1. Techniques for High Accuracy Relative and Absolute Localization of TerraSAR-X / TanDEM-X Data U. Balss, M. Eineder, T. Fritz, H. Breit, and C. Minet German Aerospace Center (DLR), Remote Sensing Technology Institute (IMF)

2. Institut für Methodik der Fernerkundung bzw. Deutsches Fernerkundungsdatenzentrum Folie 2 2 Institut für Methodik der Fernerkundung Outline Introduction Measurement Method Consideration of Continental Drift Particulars of Bistatic Acquisition Geometry Measurement Results of Absolute Localization Accuracy (TerraSAR-X) Relative Localization Accuracy (TanDEM-X) Conclusions

3. Institut für Methodik der Fernerkundung bzw. Deutsches Fernerkundungsdatenzentrum Folie 3 3 Institut für Methodik der Fernerkundung Introduction TSX-1 (launched June 2007) and TDX-1 (launched June 2010) constitute first bistatic SAR system in space. One satellite transmits radar pulses. Both satellites receive the echoes. High geometric accuracy requires consideration of signal propagation effects geodynamic effects in geolocation of ground targets. passive channel: Geolocation is additionally complicated by bistatic acquisition geometry

4. Institut für Methodik der Fernerkundung bzw. Deutsches Fernerkundungsdatenzentrum Folie 4 4 Institut für Methodik der Fernerkundung Measurement Method 6 Corner Reflectors (CR) at Oberpfaffenhofen Test Site Comparison of measured radar positions their expected values Expected values are based on precise GPS measurements of CR positions orbit interpolation of satellites positions The following effects have to be taken into account: additional signal delays (caused by electrons in ionosphere and water vapor in atmosphere) solid earth tides continental drift

5. Institut für Methodik der Fernerkundung bzw. Deutsches Fernerkundungsdatenzentrum Folie 5 5 Institut für Methodik der Fernerkundung Continental Drift and Geodetic Coordinate Systems * TSX-1 / TDX-1 orbit in ITRS-2005/08 coordinates * GPS position of CR in tectonic plate fixed system (e.g. ETRS89) misinterpretation of GPS coordinates offset between expected and true CR position (e.g. approx. 60 cm if ETRS89 is taken for ITRF-2005)

6. Institut für Methodik der Fernerkundung bzw. Deutsches Fernerkundungsdatenzentrum Folie 6 6 Institut für Methodik der Fernerkundung Effect of Continental Drift on Radar Coordinates (e.g. corner reflector CR moved to northeast) expected position of CR true position of CR flight path continental drift azimuth range ascending orbit (right looking): CR occurs more in late azimuth and far range than expected W S N E height t1t1 t 2 >t 1

7. Institut für Methodik der Fernerkundung bzw. Deutsches Fernerkundungsdatenzentrum Folie 7 7 Institut für Methodik der Fernerkundung Effect of Continental Drift on Radar Coordinates (e.g. corner reflector CR moved to northeast) expected position of CR true position of CR flight path continental drift azimuth range descending orbit (right looking): CR occurs more in early azimuth and near range than expected W S N E height t 2 <t 1 t1t1

8. Institut für Methodik der Fernerkundung bzw. Deutsches Fernerkundungsdatenzentrum Folie 8 8 Institut für Methodik der Fernerkundung Effect of a Coordinate System Mismatch Wrong: ETRS89 coordinates are misinterpreted as ITRS- 2005 Correct: ETRS89 coordinates are transformed to ITRS-2005 system

9. Institut für Methodik der Fernerkundung bzw. Deutsches Fernerkundungsdatenzentrum Folie 9 9 Institut für Methodik der Fernerkundung Bistatic Acquisition Geometry of TanDEM-X (Passive Imaging Channel) bistatic range depends on signal travel time which again depends on bistatic range

10. Institut für Methodik der Fernerkundung bzw. Deutsches Fernerkundungsdatenzentrum Folie 10 10 Institut für Methodik der Fernerkundung Computation of Bistatic Closest Approach iteration step (n=0, 1, 2, …) initialization 2) Coordinate of hyperbola apex results by nested intervals. 1) Recursive computation of bistatic range for given slow time t:

11. Institut für Methodik der Fernerkundung bzw. Deutsches Fernerkundungsdatenzentrum Folie 11 11 Institut für Methodik der Fernerkundung Bistatic Acquisition Geometry of TanDEM-X (Active Imaging Channel) Satellite moves during signal travel time. Satellite position differs between instants of pulse transmission and echo reception. Thus, even this acquisition geometry is strictly speaking bistatic. Results of monostatic and bistatic computation scheme differ by some tenth of a millimeter.

12. Institut für Methodik der Fernerkundung bzw. Deutsches Fernerkundungsdatenzentrum Folie 12 12 Institut für Methodik der Fernerkundung Absolute Pixel Localization Accuracy of TSX-1 (Based on TSX-1 Calibration Datatakes 2007/09) L1B products created before 2011-07-15: bandwidth dependent range offset: 100 MHz : -33 cm 150 MHz : -14 cm 300 MHz : +12 cm azimuth offset: +8 cm Meanwhile bandwidth dependency is solved by code change in SAR processor and instrument is recalibrated.

13. Institut für Methodik der Fernerkundung bzw. Deutsches Fernerkundungsdatenzentrum Folie 13 13 Institut für Methodik der Fernerkundung Absolute Localization Accuracy of TSX-1 (Reprocessed by Actual SAR Processor Version) σ azimuth = 6.3 cm σ range = 3.8 cm The following offsets are subtracted: azimuth offset: +8 cm range offset : -29 cm

14. Institut für Methodik der Fernerkundung bzw. Deutsches Fernerkundungsdatenzentrum Folie 14 14 Institut für Methodik der Fernerkundung Absolute Localization Accuracy of TSX-1 and TDX-1 (Based on Calibration Datatakes 2010) TDX-1 σ azimuth = 5.5 cm σ range = 3.5 cm TSX-1 σ azimuth = 5.3 cm σ range = 3.5 cm

15. Institut für Methodik der Fernerkundung bzw. Deutsches Fernerkundungsdatenzentrum Folie 15 15 Institut für Methodik der Fernerkundung Relative Localization Accuracy of Bistatic TanDEM-X Acquisitions different scaling of axes azimuth: -100 … +60 mm range : -10 … +6 mm ! mean value: m azimuth = -18 mm m range = -2.1 mm standard deviation: over all acquisitions: σ azimuth = 40 mm σ range = 4.4 mm within an acquisition: σ azimuth = 16 mm σ range = 1.0 mm

16. Institut für Methodik der Fernerkundung bzw. Deutsches Fernerkundungsdatenzentrum Folie 16 16 Institut für Methodik der Fernerkundung Conclusions Geolocation of ground targets has to accurately consider signal propagation and geodynamic effects. Taking these effects into account, we reveal a much better absolute pixel localization accuracy of TSX-1 / TDX-1, than previous studies. The increased measurement accuracy also helped us to identify and solve a small systematic effect in SAR processing. TerraSAR-X: Absolute pixel localization accuracy in the order of magnitude of just few centimeters. TanDEM-X: Relative pixel localization accuracy in range direction even at sub-centimeter level.

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