I GHz. degrees. I pixels. I km. A High-Resolution, Four-Band SAR Testbed with Real-Time Image Formation. de3 db m. < 45 to > 135 < -30

Transcription

1 A High-Resolution, Four-Band SAR Testbed with Real-Tie age Foration Bruce Walker, Grant Sander, Marty Thopson, Bryan Burns, Rick Fellerhoff, and Dale Dub&C Sandia National Laboratories, P.. Box 58 Albuquerque, NM (55) ; fax: (55) ; e-ail: ABSTRACT This paper describes the Twin-Otter SAR Testbed developed at Sandia National Laboratories. This SAR is a flexible, adaptable testbed capable of operation on four frequency bands: Ka, Ku, X, and VHFAJHF bands. The SAR features real-tie iage foration at fine resolution in spotlight and stripap odes. Highquality iages are fored in real tie using the overlapped subaperture (OSA) iage-foration and phase gradient autofocus (PGA) algoriths. NTRODUCTON n cooperation with nuerous sponsors, Sandia National Laboratories has developed a ultiode SAR testbed capable of operation on four bands: Ka band ( GHz), Ku band (14-16GHz), X band ( GHz), and VHF/uHF ( MHz). The S A R achieves state-of-the-art resolutions on each band while foring the S A R iages in real tie. Exceptional realtie iage quality is achieve through the use of innovative iage-foration and autofocus algoriths as well as highaccuracy otion easureent and copensation. A TwinOtter aircraft (Figure 1) provides a flexible, low-cost platfor for the SAR. The S A R testbed is designed to be very adaptable -- hundreds of paraeters ay be readily changed to eet the needs of new experients. Table 1 suarizes the paraeters for the Twin-Otter SAR. The SAR operates over a wide range of resolutions, depression angles (2 to 9 degrees), squint angles (greater than k45 degrees fro broadside depending on operating paraeters), and ranges (1 to 16 k). Both W and HH polarizations are available on each frequency band, except Ka band which currently uses only W. Cross polarization, E, ~ HV, is available at the VHF'/UHF band. nwthe &ight ode, dwell ties in excess of 6seconds have been deonstrated with excellent iage quality. Table 1. Twin-Otter S A R Paraeters Paraeter Operating frequency VHF X band Ku band Ka band Range Aircraft velocity Spotlight resolution Stripap resolution Value VHF X Ku. and Ka bands Swath width Depression angle Squint angle Noise equivalent reflection coefficient (band dependent) Peak sidelobes Multiplicative noise ratio Dynaic range Absolute RCS calibration, 3<r 125to to to to to to 7 1 to 3 2 to 1 1 to to 9 < 45 to > 135 < -3 < -35 < -1 > 75 < +3 Units MHz GHz GHz k pixels degrees degrees de3 dbc de3 db OPERATNG MODES The Twin-Otter S A R can operate in either of the conventional stripap or spotlight S A R odes. Figure 2 is an exaple of the stripap ode where four stripap passes of the Washington, DC area have been osaiced. n addition to stripap and spotlight odes, the S A R is capable of flying circles around targets in a spotlight ode. Circular data collections provide a eans of efficiently characterizing targets over a wide range of aspect angles. Several enhanceents have been added to the Twin-Otter SAR beyond the conventional SAR odes. At Ku-band, a two-antenna configuration provides 3-D or interferoetric S A R (FSAR) capability. Using careful calibration procedures, height accuracies of sub-eter rs are Figure 1. Twin-Otter Aircraft 11

2 Figure 2. SAR age of Washington, DC at 1- Resolution, Ku Band achieved. The FSAR has been used to ap rural as well as urban regions. A novel phase-unwrapping technique [l] uses an aplitude-onopulse easureent to aid in unwrapping the phase abiguities. This technique provides unabiguous height easureents and greatly iproved coputational efficiency copared to estiation-based phase-unwrapping algoriths. Figure 3 shows an FSAR iage of a rural area near Albuquerque, NM, yielding height noise of about.5- rs. Additional enhanceents include coherent change detection for detecting decorrelation or inute changes in radar scattering and a bistatic S A R ode which does not require direct-path synchronization between the transitter and receiver. RF tags or transponders have been deonstrated which can be encoded within a S A R iage. HARDWARE DESCRPTON Figure 4 shows a siplified block diagra of the base Ku-band SAR. The antenna, inertial easureent unit (MU), and rf front end are located on the gibal as shown in Figure 5. Figure 6 shows the Radar Assebly which contains the digital-wavefor synthesizer, frequency converter (exciter), twothannel receiver, A D converter, and iage-foration processor. Four frequency bands are achieved by frequency translating to the desired frequency band fro the base Ku-band SAR. The addition of an antenna, rf front end, transitter, and frequency translator allow the Ku-band S A R to be readily adapted to different frequencies. This technique iniizes the new hardware required. To date, three additional frequency bands have been added: Ka, X, and VHFhJHF. The V H F band is actually designed to allow noncontinuous coverage fro 5 MHz to 2 GHz; however, the current antenna liits the lowend frequency at 125 MHz and the transitter liits the highend frequency at 95 MHz. The Sandia S A R platfor cobines five priary technologies which are each essential to the foration of highquality, fine-resolution iages in real-tie: Figure 3. Rendering of nterferoetric SAR age of Black Mesa, NM

3 .' receive LO (derap) signals. The DWS consists of two digital-phase-generator GaAs ASCs, each driving a GaAs sine look-up-table read-only eory (ROW. Each phase-generator and sine-rom pair operates at 4 MHz, both driving an inputultiplexed GaAs digital-to-analog converter (DAC) to yield the analog chirp output. The two circuits are synchronized and clocked at 8 O O M H z to yield a theoretical bandwidth of DC to 4MHz for the chirp output. The frequency converter, transitter, and receiver subasseblies achieve phaselinearity across wide bandwidths through Figure 4. Siplified Block Diagra of the Twin-Otter SAR stringent phaseerror allocations and novel phasecancellation techniques. n addition io predistorting the trans&tted wavefor to Digital-wavefor synthesizer with RF phase-error copensate for phase nonlinearities, rf circuits have been correction, developed which cancel the phase nonlinearities of other rf Linear-phase, wide-bandwidth icrowave subsystes, coponents. These technologies achieve excellent iage Real-tie iage-foration processor which quality, with aziuth and range sidelobes below -35 B c. ipleents both the overlapped-subaperture (OSA) and phase-gradient autofocus (PGA) algoriths, The real-tie iage foration processor is a custo design High-accuracy otion easureent and precision based on the digital-array signal processor and GPS-aided navigation, and prograable-array controller (DASPlPAC) FFT chipset Real-tie otion copensation of the transitted fro Signal Processing Technologies (SPT). Six wavefor and received saples. DASPPAC FFT boards provide the priary coputational throughput required. Two TMS32C3 control coputers The digital-wavefor generates a provide the otion copensation, autofocus, and control linear-fm or chirp wavefor for both the transit and functions. Figure 5. Antenna Assebly Figure 6. Radar Assebly

4 , Three short 19-in racks contain the navigation operator interface, radar operator interface, iage display, highdensity tape recorders, and power supplies. Two coercial PCs provide the navigation and radar operator interfaces, allowing ission or radar operation changes to be easily ade. The SAR iage is displayed in real-tie on a 248 x 256-pixel MegaScan onitor. An Apex DCRSi recorder stores the radar phase history data, the coplex iage data, and other auxiliary data, such as otion easureent, state-of-health, etc. A Metru VLDS recorder ay also be used as a backup recorder in storing the iage data. MAGE-FORMATON ALGORTHMS The iage-foration processor ipleents the overlapped subaperture (OSA) iage-foration and phase-gradient autofocus (PGA) algoriths in real tie. The OSA algorith [2] enables real-tie iage foration through innovations in digital-wavefor synthesis, A/D sapling, and high-accuracy otion easureent. These technologies allow real-tie otion copensation of the transitted wavefor and received saples, siplifying real-tie iage foration. To for fine-resolution SAR iages, an algorith that can copensate for sigfkant non-straight-line otion is required. The OSA algorith was designed to be a realtie solution to this proble. The algorith is constructed entirely with FFT and vector-ultiplication operations. Soe otion-copensation steps are carried out before the return signal is processed by changing the radar center frequency and phase, PRF, and A D converter saple rate. Az HQ Order FEU3 The radar coputes these paraeters as functions of otion data provided by the otion-easureent syste. Figure 7 is a siplified functional diagra of the OSA algorith. The algorith is coposed of three basic steps: coarse-resolution aziuth processing, fine-resolution range processing, and fine-resolution aziuth processing. n OSA, a synthetic aperture is divided into overlapping subapertures which are processed individually to produce a sequence of iages which have coarse resolution in aziuth and fine resolution in range. n the final stage of processing, the coarse-resolution iages are coherently cobined to produce the final fine-resolution iage. n OSA, range and aziuth igration are corrected using coplex ultiplies; inefficient interpolation operations are not required. The real-tie S A R also incorporates the PGA autofocus algorith [3] to estiate and reove any residual otioneasureent error that would cause searing in the aziuth diension of the iage. Autofocus is done in two steps: point selection and phase-error extraction. At the point-select process, the data has only been OSA processed to the coarse-resolution stage in aziuth and fine-resolution in range. The point-select process deterines which points (range-aziuth bins) are ost suited for extraction of the phase error, and gives the coordinates of these bins to the PGA algorith. n this anner, autofocus requires less than 1% of the coputational load of the iage-foration processor. The PGA algorith finds the doinant scatterer in each bin and shifts it to zero Doppler frequency in the iage doain. The algorith then assues any variation fro an ideal aziuth-ipulse response centered at zero-doppler is due to residual phase errors. This error is averaged with those errors found fro other selected points to yield an estiate for the actual error. The algorith is iterated until the average detected error level falls below a specified threshold, or until a axiu nuber of iterations have been perfored. The efficiency and robustness of the PGA algorith is ideally suited for real-tie iage foration. MOTON MEASUREMENT Center in Fundion age output Fine Aziuth VVhdow Fundi Range Dependent AZHgb Order Foar s Residual Range Walk Aplitude Corredion Figure 7. Functional Diagra of Overlapped Subaperture Algorith The S A R otion-easureent and navigation syste consists of a iniaturized, high-accuracy, ring-laser-gyro MU;a 3-axis gibal antennapointing and stabilization assebly; and an autonoous 6channel P(Y)-code GPS receiver [4]. The syste provides four ajor functions:

5 Navigation, Motion easureent calculations for S A R otion copensation, Antenna pointing and stabilization, and pilot guidance. The lmu is closely ounted to the antenna to allow easureent of high-frequency otion due to turbulent flight conditions and structural resonances excited by the aircraft engines. The gibal ounting of the MU is ade possible by its sall size and weight. Output fro the MU is fed into the MoCop coputer which ipleents the digital-signal processing and otion calculations of the otion-easureent syste. The output of the GPS receiver is also fed into the MoCop coputer which ipleents a nine-error-state Kalan filter to copensate for long-ter drift in the MU easureent. The resulting velocity error of the syste is less than 5 d s e c standard deviation, with an absolute position accuracy of less than 5- rs for daerential-gps ode. The resulting position and velocity data is fed fro the MoCop coputer to the radar control and interface coputer for S A R otion copensation. The antenna pointing and stabilization function aintains the boresight of the antenna on either a straight line on the ground (stripap ode) or a fixed point on the ground (spotlight ode) during each aperture. The pointing accuracy has been easured at less than.1-degrees rs. Navigation data fro the MoCop coputer is provided to the pilots via an aircraft waypoint guidance display (located in the cockpit) which directs the pilot to fly precise trajectories. SUMMARY The Twin-Otter S A R is a state-of-the-art testbed, encopassing four frequency bands and operating over a wide paraeter space in resolution and geoetry. Exceptional iage quality is produced in real-tie and in a dynaniic flight environent. The testbed is designed to be flexible and can be readily adapted to future experients. Nuerous innovations in real-tie hardware and algoriths have been deonstrated and are being transitioned into other progras. ACKNOWLEDGMENTS n addition to the authors of this paper, any individuals at Sandia National Labs were instruental in the design, developent, and support of this syste. Sponsors in the developent and use of the Twin-Otter SAR testbed include the US Departent of Energy, Ary, Air Force, Navy, ARPA, Coast Guard, and industry. REFERENCES [l] D. L. Bickel and W. H. Hensley, Deterination of Absolute nterferoetric Phase Using the BeaAplitude Ratio Technique, 1996 nternational Geoscience and Reote Sensing Syposiu, May [2] B. L. Burns and J. T. Cordaro, A S A R ageforation Algorith that Copensates for the Spatially-Variant Effects of Antenna Motion, SPE Conference Proceedings, April [3] P. H. Eichel, D. C. Ghiglia, and C. V. Jakowatz Jr., Speckle Processing Method for Synthetic-ApertureRadar Phase Correction, Optics Letters, Volue 14, Nuber 1, January 1,1989. [4] J. R Fellerhoff and S. M. Kohler, Developent of a GPS-Aided Motion Measureent, Pointing and Stabilization Syste for a Synthetic Aperture Radar, Proceedings of the 48th Annual Meeting of The nstitute of Navigation, June 1992.

6 Portions of this docuent ay be illegible in electronic iage products. ages are produced fro the best available original dor?trent