Lynx: A High-Resolution Synthetic Aperture Radar

S. I. Tsunoda, F. Pace, J. Stence, M. Woodring;
General Atomics PO Box 85608, San Diego, CA, 92121-5608

W. H. Hensley, A. W. Doerry, B. C. Walker;
Sandia National Laboratories PO Box 5800, MS 0529, Albuquerque, NM, 87185-0529

Abstract- Lynx is a high resolution, synthetic aperture radar (SAR) that was designed and built by Sandia National Laboratories in collaboration with General Atomics (GA). The Lynx production weight is less than 120 lb. and has a slant range of 30 km moderate rain (4 mrn/hr). It has operator selectable resolution and is capable of 0.1 m resolution in spotlight mode and 0.3 m resolution in stripmap mode. In ground moving target indicator mode, the minimum detectable velocity is 6 knots with a minimum target cross-section of 10 dBsm. In coherent change detection mode, Lynx makes registered, complex image comparisons either of 01 m resolution (minimum) spotlight images or of 0.3 m resolution (minimum) strip images. The Lynx user interface features a view manager that allows it to pan and zoom like a video camera.

Keywords: Synthetic Aperture Radar, SAR, Remote

Sensing, UAV, MTI, GMTI, CCD

INTRODUCTION
SYSTEM DESIGN
HARDWARE
IMAGE FORMATION
MOTION MEASUREMENTS/COMPENSATION
FLIGHT TESTS
SUMMARY
ACKNOWLEDGEMENTS
REFERENCES

1. INTRODUCTION

Lynx is a state of the art, high resolution synthetic aperture radar (SAR). Lynx was designed and built by Sandia National Laboratories and incorporates General Atomics' design requirements to address a wide variety of manned and u:rimanned missions. It may be operated on the Predator, I-GNAT, or Prowler II platforms which are manufactured by General Atomics (GA). It may also be operated on manned platforms. It has been integrated into a Twin Otter and into an Army U-21. Lynx was developed entirely on GA corporate funds. GA is presently beginning the manufacture of Lynx and intends to sell Lynx units and Lynx services to military and commercial customers

Figure 1. General Atomics I-GNAT

Lynx is a multimode radar. Its SAR modes include a spotlight mode and two stripmap or search modes. In addition, Lynx has a ground moving target indicator (GMTI) mode. Lynx also features a coherent change detection (CCD) mode which can indicate minute changes in two SAR images taken at different times. CCD may be performed with either spotlight or stripmap images. Lynx also features a uniquely flexible user interface. The user interface features a view manager that allows Lynx to pan and zoom like a video camera. Lynx also features a conventional waterfall display for stripmap display.

Lynx operates at Ku band and is capable of 0.1 m resolution in spotlight mode and 0.3 m resolution in tripmap mode. It has a slant range of 30 km in weather and weighs less than 125 lb.

Table 1. Stripmap SAR Mode specifications.

Resolution	0.3 to 3.0	m	Both slant range and azimuth
Range	7 to 30	km	Slant range (3-60 km at reduced performance)
Ground swath width	2600	pixels	Only with 16-node system (to 3500 pixels at coarser resolutions)
View size	934	m	At 0.3 m resolution 45 deg. depression
Squint angle	+/- (45 to 135)	deg	Squint is difference between scene center-line and aircraft velocity vector

2. SYSTEM DESIGN

The Lynx SAR was designed for operation on a wide variety of manned and unmanned aircraft. In particular, it can be operated from the Predator, I-GNAT, and Prowler II platforms manufactured by GA. During System Integration testing it was operated on board Sandia's DOE DeHavilland DH-6 Twin-Otter aircraft.

The Lynx SAR operates in the Ku-Band anywhere within the range 15.2 GHz to 18.2 GHz, with 320 W of transmitter power. It is designed to operate and maintain performance specifications in adverse weather, using a Sandia derived weather model that includes 4 mmlhr rainfall. It forms fine-resolution images in real-time and outputs both NTSC video as well as digital images. The Lynx SAR has four primary operating modes. These are described as follows.

Figure 2. SAR Geo-Referenced Stripmap Mode

Geo-ref Stripmap SAR Mode

In Geo-ref Mode, the operator specifies a precise strip on the ground to be imaged. The SAR then patches together a continuous and seamless string of images to yield the strip until the specified end-point is reached or the radar is commanded to do otherwise. The aircraft is not constrained to fly parallel to the strip, and images can be formed on either side of the aircraft. Specifications for this mode are given in Table 1.

Figure 3. SAR Transit Stripmap Mode

Transit Stripmap SAR Mode

In Transit Mode, the operator specifies a range from the aircraft to the target line and the SAR forms a Stripmap parallel to the aircraft's flight path. The SAR then patches together a continuous and seamless string of images to yield the strip, and will continue to do so until commanded otherwise, or until the vehicle deviates too far from the original flight path. In the event of such a deviation, a new Transit Stripmap will begin immediately. In all other respects, the performance of Transit Mode Stripmap is identical to Geo-ref Stripmap Mode.

Figure 4. SAR Spotlight Mode

SAR Spotlight Mode

In Spotlight Mode, the operator specifies the coordinates of a point on the ground and the SAR dwells on that point until commanded otherwise, or until the imaging geometry is exceeded. As with Stripmap modes, imaging may be on either side of the aircraft.

This mode allows finer resolutions than the Stripmap modes. Performance is summarized in Table 2.
In addition, an auto-zooming feature is also supported, whereby subsequent images are formed at ever finer resolutions. This mode continues until the SAR's limits are reached, or the system is commanded to do otherwise.

Resolution	0.1 to 3.0	m	Select one of five resolutions
Range	4 to 25	km	Slant range (3-60 km at reducedperformance)
Patch Size	2 X (640 X 480)	pixels
View size	640 X 480	pixels	Over NTSC video link
Squint angle	+!-(50 to 130)	deg
	+I- ( 45 to 135)	deg	0.15 m resolution and coarser

Table 2. Spotlight SAR Mode specifications.

Ground Moving Target Indication- GMTI

The relatively slow velocities of UAVs allow fairly simple exo-clutter GMTI schemes to offer reasonably good performance. The Lynx GMTI mode allows scanning over 270 degrees with performance summarized in Table 3.

Table 3. GMTI Performance

Min. Detectable Velocity	5.8	kts	At 35 m/s (near range)
Range	4 to 25	km	Slant range
Angular Coverage	-135 to+l35	deg	Total possible swept angle
Ground swath	10	km	Less at nearer ranges
Min. detectable target	+10	dBsm
Max. Clutter	-10	dBsm/rrf	Average distributed clutter

Coherent Change Detection - CCD

Coherent Change Detection is a technique whereby the phase correlation between two SAR images of the same scene is computed. Any changes in the complex reflectivity function of the scene are manifested as a decorrelation in the phase of the appropriate pixels between the two images. In this manner, very subtle changes in the scene from one image to the next can be
detected. Necessarily, the images themselves must remain complex for this to work.[3J
In the SAR modes, the radar can output complex (un detected) images that are necessary for Coherent Change Detection to work. These images can be transmitted to the ground station where ground-processing of the current image along with a library image allows near-real-time detection of changes in the scene. CCD can be performed with either Stripmap or Spotlight SAR images.

Figure 5. UAV Ground Station

User Interface

Consistent with the philosophy for other sensors of the GA UAV family, the user interface for the SAR was designed to allow easy operation by an operator with minimal radar-specific knowledge. The operator selects resolution and operating mode, and then basically 'points and shoots' the radar, much like the optical sensors.
detected images can be exported to View Manager software

Radar images are transmitted to the radar operator by any of two means. The first is an NTSC video link which allows the SAR to be treated as "just another sensor" to a UAV payload operator. The radar actually forms larger images than can be displayed over the NTSC video link, but novel View Manager software allows the operator to pan and zoom within its memory. Images may be saved in on-board buffers for later viewing.

The second means of image transmission is a digital data link that can transmit an entire image at full resolution. This data can then be formatted to comply with the National Imagery Transmission Format, NITFS 2.0.

Target coordinates are easily extracted from any SAR image to facilitate pointing the SAR for new images. In GMTI mode, locations of detected movers are transmitted for display on map overlays.

3.HARDWARE

While SARs tend to be fairly complex instruments, a primary goal for Lynx was ease of manufacture. This drove all aspects of design.
The system has been designed as two relatively generic packages. These are the Radar Electronics Assembly (REA) and the Sensor Front-End or Gimbal Assembly. The combined weight is currently about 125 lb with some variance due to different cable assemblies for different platforms.

Figure 6. Lynx Radar Electronics Assembly

Radar Electronics Assembly- REA

The REA contains radar control, waveform generation, up-conversion, receiver, video, ADC, and signal processing functions. These functions exist in a custom VME chassis, with individual boards/assemblies roughly divided as follows.

The RF/Microwave functions are within a set of five VME boards/assemblies. These include the STALO module, Up-converter module, Ku-Band module, Receiver module, and the RF interconnect module. The only major RF/microwave functions not found in these modules are the traveling wave tube amplifier (TWTA) and the low noise amplifier (LNA).

Digital Waveform Synthesis (DWS) is accomplished by a custom VME board that generates a chirp waveform with 42-bit parameter precision at 1 GHz. Although the board is custom, all components are off-the-shelf. The Analog-to-Digital Conversion (ADC) is also accomplished by a custom VME board that operates at 125 MHz sampling rate and provides 8-bit data. This data can be presummed and otherwise pre-processed before being sent across a RACEway bus to the signal processor.

The Signal Processor consists of 16 nodes of Mercury Computer Systems RACEway connected 200 MHz Power PCs. These implement a scalable architecture for image formation. Fewer nodes may be installed for a less capable SAR system. Four additional nodes are used for other radar functions including Motion Measurement, Radar Control, and optional data recording.

Gimbal Assembly

Figure 7. Gimbal Assembly

The radar antenna, motion measurement hardware, and front-end microwave components including the TWTA are mounted on a 3-axis gimbal. The a 3-axis gimbal was custom designed and built by Sandia specifically for the Lynx radar. All components are mounted on the inner gimbal. The antenna was custom designed at Sandia specifically for the Lynx radar. It is a vertically polariz ed hom-fed dish antenna with a 3.2 degree azimuth beamwidth and a 7 degree elevation beamwidth.

Motion measurement is a Carrier-Phase-GPS-aided Inertial Navigation System centered around a Litton LN-200 Fiber Optic IMU. This is augmented by an Interstate Electronics Corporation GPS receiver.

Figure 8. Litton LN-200 IMU

The transmitter power amplifier is a TWTA capable of outputting 320 W output at 35% duty factor averaged over the Lynx frequency band. The receiver LNA provides an overall system noise figure of about 4.5 dB.

4. IMAGE FORMATION

Image formation in all SAR modes is accomplished by stretch processingl2l, that is, de-ramping the received chirp prior to digitizing the signal. After analog to digital conversion (ADC), presumming is employed to maximize SNR in the image and minimize the load on the subsequent processors. The algorithm used thereafter is an expanded version of the Sandia developed Overlapped Subaperture (OSA) processing algorithm. OSA processing is followed b Sandia developed Phase Gradient Autofocus (PGA). Either complex images or detected images can be exported to View Manager software

5. MOTION MEASUREMENT/COMPENSATION

Motion measurements are received from an Inertial Measurement system mounted on the back of the antenna itself. These are augmented by carrier-phase GPS measurements and combined in a Kalman filter to accurately estimate position and velocity information crucial to proper motion compensation in the SAR. This processing is done on a single Power PC processing node.
The Motion Compensation philosophy for this radar is to perform compensation as early as possible in the signal path. Transmitted waveform parameters are adjusted, as well as pulse timing, to collect optimal data on the desired space-frequency grid. This is prior to digital sampling, and m1mm1zes the need for subsequent data interpolation.r4l During image formation, residual spatially variant phase errors are compensated as spatial coordinates become available during OSA processing. Finally, any errors due to unsensed motion are mitigated by an autofocus operation.

6. FLIGHT TESTS

Flight tests began in July 1998 with the radar mounted in Sandia's DOE Twin-Otter manned aircraft, and continued through February 1999. The first flights in a GA Aeronautical Systems, Inc. I-GNAT UAV occurred in March 1999. To date, two Lynx SARs have been built by Sandia. GA is currently constructing a third unit, and will build all subsequent units.

The SAR currently meets its image quality goals and routinely makes high-quality fine-resolution images. The first CCD images have been processed at the time of this writing. For GMTI mode, data has been collected and is undergoing analysis to adjust the processing for optimal

Figure 10. Belen railroad bridge over Rio Grande river (1 ft resolution Spotlight Mode performance

Figure 9. Sandia National Laboratories DOE DeHavilland DH-6 Twin-Otter

7. SUMMARY

We have described Lynx a lightweight, high performance SAR. Lynx operates in Ku band and features spotlight and stripmap SAR modes, a GMTI mode, and CCD. In spotlight mode it is capable of 0.1 m resolution, while in stripmap mode it is capable of 0.3 m resolution. Lynx can achieve a slant range of 25 km at the finest resolution in moderate rain. It can achieve a slant range of 45 krn at coarse resolution. It is designed be operated on a variety of manned and unmanned platforms. All the image processing is done in the air and the imagery (real or complex) is downlinked from an unmanned platform. Phase histories and/or imagery may be recorded in a manned platform. No post-processing of the imagery is required (except CCD). The Lynx production weight is less than 120 lb. Lynx also features a user friendly mode of operation that allows the SAR to be used like a video camera.
At the time of this writing all SAR image specifications have been met or exceeded in manned flight tests. In addition, the radar has manned test flights. In addition, the radar has successfully flown in the I-Gnat and its specifications verified on that platform as well. High quality 4" resolution imagery can also be made routinely.

8. ACKNOWLEDGMENTS

Many other individuals, both at Sandia and at GA, provided crucial efforts towards making the Lynx SARa reality. This was truly a team effort. The authors wish to acknowledge them and thank them.
Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000

Figure 11. Albuquerque Atomic Museum, KAFB (1 ft resolution Spotlight Mode)

9. REFERENCES

Bums, B. L., J. T. Cordaro, "SAR image formation algorithm that compensates for the spatially variant effects of antenna motion", SPIE Proceedings, Vol 2230, SPIE's International Symposium on Optical Engineering in Aerospace Sensing, Orlando, 4-8 April 1994.
Caputi, William J. Jr., "Stretch: A Time Transformation Technique", IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-7, No 2, pp 269-278, March 1971.
Jakowatz, Charles V., Daniel E. Wahl, Paul H. Eichel, Dennis C. Ghiglia, Paul A. Thompson, "Spotlight-Mode Synthetic Aperture Radar: A Signal Processing Approach", ISBN 0-7923-9677-4, Kluwer Academic Publishers, 1996.
Lawton, Wayne, "A New Polar Fourier Transform for Computer-Aided Tomography and Spotlight Synthetic Aperture Radar", IEEE Transactions on Acoustics, Speech, and Signal Processing, Vol. 36, No.6, pp 931-933, June 1988.

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