Christensen, Erik Lintz

Dall, Jørgen

Granholm, Johan

Madsen, Søren Nørvang

Skou, Niels

Woelders, Kim

EMISAR is a C- and L-band fully polarimetric (i.e. 4 complex channel per frequency) synthetic aperture radar designed for remote sensing with high demands for resolution (2 m), polarization discrimination, and absolute radiometric and polarimetric calibration. The present installation has one 3-axes stabilized antenna (C- or L-band) and two flush mounted C-band antennas providing the system with cross track and repeat track interferometric capabilitie

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Downloaded from orbit.dtu.dk on: Dec 17, 2017EMISAR: C- and L-band polarimetric and interferometric SARChristensen, Erik Lintz; Dall, Jørgen; Skou, Niels; Woelders, Kim; Granholm, Johan; Madsen, SørenNørvangPublished in:Proceedings of the International Geoscience and Remote Sensing SymposiumPublication date:1996Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Christensen, E. L., Dall, J., Skou, N., Woelders, K., Granholm, J., & Madsen, S. N. (1996). EMISAR: C- and L-band polarimetric and interferometric SAR. In Proceedings of the International Geoscience and Remote SensingSymposium: Remote Sensing for a Sustainable Future (Vol. Volume 3, pp. 1629-1632). IEEE.EMISAR: C- AND L-BAND POLARIMETRIC AND INTERFEROMETRIC SAR E. Lintz Christensen, J. Dall, N. Skou, K. Woelders, J. Granholm, and S. N~rvang Madsen Danish Center for Remote Sensing, mu, Dept. for Electromagnetic Systems' B. 348, Technical University of Denmark, DK-2800 Lyngby, Denmark. Phone: +45 4588 1444. Fax: +45 4593 1634. E-mail: lintz@emi.dtu.dk ABSTRACT EMISAR is a C- and L-band fully polarimetric (i.e. 4 com- plex channel per frequency) synthetic aperture radar designed for remote sensing with high demands for resolution (2 m), polarization discrimination, and absolute radiometric and polarimetric calibration. The present installation has one 3- axes stabilized antenna (C- or L-band) and two flush mounted C-band antennas providing the system with cross track and repeat track interferometric capabilities. INTRODUCTION Electromagnetics Institute (EMI) began operating its C-band, vertically polarized, Synthetic Aperture Radar (SAR) in 1989. The radar has a 100 MHz bandwidth and an 80 km range. A full swath, full resolution real-time processor was completed in 1992. The C-band system has since been up- graded to fully polarimetric capability, and the first test flights took place in the fall of 1993. An additional L-band system with full polarimetric capability and the same resolu- tion and image quality was completed and tested early 1995. The system was upgraded to interferometric capability in 1995 and the work on perfecting the installation and the processing software is ongoing. The major application of the system is data acquisition for the research of the Danish Center for Remote Sensing (DCRS) which has been established at EM1 with funding from the Danish National Research Foundation. The upgrade to polarimetry has been supported by the Joint Research Centre (JRC) of the European Community with the intention that EM1 will operate the polarimetric SAR for JRC in con- nection with the intended EARSEC (European Airborne Re- mote Sensing Capabilities) campaigns. During 1994 and 1995 the SAR system has been used to acquire polarimetric data for EMAC (European Multi-sensor Airborne Cam- paigns) organized and sponsored by ESA [ 1 ]. SAR SYSTEM RADAR HARDWARE The SAR consists of an airborne system (the SAR sensor including 2 sensors with antennas), and a ground segment (the off-line processing facility). Figure 1 gives an overview of the complete system while Table 1 summarizes the more important performance parameters of the sensor. AIRCRAFT The EMISAR is operated on a Gulfstream G-3 aircraft of the Royal Danish Air Force (RDAF). The G-3 is a twin enginc jet, with a 6000 km range. The radar is usually operated at an altitude of 41,000 ft. but lower altitudes are used in some modes. The size, weight, and power consumption allows the com- plete dual frequency system to be transferred to a smaller jet aircraft of the size of a Falcon 20 but the large range and high ceiling of the G-3 has advantages with regard to data acquisi- tions at distant sites and flexibility in imaging gcometry. The G-3 can reach most sites in Europe, carry out the mapping, and return to the home base within one or two days. The aircraft flight director gets its navigation information from an inertial navigation system and thus has a limited ac- curacy which is sufficient for normal flights. The SAR con- trol computer gets navigation information from the aircraft P- code GPS receiver and emulates an instrument landing sys- tem (ILS) receiver which is connected to the aircraft flight di- rector computer. In this way the actual flight track during mapping can be kept within a few meters of the desired track. ANTENNA POD INSTALLATION A polarimetric antenna system and the radar INU are in- stalled in a pod mounted below the fuselage, which facilitates rapid system installation and dismount. The pod installation includes a 3-axes stabilized antenna that allows zero Doppler Table 1 .  EMISAR nerformance Dai imeters 1.25 GHz Polarimetric < -35 dB < -30 dB 2, 4 or 8 m 2 , 4  or 8 m 12 or 24 km Max. 64 km < -35 dB Calibrated cross-talk < -35 dB terms Electromagnetics Institute (EMI, TUD) changed name per 1.1.96 to Dept. for Electromagnetic Systems (EMI, DTU) 1629 0-7803-3068-4/96$5.000 1996 IEEE Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on March 02,2010 at 07:23:52 EST from IEEE Xplore.  Restrictions apply. i ~ Power supply: i 6.0 kVA 1 ~ L- and ~ L q D i g i t a l  L- and subsystem C-bard ~~~ ,";:AzF ~ +b and control Analog subsystem I I I i I I A mounted antennas U AIRBORNE L- or C-band Antenna I z (Ground segment ] Work stations and parallel computer I I DSPU F Tape transcript LA- GROUND BASED Figure 1. EMISAR C- and L-band SAR system overview with airborne and ground based segments. mapping. However, the present pod has only room for one antenna, i.e. either L- or C-band. The radar electronics has been designed for simultaneous L- and C-band operation. DUAL FREQUENCY AND INTERFEROMETRY In 1995 one RDAF Gulfstream aircraft was augmented with two flush mounted antennas primarily intended for interfer- ometric SAR applications. However, both antennas are fully polarimetric thus this installation offers a possibility for si- multaneous dual frequency operation, although not at zero Doppler. The data rate needed for the squinted operation with the present hardware does not permit full resolution/swath The flush mounted installation presently has a physical baseline of 1.1 m and topographic mapping (aiming at pro- ducing digital elevation models) with this system has already been demonstrated [2]. The complete installation, having one antenna in the pod and two flush mounted antennas, also has the capability of along track interferometry although that feature has not yet been tested. The system has been extended to support repeat track inter- ferometry (RTI) but this mode is still under development. Very accurate track geometry is possible by the SAR control computer emulating an ILS receiver making actual tracks de- viate only a few meter from the desired. Very high resolution dual frequency operation with full polarimetry. topographic mapping and change detection are possible al- though the method does require a challenging off-line processing [ 31. ANTENNAS The antenna for C-band is a dual polarized 32x7 element mi- crostrip antenna designed to approximate a modified cosec- squared elevation radiation pattern for optimum illumination of the ground. The design has achieved high polarization dis- crimination. [4]? [ 5 ] .  The L-band antenna is a dual polarized 8x2 element micro- strip antenna with even better high polarization discrimina- tion. In order to obtain the required bandwidth the elements are stacked microstrip patches [6] .  The antenna designs have furthermore been optimized to improve the system azimuth sidelobe performance [7].  Some of the more important antenna parameters are listed for both frequencies in Table 1. The antenna attitude is automatically controlled. The an- tenna depression is set-up before each mapping to give the appropriate illumination of the target area. The aircraft atti- tude is measured by an inertial navigation unit (INU) located next to the antenna, and the antenna pointing is continuously updated to compensate for aircraft motion during mapping. 1630 Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on March 02,2010 at 07:23:52 EST from IEEE Xplore.  Restrictions apply. THE SENSORS L-band 3 dB resolution (m) PSLR (dB) The two sensors, one for C-band and one for L-band are identical except for the microwave subsystems. Digitally generated modulation waveforms are converted to an I, Q pair of baseband signals which are upconverted, amplified, and then guided to the antennas. The signals re- ceived by the antennas are amplified and downconverted to an I, Q pair of baseband signals. The analog subsystems are temperature regulated to assure high stability. The I, Q baseband signals are digitized to 8 bit samples at 100 Mhz and the received baseband signals is range pre-fil- tering (if reduced bandwidth and wider swath has been re- quested), double buffered, first order motion compensated, and azimuth pre-filtered and decimated to the requested pixel spacing. The prc-processed data are sent to the high density digital tape recorder (HDDT) which has sufficient capacity for the necessary ancillary information plus the output of both sensors at 1.5 m pixel spacing (240 Mbith). The data are also sent to the real time processor, [8], [9], which performs the SAR processing at full swath and resolution of one channel (although with some limitations at L-band and for squinted data). The HDDT can accept all 8 channels at 1.5 m pixel spac- ing which is sufficient only when the signals are recorded at zero Doppler (a limitation of the existing azimuth pre-filter). With the present installation one sensor will use the flush mounted antennas and thus simultaneous dual frequency po- larimetric operation is not yet supported at the highest reso- lution with a full swath. azimuth range 2.05 f 0.02 1.98 + 0.02 -25.2 k 2.0 -25.8 f 1.3 ABSOLUTE CALIBRATION C-band 3 dB resolution (m) PSLR (dB) ISLR (dB) The SAR system has been designed and thoroughly tested to provide absolute radiometrically and polarimetrically cali- brated data. This is accomplished by 1) having a very stable system, 2) performing internal calibration of this system im- mediately before and after each data take, and 3) by per- forming an absolute calibration at each mission using exter- nal standards. EMISAR employs a unique internal calibration system which greatly relieves the dependence on external calibration targets: by measuring system parameters via internal signal loops just before and after mapping a scene, a calibrated im- age can be generated assuming knowledge and stability of a few passive components [IO], [1 I] ,  1121. The internal calibration procedure comprises channel am- plitude- and phase-imbalance correction, absolute radiometric calibration, and it has a potential for range delay calibration, STC calibration, and noise estimation, as well. Furthermore, the built-in calibration paths allow the pulse-based procedure to calibrate almost the complete system, including both the transmitter and the receiver, but not the antenna. A 3-axes stabilized antenna has been chosen for both fre- quencies (the flush mounted antennas are not used for pola- rimetric calibrated modes) to avoid the calibration uncertainty otherwise caused by temporal variation of the antenna point- ing error. The antenna is pointed perpendicular to the desircd azimuth range 1.97 f 0.01 2.09 f 0.01 -3 1.9 k 2.4 -30.8 f 0.8 -30.9 f 2.1 -29.0 k 1.2 track to permit zero Doppler processing. The antennas which are not included in the internal calibration loops have been calibrated to an accuracy of 0.1 dB on the directivity and 0.2 dB on the absolute gain by using the ESA-TUD Spherical Near-Field Antenna Test Facility [ 131. The cross polarization terms of the antenna gains have not been included in the cali- bration. To facilitate the absolute (external) calibration, a calibration test site (with 3 trihedral and 4 dihedral corner reflectors) has been established in cooperation with Research Centre Foulum which is also collaborating with DCRS on research in map- ping soil and plants. This site is mapped on all missions and a very significant number of calibration data sets are available. The results obtained for the 3 dB resolution, the peak side lobe ratio (PSLR) and the integrated side lobe ratio (ISLR) using the co-polar trihedral responses are summarized in Table 2. The calibration stability has been examined based on a three day campaign where first two C-band calibration scenes were acquired, then four L-band scenes, and finally two ad- ditional C-band scenes, i.e. the antenna was dis-mounted and re-mounted between the two C-band missions. Using internal calibration only, the standard deviations of the absolute cali- bration and the channel imbalance was calculated, Table 3 .  Obviously a single external calibration per mission suffice. For absolute calibration and channel imbalance correction, the internal calibration is complemented by corrections de- rived from external calibration results. These corrections arc small and vary little from mission to mission. They were de- termined in an initial calibration experiment, and on most missions they are checked and adjusted if necessary by cxter- nal calibration. Cross-talk calibration may be carried out using distributed targets for which the true CO- and cross-polarized returns are uncorrelated. This requirement is met by natural targets with azimuthal symmetry. Unlike most other algorithms, the Que- gan algorithm [IS] is non-iterativc, and since it has previ- Table 2. L-band and C-band impulse response statistics based on 2.0 m trihedrals (average fstandard deviation). Table 3. Standard deviation of residual calibration errors after internal calibration.. I Frequency I TX imbal- I RX imbal- I Absolute 1 Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on March 02,2010 at 07:23:52 EST from IEEE Xplore.  Restrictions apply. ously shown good results for AIRSAR data, it has been used - in an enhanced version providing 4 channels - to estimate the rather small residual cross-talk from the EMISAR data [ 11. Recent results seem to question that natural targets in general can be assumed to have sufficient azimuthal symme- try. Therefore EMISAR data, considering their low intrinsic cross talk (Table I ) ,  are usually not cross-talk corrected. PROCESSING CAPACITY The processing of SAR data to quality images used to be a very time consuming task for standard computers. However. with 3 of todays fast RISC work stations with adequate RAM the computer capacity for processing polarimetric SAR data is in excess of 1000 fully polarimetric (i.e. 4 channels. 12 by 12 km, 2 by 2 m resolution) scenes a year. With the recent acquisition of a parallel computer facility the computer ca- pacity is not the problem. However, bottlenecks during HDDT transcription, lack of automated set-up in parts of the processing chain and output product generation presently limits the actual throughput to a somewhat lower number. The HDDT reliability is a weak link. Presently the actual processing throughput is around 10 polarimetric scenes per week. Work on automating the entire processing chain is in progress and further improvements of the capacity is expected during the summer of 1996. REFERENCES J. Dall, S. Ngrvang Madsen, H. Skriver, S. Savstrup Kristensen, and E. Lintz Christensen. “EMISAR De- ployment in the EMAC ‘94 Campaign”, Electromag- netics Institute, The Technical University of Denmark, Lyngby, Denmark, R 600, Sept. 1994. S.N. Madsen, N. Skou, J. Granholm, K. Woelders, E.L. Christensen, “A System for Airborne SAR Interferome- try”, European Conference on Synthetic Aperture Radar, EUSAR ‘96, Konigswinter, Germany, 26-28 March, 1996, and in the International Journal of Electronics and Communication, Germany (AEU), March 1996. Both in press. J.J. Mohr and S. Ngrvang Madsen, “Parametric Estima- tion of Time Varying Baselines in Airborne IFSAR”. International Geoscience and Remote Sensing Sympo- sium, IGARSS ‘96, Nebraska, USA, 27-31 May 1996. In press. K. Woelders and J. Granholm, “Design and Perform- ance of a Dual-Linearly Polarized C-band Microstrip Array Antenna”, Second International Workshop on Radar Polarimetry, Nantes, France, Sept. 1992, pp. 369- 378. J. Granholm, K. Woelders, M. Dich and E. Lintz Chri- stensen. “Microstrip Antenna for Polarimetric C-band SAR’‘. IEEE AP-S International Symposium and URSI Radio Science Meeting, Seattle, Washington, June 19- 24. 1994 N. Skou. J. Granholm, K. Woelders, J. Rohde, J. Dall, and E. Lintz. Christensen, “ A  high Resolution Polarimet- ric L-Band SAR - Design and First Results”, IGARSS 9 5. 1995 International Geoscience and Remote Sensing Symposium, Firenze, July 10-14, 1995, Vol. 3, pp 1779- 1782. E. Lintz Christensen and M. Dich, “SAR Antenna De- sign for Ambiguity- and Multipath Suppression”, IGARSS ‘93, 1993 International Geoscience and Re- mote Sensing Symposium, Tokyo, August 18-21, 1993, J. Dall, J. Hjelm Jgrgensen, E. Lintz Christensen and S. Ngrvang Madsen, “Real-Time Processor for the Danish Airborne SAR”, IEE Proceedings Part F, Special part on Remote Sensing, pp 115-121, April 1992. J. Dall, J. Hjelm Jgrgensen, A. Netterstrgm, N. Vardi, E. Lintz Christensen, and S. Nflrvang Madsen, “The Danish Real-Time SAR Processor: First Results”, IGARSS ‘93, 1993 International Geoscience and Remote Sensing Symposium, Tokyo, August 18-21, Vol. 2, pp 784-787. 1993. Vol. 3, pp 1401-1403. [lo] A.L. Gray. P.W. Vachon, C.E. Livingstone, and T.I. Lukowsh, “Synthetic Aperture Radar Calibration Using Reference Reflectors”, IEEE Transactions on Geosci- ence and remote Sensing, Vol. 28, No 3, pp. 374-383, May 1990. [ l l ]  N. Skou, “The Danish Polarimetric SAR System”, Second International Workshop on Radar Polarimetry, Nantes, France, 8-10. September 1992, pp 525-533. [ 121 J. Dall. Niels Skou, E. Lintz Christensen, “Pulse-based Internal Calibration of Polarimetric SAR’, IGARSS’94, 1994 International Geoscience and Remote Sensing Symposium, California, USA, 8- 12 August 1994. 1131 J.E. Hansen (Ed), “Spherical Near Field Antenna Meas- urements“. Peter Peregrinus, London, 1988 [ 141 Henning Skriver, Jgrgen Dall, Sgren Ngrvang Madsen, “External Polarimetric Calibration of the Danish Polari- metric C-band SAR”, IGARSS’94, 1994 International Geoscience and Remote Sensing Symposium, Califor- nia, USA, 8-12 August 1994 [15] S. Quegan, “A Unified Algorithm for Phase and Cross- Talk Calibration of Polarimetric Data - Theory and Observations”, IEEE Transaction on Geoscience and Remote Sensing, Vol. 2, No. 1, pp. 89-99, January 1993. 1632 Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on March 02,2010 at 07:23:52 EST from IEEE Xplore.  Restrictions apply. 

EMISAR: C- and L-band polarimetric and interferometric SAR

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EMISAR: C- and L-band polarimetric and interferometric SAR

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