Investigations with Sentinel-1 IW data
www.DLR.de/HR > SEOM INSARAP • INSARAP Workshop • December 10, 2014 > Slide 1 SEOM – INSARAP: Sentinel-1 InSAR Performance Study with TOPS Data ESA-ESRIN Contract 4000110587/14/I-BG Investigations with Sentinel-1 IW Data Pau Prats, Marc Rodriguez-Cassola DLR-HR, Microwaves and Radar Institute, Pau.Prats@dlr.de www.DLR.de/HR > SEOM INSARAP • INSARAP Workshop • December 10, 2014 > Slide 2 Statistical Analysis with Sentinel-1 Interferograms • Analysis based on IW acquisitions over pilot sites (54 images, 40 interferograms). • All acquisitions after October 2nd (due to new synchronization strategy). • Sentinel-1 burst synchronization requirement: 5 ms. • Total zero Doppler steering (Doppler around 0 Hz from beginning of October). www.DLR.de/HR > SEOM INSARAP • INSARAP Workshop • December 10, 2014 > Slide 3 Statistical Analysis with Sentinel-1 Interferograms Burst mis-synchronization [ms] 5.00 Perpendicular baseline [m] 150.00 4.00 3.00 100.00 2.00 50.00 1.00 0.00 0.00 -1.00 -50.00 -2.00 -100.00 -3.00 -150.00 -4.00 -5.00 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 -200.00 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 Doppler centroid [Hz] 80.00 Std.Dev. Burst mis: 2 ms Std.Dev. 𝐵𝐵⊥ = 69 m Std.Dev. Doppler = 20 Hz (6.5% az. Bandwidth) 60.00 40.00 20.00 0.00 -20.00 08/10/2014 10/10/2014 12/10/2014 14/10/2014 16/10/2014 18/10/2014 20/10/2014 22/10/2014 24/10/2014 26/10/2014 28/10/2014 30/10/2014 01/11/2014 03/11/2014 05/11/2014 07/11/2014 09/11/2014 11/11/2014 13/11/2014 15/11/2014 17/11/2014 19/11/2014 21/11/2014 23/11/2014 25/11/2014 27/11/2014 29/11/2014 01/12/2014 03/12/2014 05/12/2014 07/12/2014 -40.00 www.DLR.de/HR > SEOM INSARAP • INSARAP Workshop • December 10, 2014 > Slide 4 Azimuth Spectrum Shape in the IW Mode - Below are shown the (along range) averaged profiles of deramped and demodulated spectra for one single burst of several independent data takes (the blue line is the estimated; the purple one is the expected). - Effect still observed in the latest products (December 2014). - If effect corrected at some point (assuming the processor is the cause), time series might have to be reprocessed due to impact in radiometric accuracy (amplitude dispersion index). www.DLR.de/HR > SEOM INSARAP • INSARAP Workshop • December 10, 2014 > Slide 5 Inconsistency of Azimuth Shifts • The shift between bursts (computed using GPS tag or time from ascending node, azimuthAnxTime) results in a non-integer number of azimuth samples. • Both time tags are truncated to microsends ⇒ almost one centimetre in along-track. • Residual azimuth coregistration error consistency: in stationary scenes, the estimated residual error should be almost constant for the whole image. • However, the analysis reveals quite different estimations from burst to burst and from sub-swath to sub-swath. • The above analysis might indicate an improper alignment of azimuth bursts during L1 processing. www.DLR.de/HR > SEOM INSARAP • INSARAP Workshop • December 10, 2014 > Slide 6 azimuth → Azimuth Ambiguities Analysis www.DLR.de/HR > SEOM INSARAP • INSARAP Workshop • December 10, 2014 > Slide 7 Azimuth Ambiguities Analysis (Master 15.10.2014) 27.10.2014 Non-homogeneous azimuth ambiguities. Biased Doppler estimation over sea? azimuth → 08.11.2014 azimuth → www.DLR.de/HR > SEOM INSARAP • INSARAP Workshop • December 10, 2014 > Slide 8 The Stop-and-Go Effet in the TOPS Mode* *M. Rodriguez-Cassola, P. Prats-Iraola, F. De Zan, R. Scheiber, A. Reigber, D. Geudtner, A. Moreira, “Doppler-related Distortions in TOPS SAR Images,” IEEE Trans. Geosci. and Remote Sensing, vol. 53, no. 1, Jan. 2015. www.DLR.de/HR > SEOM INSARAP • INSARAP Workshop • December 10, 2014 > Slide 9 Distortions in the TOPS mode due to the Stopand-Go approximation - We differentiate two stop-and-go effects: - Slow-time stop-and-go: between transmission and reception of the chirp signal. Corrected in Sentinel-1 (bistatic correction). - Fast-time stop-and-go: during transmission of chirp signal [2] (real Doppler). Not corrected within the processing of Sentinel-1 TOPS data. Chirp duration: 50 µs ⇒ the satellite moved about 40cm! Effect well-known in airborne FMCW SAR - If fast-time stop-and-go not corrected in the TOPS mode, undesired systematic shifts in range occur (azimuth-variant!): Δ𝜏𝜏 = 𝑓𝑓𝐷𝐷𝐷𝐷 (𝑡𝑡) [𝑠𝑠] Kr - Easy correction in the wavenumber domain. www.DLR.de/HR > SEOM INSARAP • INSARAP Workshop • December 10, 2014 > Slide 10 Stop-and-Go in the TOPS Mode (cont.) - Shift depends on Doppler centroid ⇒ The bursts are skewed in range. Range shift before and after stop-and-go correction Maximum shift of ~35 cm for Sentinel-1 (70 cm among bursts) Correlated for master and slave acquisitions but critical for accurate geolocation! www.DLR.de/HR > SEOM INSARAP • INSARAP Workshop • December 10, 2014 > Slide 11 Results with Sentinel-1 TOPS Data - Analysis of CR located at the overlap region Data take: S1A_IW_SLC__1SDH_20140604T170819_20140604T170849_000903_000DFA_4472.SAFE Azimuth IRFs Range IRFs Shift of 0.3 samples in range as predicted. —— Burst i —— Burst i+1 www.DLR.de/HR > SEOM INSARAP • INSARAP Workshop • December 10, 2014 > Slide 12 Summary • TOPS InSAR processing: Geometric approach + (optional!) global offset. • TOPS special considerations: • Phase interpretation: phase jumps will appear in scenes with azimuthal motion ⇒ more sensitivity to along-track motion, but only at overlap areas. • • Ionospheric scintillations might produce phase discontinuities between bursts. • Role of orbital tube. • Azimuth spectral filtering for speckle tracking. Investigations with Sentinel-1 IW interferograms: • Very good burst synchronization at data take start (2 ms std.dev.). • Total zero Doppler steering (20 Hz std.dev. ≈ 6.5% az. bandwidth). • Minor issues found: shape of azimuth spectrum, inconsistency of azimuth shifts, fast stop-and-go correction not applied, non-homogeneity of azimuth ambiguities.
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