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3_Xorbits_InSAR_IGARSS2011.ppt 3_Xorbits_InSAR_IGARSS2011.ppt Presentation Transcript

  • 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)
  • Theory: InSAR under crossing orbits
    • Consider a pair of SAR acquisitions with crossing ground-tracks
    • In general, different observation geometry implies that different portions of the 2-D ground spectra are sampled
      • Coherence loss
    • For small crossing angles, common spectrum can be achieved/increased by squinted acquistions
    Ground projected squint angle Spectral overlap achieved by applying opposed squints i.e. a Doppler Centroid offset is required!!!
  • Experiments
    • First TanDEM-X DEM
      • Acquisition while TDX was chasing TSX (before pursuit monostatic commissioning phase)
      • Crossing tracks due to Earth rotation
      • Small crossing angle -> small squints (< beam width)
      • Motivation: scientific impatience and proof of concept
    • Repeat-pass InSAR using different (crossing) tracks
      • Large crossing angles (1° - 2°) -> large squints (>> beam width)
      • Allows 1,5 and 6 day repeat-pass acquisitions with 11 day repeat cycle
        • for restricted regions (where ground tracks converge)
      • Motivation: proof of concept, scientifically relevant data
      • Time series being acquired
  • 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)
  • TanDEM-X first DEM
    • Along track (~200 km) separation results in crossing ground-tracks due to Earth Rotation
    • 0.13° relative squint required
    • ~2 km resulting effective baseline
    • h amb = 3.8 m
    • Almost out of the box processing with TAXI
  • TanDEM-X first DEM (October Revolution Island)
  • DEM Error/Performance High-pass filtered DEM Coherence to point-to-point error Scaling to restore white noise powe
  • Repeat-pass using different tracks (AKA 1/5-day repeat-pass)
    • Closest (non identical) tracks after (approximately) whole number of days
      • closest after 5 or 6 (= -5) days
      • next after 1 day
    • Near equator
      • Very large baselines ( InSAR not possible )
      • Tracks nearly parallel
    • Somewhere near poles
      • Tracks cross -> baseline vanishes
      • Large crossing angles
        • ~4° for 1 day interval
        • ~2° for 5 day interval
    For 30° incidence
  • Ronne #1 (lat/lon: -78°/-56.5°) Ronne #2 (lat/lon: -78.1°/ -48.2525°)
  • Acquisition Timeline Snow and ice precipitation (800km away) (28/06) High wind speeds (up to 50km/h, monthly mean 16km/h) (26/06)
  • Ronne #1 ( 150 MHz Bandwidth)
    • XTI baselines go through zero for all acquisitions
    • Extreme baseline variation with azimuth!
    • Scene consists of very flat and homogeneous floating ice.
  • Ronne #1, 300 MHz strip-map (experimental) 1st cycle 5d 1d 6d
  • 5d 1d 6d Ronne #1, 300 MHz strip-map (experimental) 2nd cycle
    • Are fringes x-orbits processing artifacts?
      • No! Same fringe pattern appears also in zero-squint 11-day repeat.
      • 11 day coherence is very bad.
      • Since baseline is small and constant -> high temporal decorrelation
    Ronne #1, 300 MHz strip-map (experimental) 11 day repeat 11d
    • Fringe rate is proportional to time-lag
      • Can be explained by surface velocity
      • Fringes indicate velocity gradients (velocity itself is ambiguous).
      • Velocity can be estimated from radargrammetric shifts
    • Coupling between height uncertainty and varying baseline introduces azimuth phase ramp
    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
  • 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.)
  • 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
  • Varying baseline
    • Joint histogram (pdf) of k z and coherence shows well defined relation.
    • Due to homogeneity of scene, mean coherence vs. kz can be directly related to volume decorrelation.
      • Mean SNR and temporal decorrelation introduce a constant factor.
  • Outlook
    • The feasibility of InSAR under crossing orbits has been experimentally validated
    • TanDEM-X experiment shows potential for XTI under very long baselines
    • Repeat-pass acquisitions under crossing tracks demonstrated
      • 1/5/6-day “repeat-pass” acquisitions possible
      • Possible scenes constrained to very narrow band of latitudes…
      • … yet scientifically very interesting areas can be addressed
    • Outlook
      • Longer time-series over Ronne Ice Shelf and other regions in Antarctica
      • Some InSAR processing challenges remain (improve coregistration under low coherence areas, eliminate some systematic phase errors).
      • Study of Ice structure exploiting variation of baseline.