designs. SES is designed to support advanced concepts and component development and testing. The facility performs standard calibration and system verification for in-house programs such as the Fly Away Sensor Package (FASP) and Captive-Carry SM3 sensor, but the primary mission of SES is to provide a proving ground for technology that is five or more years from tactical implementation. SES supports a broad scope of programs at MIT LL (see Figure 1-2) ranging from tactical ground-based MWIR systems through strategic space-based LWIR architectures. Support for these programs is provided by two distinct testing environments; the Ambient Scene Projection System (ASPS) and the Cryogenic Scene Projection System (CSPS). The ASPS (see Figure 1-3) is a collection of optical benches (both standard and custom), collimators, blackbody sources, filter/aperture wheels, steering mirrors, and dynamic scene generators. In 2000 SES installed the CSPS (see Figure 1-4),

a moderately sized thermal vacuum chamber, to evaluate LWIR seekers that view cold space. The interior of the CSPS houses a projection collimator, NIST traceable cavity source blackbody, 17 position filter/aperture wheel, and two dynamic scene generators. As part of the OSTF, SES is available to any government-funded program at MIT LL and may also be made available to government contractors on a limited basis. The objective of all research ongoing in SES is to transfer both intellectual property and hardware design to industry for real-world application.

• Simulated Seeker• Simulated Target• Simulated Flight

Environment

• Actual Seeker• Simulated Target• Simulated Flight

Environment

• Seeker Emulation• Actual Target• Simulated Flight

Environment

• Actual Seeker• Actual Target• Actual Flight

Environment

SoftwareSimulation

Hardware-in-the-Loop

Captive Carry Live Fire

Scenario Variety Fidelity

FASP

• Actual Seeker• Actual Target• Simulated Flight

Environment

Complete Imagery Resolved Imagery Non-Resolved Imagery Figure 1-1 MIT LL seeker testing and data collection

methodology

MWIR Seeker Support

IR to RF Handover for BMD

LWIR Midcourse Seeker Support

Advanced algorithms and seeker signal processor architectures

CMCM and ASFT: FASP, MFASP calibration

Boost Phase LWIR Seeker Support

MWIR Optical System (bench top)

LWIR Optical System (Chamber)

1996 1998 2000 2002 2004 2006

OSTF: SES 2-color upgrade

Advanced FPA Characterization

1997 1999 2001 2003 2005

Air Defense Support

Airborne Sensor Support

Figure 1-2 SES program support

Figure 1-3 Ambient Scene Projection System (ASPS)

Figure 1-4 Cryogenic Scene Projection System (CSPS)

2. DYNAMIC INFRARED SCENE PROJECTION The heart of the SES facility is its capability to project static and dynamic infrared scenes into both room temperature and cryogenic environments. Nuclear Optical Dynamic Display System (NODDS) resistive array technology is used to provide most of the infrared scene projection. Other projection sources include point source blackbodies and high-speed turning mirrors. In some cases, complex scenes are generated by combining the emission of a NODDS array with a point source blackbody using an infrared beamsplitter. SES employs two NODDS arrays in bench-top dewar assemblies in the ASPS and houses two additional arrays in the CSPS chamber. NODDS arrays were developed by the Defense Nuclear Agency to support infrared scene projection at cryogenic background temperatures and nuclear radiation environments. Arrays come in 128×128 and 512×512 configurations (see Figure 2-1); both of which support 15% and 53% fill factor designs. The individual emitters utilize a resistive microbridge design with a display pixel (dixel) pitch of 50.9 µm (see Figure 2-2) [2]. All of the NODDS arrays used in SES are on loan from the Defense Threat Reduction Agency and monitored by ATK Mission Research Corporation (MRC).

512x512 NODDS (D70538) 128x128 NODDS (N23D-31) Figure 2-1 NODDS resitive array configurations

E

Address Line

CMOS RIIC

Serpentine Resistor

Power

Data Line

E

Address Line

CMOS RIIC

Serpentine Resistor

PowerPower

Data Line Figure 2-2 NODDS dixel element

NODDS dixel elements are heated with a current generated by a MOSFET in the unit cell. Physical chip temperatures reach 700 K, allowing for an apparent temperature of 400 K in the longwave infrared (LWIR) and 500K in the midwave infrared (MWIR) for the 15% emitter designs. The 53% fill factor arrays nominally output three times the radiance of the 15% chips, but operate at one-third the speed. Table 2-1 summarizes the most pertinent attributes of the NODDS performance. Rise times, fall times, and maximum radiance values of the NODDS emitting elements have temperature dependence, but the dixels can be nominally updated at 100 Hz for the 15% emitters and 33 Hz for the 53% design (see Figure 2-3).

Table 2-1 NODDS array parameters

0.14 0.16 0.18 0.20

0.005

0.01

W/c

m2 /s

r (3-

5 μ

m)

15% Emitter Design

0.14 0.16 0.18 0.20

0.01

0.02

0.03

Time (s)

W/c

m2 /s

r (3-

5 μ

m)

53% Emitter Design

Rise Time:5.7ms(10%-90%)

Fall Time:3.4ms(90%-10%)

Rise Time:14.9ms(10%-90%)

Fall Time:8.1ms(90%-10%)

NODDS Dixel Temporal Response

Figure 2-3 Typical NODDS temporal response

The NODDS arrays are controlled by the FIESTA 8500 Projector Array Control System (see Figure 2-4). The FIESTA was built by ATK MRC and converts 16-bit movie frames into addressed voltages between 0.86 and 3.65 V. The core of the FIESTA rack is a VME chassis that holds eleven cards: one PC with drives, one timing board, four Power PCs, and four 8-channel digital-to-analog converters (DACs). The system also requires individual RAID drives for each channel, remote digital and analog boxes, and a power supply. Movies are stored in the 400 GB RAID array and played in a scripted fashion up to 240 Hz. FIESTA supports both real-time and non-real-time nonuniformity correction (NUC) and allows a physical separation of up to 100 ft between the arrays and the electronics through the use of differential voltage signals. The differential voltage signals are converted to analog signals by MIT LL designed analog cards before being sent to the arrays. The MIT LL analog cards include digital processors to calibrate the card offsets to within 1 mV before each movie is played. The analog cards were designed to eliminate a DAC drift that was discovered in the original system. FIESTA currently supports 32 analog channels, each of which drive a group of 16 columns in the resistive array. FIESTA’s 32 analog channels can be used to drive either one 512×512 NODDS array or four 128×128 NODDS arrays in a simultaneous environment. Additional timing circuitry can be employed to drive two 512×512 NODDS arrays in a pseudo-simultaneous environment.

Parameter 15% Emitter 53% Emitter Fill Factor 15% 53% Dixel Pitch 50.9μm 50.9μm

Maximum Apparent Temperature (Nominal)

500K (MWIR) 400K (LWIR)

600K (MWIR) 500K (LWIR)

Rise Time (10%-90%) 5.7ms 14.9ms Fall Time (90%-10%) 3.4ms 8.1ms

Operating Temperatures 20K -293K 20K-293K

0.86-3.65 VAnalog Output Range

In or OutSynchronization

TCP/IPRemote Operation

100 ft.Rack/Array Separation

120V,20APower Requirements

32

16-bit

8 or 16-bit

400 GB

1 to 240 Hz

# of Analog Channels

DAC Resolution

Movie Resolutions

Hard Drive

Frame Rate

0.86-3.65 VAnalog Output Range

In or OutSynchronization

TCP/IPRemote Operation

100 ft.Rack/Array Separation

120V,20APower Requirements

32

16-bit

8 or 16-bit

400 GB

1 to 240 Hz

# of Analog Channels

DAC Resolution

Movie Resolutions

Hard Drive

Frame Rate

Figure 2-4 FIESTA 8500

Figure 2-5 NODDS projection process block diagram

Resistive array technology will play an increasing role in infrared system evaluation and development as the cost and risk of flight testing increases and new display technologies emerge. However, the array transfer functions (voltage to photon emission) are highly nonlinear (see Figure 2-6) and nonuniform, requiring the application of complex NUC techniques. Current MIT LL NUC procedure can repeatedly correct NODDS arrays to less than 1% residual nonuniformity over limited drive ranges (see Figure 2-7) [3]. SES is currently investigating upgrades to its scene projection capabilities, which include iterative NUC techniques, new projection electronics, larger format resistive arrays, and true hardware-in-the-loop operation.

Figure 2-6 Median dixel response curve (note non-linear nature of transfer function)

=.008)=.160)

Figure 2-7 NODDS raw and corrected nonuniformity (noise in low end raw nonuniformity is camera induced)

3. COMPLEX SCENE GENERATION Simulation of a radiometrically correct complex scene is a challenge for SES. Over the last 30 years a variety of codes have been written, then validated through series of laboratory measurements and flight tests. These codes and measurements have been organized by the Lexington Discrimination System at Lincoln Laboratory [4]. A scene may comprise several objects and a background, all of which require calculation of their radiometric intensity as a function of time. Figure 3-1 shows the complexity of a scene that requires modeling. Components of an object include the body itself, typified by a weapon, rocket body, or airframe, and any emissions, typified by the plume of a burning rocket or aircraft engine. Each object in the complex scene must be modeled and converted into scene generation signals. To complete the projection radiance, the trajectory of the objects in the scene must be combined with an observer location,

the result being a file that has both the observing distance and object line-of-sight geometry as a function of time. The object trajectories are generally described by the Strategic and Theater Attack Modeling Process.

The body of an object is modeled with the Optical Signatures Code (OSC), a standard code that provides radiometric intensities from visible through LWIR. OSC requires a body description and trajectory to calculate the body’s radiance. The description differs for endoatmospheric phases that have significant air heating and exoatmospheric phases where there is little drag. The description types are shown in Figure 3-2, where the left figure is typical of a model operating in the atmosphere and the right is typical of a model above the atmosphere. Notice that the exoatmospheric model allows significantly greater shape complexity than the endoatmospheric model. OSC then calculates the effects of aerodynamic heating, radiative cooling, solar heating, earthshine and molecular collisions to create radiance data files. For aircraft and ground vehicle modeling, the Spectral and Inband Radiometric Imaging of Targets and Scenes (SPIRITS) code is used. SPIRITS is a detailed and extensively validated first-principles infrared spectral imaging signature model for fixed and rotary-wing aircraft, ships, and ground vehicles. Plumes from burning rockets are modeled with two different codes, the Composite High-Altitude Radiation Model (CHARM) for high-altitude rocket plumes and the Standardized Plume Flowfield (SPF) for low-altitude rockets. Each code can predict plumes for liquid or solid rocket fuels. An example of a CHARM radiance map is shown in Figure 3-3. The output of the CHARM and SPF code is combined with information obtained from captive-carry measurements to produce realistic high flux scenes. There are several options for merging the radiance files. The Composite Hardbody and Missile Plume (CHAMP) program was developed by Kinetic Kill Vehicle Hardware-in-the-Loop Simulator facility at Eglin Air Force Base, Florida, to merge the outputs of OSC and CHARM [5]. SES has chosen to create its own scripts to merge the files. The final part of the complex scene generation process is to combine the radiance files with the observer motion file to provide a radiance history at the observer’s aperture and convert it to digital codes for the scene projector, either as blackbody temperature and aperture size or NODDS digital movies. The projected scenes can be run with minor variations in sensors or algorithms to understand the robustness of sensor parameters and processes.

Figure 3-1 Scenario components supported by OSC.

(image obtained from http://www.teledynesolutions.com/optical_signt.asp)

(a) (b)

Figure 3-2 Examples of OSC model types: (a) Endoatmospheric,

(b) Exoatmospheric

CHARM Radiance Map Features

MissileHardbody

PlumeIntrinsic Core

ShockLayer

Logarithmic false color

CHARM Radiance Map Features

MissileHardbody

PlumeIntrinsic Core

ShockLayer

Logarithmic false color Figure 3-3 CHARM example: plume with hardbody

4. CRYOGENIC SCENE PROJECTION SYSTEM The CSPS was added to SES in 2000 to support LWIR seekers that operate in a space background. The original chamber contained a 350 mm diameter, 768 mm effective focal length (EFL), projection collimator that imaged the combination of a cavity blackbody source, and a 512×512 element NODDS resistive array. The collimator EFL was chosen to optimize the projection field of view (FOV) for seekers with optical focal lengths between 100 and 200 mm. In fall 2002, the CSPS began an upgrade when SES became part of the OSTF. An internal diagram of the modified CSPS is shown in Figure 4-1. The new CSPS includes a 1700 mm EFL projection collimator that provides a clear aperture diameter of 267mm. Two 128×128 NODDS arrays replaced the single 512×512 array in order to provide simultaneous two-color projection capabilities. The larger EFL and second NODDS array were chosen to optimize testing conditions for two-color seekers and advanced algorithm techniques such as superresolution and multicolor discrimination. Table 4-1 lists the pertinent attributes of the upgraded CSPS. The system is planned to be operational in April 2006. The upgrade was designed, built, and installed by ATK MRC of Santa Barbara, California.

10” Clear ApertureExit Pupil

Blackbody Sourcewith 17 Position Filter/Aperture Wheel

Scene Generators (2x128 NODDS)(Upgradeable to 2x512)

Cold Shroud (LN2)

1.7m EFL Collimator (F6.355)

UUT Specific Ante-Chamber

16” ID Gate Valve with2” Alignment Window

Figure 4-1 Internal layout of upgraded CSPS

The CSPS has a liquid nitrogen (LN2) cooled shroud surrounding all the internal optics and sources used to project onto the seeker. LN2 cooling maintains the internal temperature of CSPS at 80 K, well above the 3 K background of deep space. In operational environments however, seeker optics are kept at ambient temperature prior to launch. The optical self-emission of the seeker is several decades higher than the 80 K self-emission of the CSPS (see Figure 4-2). In order to simulate an operational environment, the seeker is kept at ambient temperature inside an antechamber. When testing is ready to begin, a gate valve opens between the antechamber and LN2 cooled components of the projection system. While observing the projection collimator, the seeker will radiatively cool as if it were viewing deep space. The ante-chamber may be isolated from the main chamber using a large Ge window in order to test sensors that cannot operate in the < 1×10-6torr pressure of the main chamber. The unit under test (UUT) is positioned on a ±5° tip/tilt stage in the ante-chamber to move the projected scene around the UUT FOV. The projection collimator’s FOV is 1.4°, although the NODDS arrays only cover 0.22°. A small dixel pitch of 0.0017° allows for significant oversampling of the UUT IFOV.

Output Pupil Size 10.5” Collimator FOV 1.4° Optical (F/6.355)

(Cold Baffle to 11.2° ) NODDS Dixel IFOV 0.0017 ° (30 µrad) Total NODDS FOV [Upgrade to 512×512 array]

0.22° (3.8 mrad) [0.88° 15.2 mrad]

Tip/Tilt Stage Travel ± 5° Main Chamber Temperature

80K

Main Chamber Pressure 9×10-7 torr

Table 4-1 CSPS parameters

Apparent Temperature (K)

In-B

and

Phot

on F

lux

(ph/

sr/c

m2/

sec)

100 150 200 250 300

Room Temp. Flux

All-reflective Telescope

Catadioptric Telescope Flux

LN2 Cold Chamber

295°K(Room Temp)

Optics Self Emission is Dominant Source of Effective Background

Optics3°K

Background295°K

(Room Temp)

Optics Self Emission is Dominant Source of Effective Background

Optics3°K

Background

Preferred Background to Replicate Real Environment

1010

1012

1014

1016

1018

Figure 4-2 Why LN2 is adequate for uncooled optics

In addition to upgrading the collimator and projection arrays, the CSPS modifications also include improved cold-shielding and better narcissus control. The new chamber has a cold background half-cone angle of 5.6°. The background radiance is < 0.01% of ambient with gradients < 0.001% of ambient within 3 mrad of the focal plane center. A three-dimensional computer-aided drawing of the chamber configuration, which shows the positions of various cold baffles, is shown in Figure 4-3. The focal plane itself is angled relative to the gut ray of the system in order to reduce narcissus effects. Specular reflections are reduced by the inclusion of serrated cold traps throughout the optical chain. NODDS arrays are kept on 3DOF stages to allow precision alignment and brought in and out of the optical chain using a source select stage. The emitter array holders have been designed to allow seamless integration of larger 512×512 or 1024×1024 element emitter arrays when they become available. The filter/aperture wheel is positioned on a focus drive to allow the aperture plane to be moved in and out of focus (depending on whether apertures or filters are being used).

Input Baffle

Cone Baffle

Collimator Optical Bench with LN2 Tank

Calibrated Blackbody

Aperture Wheel

Source Select Stage

Primary Mirror

Short Band Emitter Array

Long Band Emitter Array

Array Beam Combiner and

Baffle

Figure 4-3 CSPS configuration

The original CSPS design included a 6-position filter/aperture wheel. The new chamber layout includes a 17-position wheel that can be populated with either spectral bandpass filters or complex geometric apertures such as knife edges, bar patterns, or various diameter circles (see Figure 4-4). Advanced sensor effects such as flux nonlinearities, spatial frequency response (modulation transfer function), banding, charge spillover, and spectral cross-talk can all be characterized by using the large aperture wheel. A 17-point approximation to a sensor’s spectral quantum efficiency (on a pixel-by-pixel basis) can be calculated by populating the wheel with narrowband filters evenly spaced throughout the UUT’s waveband of interest. Simultaneous projection of the two NODDS arrays allows for projected scenes to incorporate a piecewise approximation to a target’s spectral radiance (see Figure 4-5). For two color UUTs, a separate array is used for each sensor band to emulate a spectrally interesting target (gray or nongray) with varying distance and

temperature. Attempting to emulate even a graybody target with a single projector array would yield significant out-of-band errors when testing a two-color UUT (see Figure 4-5).

Figure 4-4 CSPS 17-position aperture/filter wheel

Wavelength

SeekerBand 1

SeekerBand 2

Target being simulated

Emitter for band 2

Emitter for band 1

Out-of-band errors with single emitter array

Inte

nsity

Wavelength

SeekerBand 1

SeekerBand 2

Target being simulated

Emitter for band 2

Emitter for band 1

Out-of-band errors with single emitter array

Inte

nsity

Figure 4-5 Piecewise approximation to blackbody radiation using two resistive arrays

5. AMBIENT SCENE PROJECTION SYSTEM The ASPS is a reconfigurable testing environment for characterizing SWIR, MWIR, and LWIR sensors operating in an ambient room environment (see Figure 5-1). The ASPS has been operational since 1995 and consists of an array of optical benches, collimators, blackbody sources, filters/aperture wheels, steering mirrors, and dynamic scene generators. The incorporation of SES into the OSTF brought significant enhancements to the ASPS, including a 15-inch high-speed (60°/s) steering mirror (see Figure 5-3), fast 2-axis UUT stages, 20-position filter/aperture wheels, additional blackbodies, and a 512×512 NODDS array bench-top dewar assembly (in addition to the legacy 128×128 NODDS dewar). The new equipment purchased during OSTF incorporation brought an array of unique testing capabilities to the ASPS. These added capabilities attracted a variety of new programs that led to the opening of an SES annex laboratory to house additional warm bench test stations.

IR camera

Dual scene collimators

Programmablesteering mirror

Targetsource

Dynamicresistor array

High speed “gimbal effect” turn mirror

“Dome heating” sourceDetonation emulation

IR camera

Dual scene collimators

Programmablesteering mirror

Targetsource

Dynamicresistor array

High speed “gimbal effect” turn mirror

“Dome heating” sourceDetonation emulation

Figure 5-1 Example of ASPS test station

MWIR Camera

NODDS Dewar

15” High Speed Mirror

Off Screen:Off-Axis-ParabolaNitrogen Dewar

Figure 5-2 Example of ASPS test station using new components

Each warm bench test station in the ASPS is configured using dual off-axis-parabolas (OAPs). Side-by-side projection collimators allow for the generation of complex scenes that incorporate point source blackbodies, high-speed mirrors, and NODDS resistive arrays. Figure 3-4 is an example of a bench configuration for an air defense program. A point source blackbody will be used to emulate a flare being dropped from a commercial aircraft. Flare motion will be simulated in collimated space by a 15-inch high-speed-mirror. The aircraft will be simulated by a 512×512 NODDS array. The flare and aircraft images are combined using a ZnSe beamcombiner. Multiple flares can be emulated using an aperture with multiple point sources drilled into it. If independent motion is required, flares can be emulated using multiple point source blackbodies in conjunction with small steering mirrors and beamcombiners in the converging beam of the OAP.

Reflective OAP Collimators

Point SourceBlack Body

15” High Speed Mirror

Sensor onTwo-Axis Stage

6ft x 12ftOptical Bench

512x512 NODDSResistive Array

Refractive Ge Collimator

Figure 5-3 ASPS projection concept

Sensor characterization capabilities in the ASPS extend beyond radiometrically accurate complex scene generation. NIST traceable plate and cavity source blackbodies are on hand that emit between -40° and 3000°C (see Table 5-1). Spectral characterization is available in the form of narrow bandpass filters, monochrometers, circular variable filters, and a Fourier transform interferometer accessible through the materials measurements facility of the OSTF. A large collection of chilled aperture plates, geometric apertures, narrow bandpass filters, and other lab equipment has been amassed over the years. Fully characterized lab cameras in the MWIR and LWIR are available to provide control data and focus information for experiments (see Figure 5-5).

Model Type Range

Mikron M390 Cavity 300° - 3000°C

Mikron M335 Cavity 300° - 1500°C

Mikron M314 Plate (6”) 5° - 350°C

Mikron M345 Plate (12”) -10° - 170°C

Mirkon M340 Plate (2”) -20° - 150°C

IR Systems IR-564 (4) Cavity 25° - 1200°C

SBIR 13132 (N2 purge) Plate (4”) -40° - 100°C

Table 5-1 Partial list of ASPS blackbodies

Figure 5-4 Examples of LWIR (left) and MWIR (right) sensors

used in the ASPS

Each warm bench test station in the ASPS is adapted to a particular program. The collimators EFLs, mirror sizes, and sources are all optimized for the FOV and optical footprint of the UUT. A generic laser alignment procedure has been developed to ensure rapid UUT and source positioning for a variety of optical configurations. A software program is under way to automate a significant portion of the testing in both the ASPS and CSPS. Blackbodies, UUT gimbal, steering mirrors, aperture wheels, source selectors, lab cameras, and in most cases the UUT itself, will be controlled through scriptable command and control software. ASPS assets and capabilities will continue to evolve with program demand and adapt to the state of the art in infrared sensor characterization.

6. SENSOR TESTING SES was developed in order to provide a test bed for proving concepts and performing component and system level development and testing for advanced sensor designs. The success of the facility is measured by its ability to transfer technology to industry for implementation. Examples of technology transfer initiated from SES programs includes multi-frame averaging algorithms, superresolution algorithms, advanced signal processor concepts, mercury cadmium telluride detector limitations, ROIC designs, and advanced discrimination architectures. The origin of characteristics that affect infrared sensor performance can be traced back to the various elements of the system (see Figure 6-1). Characterization of these effects can occur at three distinct levels; Component testing and modeling, empirical modeling using a representative test sensor, and effects that always require individual seeker measurements. Effects such as bulk emissivity, spectral transmission, and quantum efficiency can be understood by individual component testing and modeling. These effects can by translated to the finished sensor system without additional testing. Optical quality, temporal linearity, banding, saturation level, uniformity, quantization, and algorithm performance are example parameters that can be understood on the basis of empirical modeling. These parameters can be characterized on a representative sensor for model development and then extrapolated to predict performance on other sensor systems. Flux linearity, operability, blinkers, detector spillover, optical ghosts, and the unknown are examples of sensor effects that require individual sensor measurements. While a representative sensor can provide information on magnitude, complete understanding of these phenomena requires measurements on each individual system. Depending on a sensor’s operational environment, effects that require individual seeker measurements can have significant impact on system performance (see [6] for a detailed example of a flux linearity characterization).

A/D SignalProcessor

Seeker measurementsdata always required

Component data and models sufficient

Performs algorithms

Measures voltage signal and

converts to digital numbers

Accumulates charge and

converts to a voltage signal

Converts photons to electrons

Focuses photons onto FPA

Image Processor

Readout ElectronicsROICFocal Plane

ArrayOptical Path

Seeker measurementsfor model development

Spectral characteristics

Linearity

Banding

Bulk Emissivity

QuantizationQuantum efficiencySpectral transmissionand reflection

Timing of readout Adaptive offset NUC

Gain correction

Saturation/starvation

Detector samplingSpider diffraction

Point spread functionand optical aberrations Linearity

Dynamic Range Manager

Frame stabilization

Uniformity and Temporal Response

Blinkers

Scattering offoptical elements

Detector spillover

Ghosts – Multiplereflections Flux linearity

Operability

The Unknown

Multiframe Averaging

Figure 6-1 Origin of sensor effects

Test articles sent to SES include representative FPAs, integrated dewar assemblies (IDAs), and full sensor systems. For each test article, the parameters listed in Figure 6-1 are measured. Characterizing these effects will help to understand performance limitations of the system and provide information regarding potential design improvements. Example products from this process include robust calibration procedures and NUC processes, performance enhancements through advanced algorithms, and hardware upgrades, including optical baffling and thermal isolation. For programs that provide FPAs or IDAs, surrogate cameras are created that are optimized for use in the CSPS or ASPS, but preserve essential elements of the sensor design: f/# (PSF), waveband, optical attenuation, and background intensity. Examples of test articles that have been successfully put through characterization include the SM3 one-color seeker and THAAD interceptor. FPA effects such as banding and reference voltage drift were characterized in the SM3 test article and used to explain phenomena observed during flight testing. Image processing techniques, including frame average and super-

resolution, were also proven using the SM3 sensor and transferred to the contractor for implementation. During THAAD FPA characterization, a temporal nonlinearity was discovered that placed an upper limit on overall sensor accuracy. In addition to representative sensor and surrogate sensor characterization, SES also supports the testing of advanced signal processors. Signal processors are a crucial component to ballistic missile and air defense sensors. Advanced algorithms and signal processing techniques require advanced processing power. The Captive Carry Adjunct Signal Processor (CASP) is a test bed developed at MIT LL to demonstrate advanced real-time signal processing capabilities using commercial-off-the-shelf (COTS) equipment. The COTS processors are interfaced to the seeker electronics using a middleware library that adapts seeker output and algorithm processes to the COTS architecture [6]. SES provides an end-to-end testing capability for the CASP program. The signal processor is placed in the loop with a seeker and the CSPS projection system. In this test-bed algorithms can be applied to seeker data in real time to validate the performance of the signal processor.

7. SUMMARY SES has been evolving over the last 10 years from a standard bench test system to developing a full cryogenic dynamic scene projection capability. The facility has been used extensively to verify seeker performance characteristics and test new algorithm and signal processing architectures. In addition to state-of the-art calibration equipment, SES also has the ability to generate and project radiometrically accurate scenes representing a wide variety of target descriptions. The next generation of the CSPS (online in April 2006) will include two-color cryogenic dynamic scene projection to allow characterization of complex seekers and algorithms. The ASPS has been, and will continue to be, continually modified to represent the state-of-the-art in ambient infrared sensor characterization. With the establishment of the OSTF, SES has combined its assets with those of active, aerosol, and material measurements testing facilities. SES will continue to evolve with programmatic and technological demands and provide its services to a wide array of government-funded programs.

8. ACKNOWLEDGMENTS This work was sponsored by the US Air Force under Air Force Contract FA8721-05-C-0002. Opinions, interpretations, conclusions and recommendations are those of the authors and not necessarily endorsed by the United States Government.

9. REFERENCES 1. D.C. Harrison, A.G. Hayes, L.A. Jiang, E.L. Hines, and J. Richardson, “The MIT Lincoln Laboratory Optical

Systems Test Facility”, SPIE Conference 6208, Technologies for Synthetic Environments: Hardware-in-the-Loop- Testing XI, April 2006

2. M. Thomas, D. Newman, M. Froli, D. Prichett, and C. Person, “Nonuniformity correction of cryogenic 512 emitter arrays: The 5 minute 5% NUC using FIESTA”, Technologies for Synthetic Environments: Hardware-in-the-Loop Testing VI, 2001 SPIE, Vol. 4366, pp 465-474

3. A.G. Hayes, F.J. Caraco, D.C. Harrison, and J.M. Sorvari, “Characterization and Comparison of 128×128 element Nuclear Optical Dynamic Display System (NODDS) Resistive Arrays”, SPIE Conference 6208, Technologies for Synthetic Environments: Hardare-in-the-Loop- Testing XI, April 2006

4. K. Edwards and W. Kornegay, “Measurements, Phenomenology, and Discrimination”, Lincoln Laboratory Journal, Volume 13, Number 1, 2002, pp75-108

5. D.R. Crow, “Composite hardbody and missile plume (CHAMP 98) IR scene generation program”, Proceedings of SPIE -- Volume 3368 Technologies for Synthetic Environments: Hardware-in-the-Loop Testing III, R.L. Murrer, Jr., July 1998, pp. 256-268

6. D. Rabinkin, E. Rutledge, and P. Monticciolo, “Missile Signal Processing Common Computer Architecture for Rapid Technology Upgrade”, Proc. Of the SPIE Vol. 5559, pp 131-145, Aug. 2004.