Bachmann, Markus

Böer, Johannes

Castellanos Alfonzo, Gabriel

Polimeni, Donata

Schrank, Dirk

Schwerdt, Marco

Tous Ramon, Núria

English

Institute of Transport Research:Publications

Long Term System Monitoring Status of the TerraSAR-X and theTanDEM-X Satellites.Nuria Tous-Ramon, Dirk Schrank, Markus Bachmann, Gabriel Castellanos Alfonzo, Donata Polimeni, Johannes Böer,Marco SchwerdtMicrowaves and Radar Institute, German Aerospace Center (DLR), GermanyAbstractThe TerraSAR-X and the TanDEM-X satellites are operated to enable two missions in parallel: the TerraSAR-X and theTanDEM-X missions for scientiﬁc and commercial applications providing a multitude of SAR products of high accuracyand reliability. After launch the whole SAR system has been successfully calibrated and veriﬁed for both satellites duringtheir commissioning phases: the TerraSAR-X in 2007 and the TanDEM-X in 2010. In order to guarantee a stable qualityof the excellent SAR products and to monitor the correct operation of the entire SAR systems, both systems are regularlymonitored. This paper presents the status and results of all long-term system monitoring (LTSM) tasks executed sincelaunch.1 IntroductionTerraSAR-X (TSX) and TanDEM-X (TDX) are Germany’sﬁrst national remote sensing satellites implemented in apublic-private partnership between the German AerospaceCentre (DLR) and EADS Astrium GmbH. Both satel-lites feature an advanced high-resolution X-Band Syn-thetic Aperture Radar (SAR) based on the active phasedarray technology which allows the operation in Spotlight,Stripmap and ScanSAR mode with two polarizations indifferent combinations and elevation angles. Furthermore,both satellites have to enable both missions in parallel: theTerraSAR-X mission for providing single multi-mode X-Band SAR data in different operation modes [1] and theTanDEM-X mission in order to generate a new global dig-ital elevation model with a 12-meter grid and a vertical ac-curacy better than two meters in bistatic operation [2].For these various acquisition modes, their active phased ar-ray antenna electronically steers and shapes the patterns inazimuth and elevation direction. The antenna consists of12 panels in azimuth direction, each panel consists of 32sub-array radiators in elevation direction each fed by itsown active transmit and receive module. It combines theability to acquire high resolution images for detailed analy-sis as well as wide swath images for overview applications.The geometric resolution varies from 1 m for Spotlight,3 m for nominal Stripmap and 16 m for ScanSAR prod-ucts. The image width ranges from 10 km (Spotlight) to100 km (ScanSAR). There are over 1000 possible productvariations, which result from the combination of differentimaging modes, polarizations and elevation angles. TheSAR performance and the calibration status of both sys-tems is regularly analysed and monitored with respect togeometric and radiometric parameters like resolution, andside lobe ratios. Long-term monitoring of system parame-ters like Doppler centroid or instrument characteristics ver-iﬁes an excellent stability of both systems.2 SAR Calibration StatusContinuous monitoring of both instruments in orbit is re-quired to detect degradations of the satellites hardware andto compensate them by adapting the respective calibrationparameters. Therefore the calibration status of TSX andTDX is checked regularly and is divided in three mainparts: Internal Calibration, Antenna Pattern Veriﬁcationand Radiometric Calibration [3] [9].Figure 1: TDX statistical values of the amplitude and gaindeviations on transmission measured for each of the 384TRMs in ﬂight during three months in 2011. The red curveshows the standard deviation of the amplitude (above) andphase (below).For characterising individual transmit/receiver modules(TRMs) simultaneously, the PN-Gating method is appliedEUSAR 2012163[4]. Regular PN-Gating checks monitor the TRM transmitand receive gain, as well as transmit and receive phase forboth instruments in-ﬂight. Possible drifts can be found bydepicting gain and phase trends over time. For examplein Figure 1 the amplitude/phase deviations with respectto a reference value on receive are plotted for TSX versusdatatake execution time for each of the 384 TRMs. AllTRMs work within the established limits and no trend canbe observed, indicating the stability of the TSX instrumentand the TRM settings respectively. The amplitude devia-tion (1σ) stays under 0.1 dB and the phase deviation under1◦ for all TRMs .Monitoring the radar front-end temperatures shows that theinstrument is operated in its space qualiﬁed thermal condi-tions. Figure 2 shows the daily maximum temperaturesof the 12 front-end antenna panels of each instrument. Themeasured panel temperatures are far from the limit temper-ature of 30◦C for nominal performance. The temperaturepeak observed in the lower plot shows the instrument ther-mal behaviour in extreme conditions during a Hot-ColdTest executed during the TDX Comissiong Phase.Figure 2: Maximal daily front-end panels temperatures(hot-spot) over time. Upper plot: TSX, lower plot: TDX.The monitoring of the antenna patterns is required to de-tect degradation of the antenna front-end, especially the an-tenna wave guides as these are not covered by the internalcalibration. For this purpose, ScanSAR data takes over theAmazon rain forest are acquired and evaluated [10]. Us-ing ScanSAR data takes it is possible to estimate both thedeviation of the pattern shape as well as the beam-to-beamgain prediction capability. The plot of Figure 3 shows therelative deviation between the measured antenna patternsand the reference antenna patterns derived by the preciseantenna model [5]. The values in blue correspond to TSX,green to TDX and they are referenced to their individualmeans, being the maximum deviation between the antennamodel and the measurements less than ±0.2 dB.Figure 3: Antenna pattern statistic derived by acquisitionsacross the Amazon rainforest in ScanSAR operation, blue:TSX since 2007, green: TDX since 2011.Monitoring the radar cross section (RCS) of corner reﬂec-tors permanently installed over time yields the radiometricstability of the whole SAR system. For this purpose TSXand TDX are operated in StripMap mode with differentbeams. By depicting the derived RCS versus mission time(as shown in Figure 4), trends of the radiometric stabilityof the entire SAR systems can be detected and monitored.In addition, the absolute radiometric accuracy of the corre-sponding satellite, which is deﬁned by the standard devia-tion of the RCS measurements, can be determined.Figure 4: RCS derived from two Corner Reﬂectors (greenCR:B09 and blue CR:B13, strip_012 ascending - middleincidence angle), both deployed for LTSM of the TDXsatellite.However, monitoring the TSX satellite over the ﬁrst threeyears mission lifetime, a radiometric stability of better than0.2 dB could be veriﬁed by a comprehensive re-calibrationcampaign executed in 2009 [7]. And as both satellites haveachieved nearly the same accuracy [6], a similar stabilitycan be expected for the second satellite TDX.3 SAR Instrument PerformanceSAR performance parameters are monitored continuouslyin order to provide long-term statistics and to prove the164quality of the SAR data products [8]. For this purpose,the impulse response function of reference point targets,like corner reﬂectors, has been analysed within a set ofSAR images. The derived integrated side lobe ratio (ISLR,see Figure 5) and the peak-to-side lobe ratio (PSLR) aremeasures for the minimum possible detection of weak tar-gets in the vicinity of strong neighbour targets. Statisticalvalues of these measurements acquired between 2009 and2011 over two corner reﬂectors are listed in Table 1.Figure 5: Azimuth and range ISLR of TDX Satellite mea-surements during mission time.TSX-1 TDX-1µ σ µ σISLRAz -20dB 0.19dB -19.68dB 0.30dBRg -19.55dB 0.21dB -20.08dB 0.21dBPSLRAz -30.07dB 0.77dB -29.20dB 1.78dBRg -21.38dB 3.83dB -29.45dB 0.80dBTable 1: ISLR and PSLR mean/standard deviation valuesfor TSX and TDX.Also the geometric resolution in azimuth and in range arederived from point target responses, as shown in Figure 6.Both, the azimuth and the slant range resolutions are verystable.Figure 6: TDX-1 Geometric resolution in azimuth andin slant range measured in 2011 (red dotted line: productspeciﬁcation of 1.8 m in slant range, red solid line: productspeciﬁcation of 3.3 m in azimuth).The further important performance parameter is theDoppler centroid, which is estimated from SAR data byfrequent acquisitions. The mean Doppler values are con-centrated around 0 Hz mainly (99% of the total acquisi-tions) in a tube of ±120 Hz providing a stable image qual-ity over mission time (see Figure 7). Outliers could beidentiﬁed as non-nominal satellite conditions (e.g. GPSProblems).Figure 7: Maximum and minimum Doppler centroid foreach datatake / polarization channel since 2010.These results show once again the accuracy of the TSX andTDX SAR systems, as well as their high stability over themission lifetime.4 ConclusionThe measurements and the extended analyses performedfor long-term system monitoring of both SAR systemsTerraSAR-X and TandDEM-X show a very high stabilityof the instrument performance. Since launch of the re-spective satellite no degradation of both instruments havebeen observed. All parameters show a constant behaviour.Moreover, the measurements against corner reﬂectors andexecuted for TerraSAR-X approve a radiometric stability20 times better then the requirements [3]. And as bothsatellites are nearly identically designed, a similar stabil-ity can be expected for the second satellite TanDEM-X.Hence, by all these measurements performed for LTSM itcan be concluded that TerraSAR-X and TanDEM-X couldbe characterized and adjusted precisely achieving at theend a highly accurate and estable SAR System.References[1] S. Buckreuss, R. Werninghaus, W. Pitz: GermanSatellite Mission TerraSAR-X, IEEE Radar Confer-ence, Rome, Italy, 2008165[2] M. Zink, M. Bartusch, D. Miller: TanDEM-X Mis-sion Status, IEEE International Geoscience and Re-mote Sensing Symposium, Vancouver, Canada, 2011[3] M. Schwerdt, J. Hueso-Gonzalez, M. Bachmann, D.Schrank, B. Döring, N. Tous-Ramon, J. M. WalterAntony: In-Orbit Calibration of the TanDEM-X Sys-tem, IEEE International Geoscience and RemoteSensing Symposium, Vancouver, Canada, 2011[4] B. Bräutigam, M. Schwerdt, M. Bachmann, M.Stangl: Individual T/R Module Characterisationof the TerraSAR-X Active Phased Array Antenna byCalibration Pulse Sequences with Orthogonal Codes,26th International Geoscience And Remote SensingSymposium, Barcelona, Spain, 2007[5] M. Bachmann, M. Schwerdt, and B. Bräutigam: Ac-curate Antenna Pattern Modelling for Phased ArrayAntennas in SAR Applications - Demonstration onTerraSAR-X, International Journal of Antennas andPropagation - Special Issue on Active Antennas forSpace Applications, Article ID 4925054, June 2009[6] Marco Schwerdt, Dirk Schrank, Markus Bachmann,Jaime Hueso Gonzalez, Björn Döring, Nuria Tous-Ramon, John Walter Antony: Calibration of theTerraSAR-X and the TanDEM-X Satellite for theTerraSAR-X Mission, 9th European Conference onSynthetic Aperture Radar, Nürnberg, Germany, 2012[7] M. Schwerdt, D. Schrank, M. Bachmann, C. Schulz,B. Döring, J. Hueso Gonzalez: TerraSAR-X Re-Calibration and Dual Receive Antenna Campaignsperformed in 2009, 8th European Conference onSynthetic Aperture Radar, Aachen, Germany, 2010[8] D. Schulze, P. Rizzoli, B. Bräutigam, G. Krieger: In-Orbit Performance of TSX-1 & TDX-1, IEEE Interna-tional Geoscience and Remote Sensing Symposium,Vancouver, Canada, 2011[9] Marco Schwerdt and Benjamin Bräutigam andMarkus Bachmann and Björn Döring and DirkSchrank and Jaime Hueso Gonzalez: FinalTerraSAR-X Calibration Results Based on Novel Ef-ﬁcient Calibration Methods, IEEE TGRS Vol. 48,No.2, Pages 677-689, February 2010[10] Markus Bachmann and Marco Schwerdt and Ben-jamin Bräutigam: TerraSAR-X Antenna Calibrationand Monitoring Based on a Precise Antenna Model,IEEE TGRS Vol. 48, No.2, Pages 690-701, February2010166

Accurate Antenna Pattern Modelling for Phased Array Antennas

German Satellite Mission TerraSAR-X,

Hueso Gonzalez: Final TerraSAR-X Calibration Results Based on Novel Efﬁcient Calibration Methods,

Hueso Gonzalez: TerraSAR-X ReCalibration and Dual Receive Antenna Campaigns performed in 2009,

Individual T/R Module Characterisation of the TerraSAR-X Active Phased Array Antenna by CalibrationPulseSequences withOrthogonal

Jaime Hueso Gonzalez, Björn Döring, Nuria TousRamon, John Walter Antony: Calibration of the TerraSAR-X and the TanDEM-X Satellite for the TerraSAR-X Mission,

TerraSAR-X Antenna Calibration and Monitoring Based on a Precise Antenna Model,

Walter Antony: In-Orbit Calibration of the TanDEM-X System,

Long Term System Monitoring Status of the TerraSAR-X/TanDEM-X Satellites

Long Term System Monitoring Status of the TerraSAR-X/TanDEM-X Satellites

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