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.