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  1. 1. Demonstration of SAR Interferometry under Crossing Orbits using TerraSAR-X and TanDEM-X P aco López-Dekker , Pau Prats, Francesco De Zan, Steffen Wollstadt, Daniel Schulze, Gerhard Krieger and Alberto Moreira Microwaves and Radar Institute, German Aerospace Center (DLR)
  2. 2. Theory: InSAR under crossing orbits <ul><li>Consider a pair of SAR acquisitions with crossing ground-tracks </li></ul><ul><li>In general, different observation geometry implies that different portions of the 2-D ground spectra are sampled </li></ul><ul><ul><li>Coherence loss </li></ul></ul><ul><li>For small crossing angles, common spectrum can be achieved/increased by squinted acquistions </li></ul>Ground projected squint angle Spectral overlap achieved by applying opposed squints i.e. a Doppler Centroid offset is required!!!
  3. 3. Experiments <ul><li>First TanDEM-X DEM </li></ul><ul><ul><li>Acquisition while TDX was chasing TSX (before pursuit monostatic commissioning phase) </li></ul></ul><ul><ul><li>Crossing tracks due to Earth rotation </li></ul></ul><ul><ul><li>Small crossing angle -> small squints (< beam width) </li></ul></ul><ul><ul><li>Motivation: scientific impatience and proof of concept </li></ul></ul><ul><li>Repeat-pass InSAR using different (crossing) tracks </li></ul><ul><ul><li>Large crossing angles (1° - 2°) -> large squints (>> beam width) </li></ul></ul><ul><ul><li>Allows 1,5 and 6 day repeat-pass acquisitions with 11 day repeat cycle </li></ul></ul><ul><ul><ul><li>for restricted regions (where ground tracks converge) </li></ul></ul></ul><ul><ul><li>Motivation: proof of concept, scientifically relevant data </li></ul></ul><ul><ul><li>Time series being acquired </li></ul></ul>
  4. 4. TanDEM-X Commissioning Phase First bi-static DEM 8 October (MET +113) June‘10 July‘10 Aug‘10 Sep‘10 Oct‘10 Nov‘10 Dec‘10 Early Orbit Phase Grg Segment Checkout Bi-static Commissioning Phase 20 km Formation TDX Orbit Drift 16.000 km  20 km Close Helix-Formation 300-400 m Launch 21 June DEM Acquisition First SAR Image 24 June (MET +3.6) First DEM 16 July (MET +25) TDX Monostatic Comm. Phase 6 Months Commissioning Phase First close formation DEM 19 October (MET +124) First bi-static SAR image 8 August (MET +48)
  5. 5. TanDEM-X first DEM <ul><li>Along track (~200 km) separation results in crossing ground-tracks due to Earth Rotation </li></ul><ul><li>0.13° relative squint required </li></ul><ul><li>~2 km resulting effective baseline </li></ul><ul><li>h amb = 3.8 m </li></ul><ul><li>Almost out of the box processing with TAXI </li></ul>
  6. 6. TanDEM-X first DEM (October Revolution Island)
  7. 7. DEM Error/Performance High-pass filtered DEM Coherence to point-to-point error Scaling to restore white noise powe
  8. 8. Repeat-pass using different tracks (AKA 1/5-day repeat-pass) <ul><li>Closest (non identical) tracks after (approximately) whole number of days </li></ul><ul><ul><li>closest after 5 or 6 (= -5) days </li></ul></ul><ul><ul><li>next after 1 day </li></ul></ul><ul><li>Near equator </li></ul><ul><ul><li>Very large baselines ( InSAR not possible ) </li></ul></ul><ul><ul><li>Tracks nearly parallel </li></ul></ul><ul><li>Somewhere near poles </li></ul><ul><ul><li>Tracks cross -> baseline vanishes </li></ul></ul><ul><ul><li>Large crossing angles </li></ul></ul><ul><ul><ul><li>~4° for 1 day interval </li></ul></ul></ul><ul><ul><ul><li>~2° for 5 day interval </li></ul></ul></ul>For 30° incidence
  9. 9. Ronne #1 (lat/lon: -78°/-56.5°) Ronne #2 (lat/lon: -78.1°/ -48.2525°)
  10. 10. Acquisition Timeline Snow and ice precipitation (800km away) (28/06) High wind speeds (up to 50km/h, monthly mean 16km/h) (26/06)
  11. 11. Ronne #1 ( 150 MHz Bandwidth) <ul><li>XTI baselines go through zero for all acquisitions </li></ul><ul><li>Extreme baseline variation with azimuth! </li></ul><ul><li>Scene consists of very flat and homogeneous floating ice. </li></ul>
  12. 12. Ronne #1, 300 MHz strip-map (experimental) 1st cycle 5d 1d 6d
  13. 13. 5d 1d 6d Ronne #1, 300 MHz strip-map (experimental) 2nd cycle
  14. 14. <ul><li>Are fringes x-orbits processing artifacts? </li></ul><ul><ul><li>No! Same fringe pattern appears also in zero-squint 11-day repeat. </li></ul></ul><ul><ul><li>11 day coherence is very bad. </li></ul></ul><ul><ul><li>Since baseline is small and constant -> high temporal decorrelation </li></ul></ul>Ronne #1, 300 MHz strip-map (experimental) 11 day repeat 11d
  15. 15. <ul><li>Fringe rate is proportional to time-lag </li></ul><ul><ul><li>Can be explained by surface velocity </li></ul></ul><ul><ul><li>Fringes indicate velocity gradients (velocity itself is ambiguous). </li></ul></ul><ul><ul><li>Velocity can be estimated from radargrammetric shifts </li></ul></ul><ul><li>Coupling between height uncertainty and varying baseline introduces azimuth phase ramp </li></ul>5d 1d Ronne #1, 300 MHz strip-map (experimental) Interpretation Zero baseline: Only noise and temporal decorelation Large baseline: Large volume decorrelation (geometric decorrelation addressed by azimuth adaptive range spectral filtering) Zero baseline: Only noise and temporal decorelation
  16. 16. Velocity measurements on the Filchner/Ronne Ice Shelf (taken from http://nsidc.org) The velocity contours are from Vaughn and Jonas, 1996 (units of m/yr). (Vaughan, D.G., and M. Jonas, 1996. Measurements of velocity of Filchner-Ronne Ice Shelf. Filchner-Ronne Ice Shelf Programme (FRISP) Report No. 10. Ed. H Oerter. Pub. AWI, Bremerhaven. 111-116.)
  17. 17. Other things to do: exploit varying baseline Ronne #1 150 MHz strip-map (3rd cycle) 1 day repeat Homogeneous scene Rapidly varying baseline Rapidly varying kz
  18. 18. Varying baseline <ul><li>Joint histogram (pdf) of k z and coherence shows well defined relation. </li></ul><ul><li>Due to homogeneity of scene, mean coherence vs. kz can be directly related to volume decorrelation. </li></ul><ul><ul><li>Mean SNR and temporal decorrelation introduce a constant factor. </li></ul></ul>Surface?
  19. 19. Outlook <ul><li>The feasibility of InSAR under crossing orbits has been experimentally validated </li></ul><ul><li>TanDEM-X experiment shows potential for XTI under very long baselines </li></ul><ul><li>Repeat-pass acquisitions under crossing tracks demonstrated </li></ul><ul><ul><li>1/5/6-day “repeat-pass” acquisitions possible </li></ul></ul><ul><ul><li>Possible scenes constrained to very narrow band of latitudes… </li></ul></ul><ul><ul><li>… yet scientifically very interesting areas can be addressed </li></ul></ul><ul><li>Outlook </li></ul><ul><ul><li>Longer time-series over Ronne Ice Shelf and other regions in Antarctica </li></ul></ul><ul><ul><li>Some InSAR processing challenges remain (improve coregistration under low coherence areas, eliminate some systematic phase errors). </li></ul></ul><ul><ul><li>Study of Ice structure exploiting variation of baseline. </li></ul></ul>