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INVESTIGATIONS WITH THE SENTINEL-1 INTERFEROMETRIC
WIDE SWATH MODE
P. Prats-Iraola a, M. Nannini a, Rolf Scheiber a, Francesco De Zan a, Steffen Wollstadt a, Federico Minati b, Mario Costantini b,
Andrea Bucarelli b, Sven Borgstrom c, Thomas Walter d, Michael Foumelis e, Yves-Louis Desnos f
a
German Aerospace Center (DLR), Microwaves and Radar Institute, 82234 Weßling, Germany – [email protected]
b
e-GEOS SpA, ASI/Telespazio, 00156 Rome, Italy
c
National Institute of Geophysics and Volcanology (INGV), Vesuvius Observatory, 80124 Napoli, Italy
d
German Research Centre for Geosciences (GFZ), Section 2.1., 14473 Potsdam, Germany
e
RSAC c/o ESA-ESRIN, EO Science, Applications and Future Technologies Department, 00044 Frascati, Italy
f
ESA-ESRIN, EO Science, Applications and Future Technologies Department, 00044 Frascati, Italy
THEME: InSAR Theory and Techniques: InSAR with Sentinel-1 (TOPS)
KEY WORDS: Sentinel-1, TOPS, SAR interferometry, differential SAR interferometry.
1. INTRODUCTION
The contribution focuses on the current status of the ESA study entitled “INSARAP: Sentinel-1 InSAR Performance study with
TOPS Data”. The study investigates the performance of the interferometric wide swath (IW) mode of Sentinel-1, which is
implemented using the terrain observation by progressive scans (TOPS) mode [1]. The key aspects of the TOPS mode that need to be
considered for accurate interferometric processing will be presented, and first analyses with Sentinel-1 time series will be shown. The
interferometric processing chain used to process the data will be expounded in detail. The interferometric results will focus on
different pilot sites, namely, Campi Flegrei/Vesuvius area and Mount Etna, both located in Italy, Istanbul city in Turkey, and Mexico
City. The evaluation of the results will be performed using in-situ geodetic measurements based on continuous GPS stations located
on the different sites. For Mexico City and Istanbul a cross-check using TSX TOPS time series will be also used. Other relevant
interferometric aspects will be also addressed, e.g., phase jumps due to azimuthal motion and ionospheric artefacts.
2. INTERFEROMETRIC PROCESSING
In the first part, the processing chain to perform the interferometric processing will be shown in detail. The approach is based on the
computation of the geometric offsets based on the geometry, namely, by exploiting the state vectors and an external DEM [2]. This
approach has been shown to be quite appropriate for the TOPS mode due to the high requirements in terms of azimuth coregistration
accuracy, in the order of 0.001 azimuth samples (about 2 cm). The geometric approach allows a very accurate coregistration
performance in relative terms, enough to satisfy the TOPS coregistration requirements even if only the ellipsoid is used to compute
the offsets. However, a global offset is still missing, which can be estimated using conventional cross-correlation techniques or by
exploiting the overlap area [3]. The latter approach will be presented and results with Sentinel-1 will be shown, where, thanks to the
proposed chain, no phase jumps will be observed in scenes without azimuthal motion. Figure 1 shows some examples of Sentinel-1
interferograms.
Figure 1. The figure below shows some Sentinel-1 interferograms in the IW mode (coherence and flattened phase with overlaid
reflectivity) computed within the study over the pilot areas. From left to right and top to bottom: Mount Etna, Campi
Flegrei/Vesuvius, Istanbul, Mexico City.
3. INTERPRETATION OF PHASE JUMPS
Indeed, the presence of azimuthal motion will introduce phase jumps in the interferograms. It will be shown that the existence of
these jumps in the TOPS mode between bursts in the presence of azimuth coregistration errors are, rather than an artefact, a matter of
interpretation [4]. Assuming the stationary targets have been perfectly coregistered, the presence of phase jumps is an indication of
true azimuthal motion, e.g., glaciers. This occurs due to the variation of the line of sight (LOS) coming from the antenna steering, so
that at burst edges the same target is observed from two different LOS angles, hence producing a small jump even for very small
azimuthal motions in the range of decimetres. Also, it will be shown that this spectral diversity at burst edges makes it more sensitive
to other physical phenomena like, e.g., ionospheric scintillations. Therefore, not only azimuthal motion can introduce phase jumps
between bursts at a local scale, also ionospheric effects can produce them. Figure 2 shows an example of the effect of azimuthal
motion in the case of a glacier, where the phase jumps between bursts and between sub-swaths are clearly visible.
Figure 2. Interferogram over the Pine Island glacier, West Antarctica. The zoom shows the phase jumps between bursts and subswaths due to the change in LOS at these places. The jumps are an indication of azimuthal motion.
4. DINSAR AND PSI ASSESSMENT
The DInSAR assessment will consist in the evaluation of scenes with azimuthal motion, e.g., glaciers. The retrieval of the motion of
the slow parts of the glacier will be done by exploiting the DInSAR phase. The phase jumps explained in the previous section will be
critical, and several approaches to overcome them and properly perform the phase unwrapping will be investigated. Similar effects
are expected to occur in the case of earthquakes, and results will be presented provided Sentinel-1 happens to acquire data over one
with the TOPS mode.
The evaluation of differential interferometric results and persistent scatterer interferometry (PSI) exploiting time series will focus on
several aspects. On the one hand, the interferometric performance of Sentinel-1 in terms of burst synchronization and interferometric
performance (e.g., NESZ, coherence over different land types, phase unwrapping) will be presented. On the other hand, the PSI
performance for surface deformation will be assessed using the in-situ geodetic measurements, as well as with the cross-comparison
between different PSI processing chains.
5. REFERENCES
[1] F. De Zan and A. Monti Guarnieri, “TOPSAR: Terrain Observation by Progressive Scans,” IEEE Trans. Geosci. Remote Sens.,
vol. 44, no. 9, pp. 2352–2360, Sep. 2006.
[2] E. Sansosti, P. Berardino, M. Manunta, F. Serafino, and G. Fornaro, “Geometrical SAR image registration,” IEEE Trans. Geosci.
Remote Sens., vol. 44, no. 10, pp. 2861–2870, Oct. 2006.
[3] P. Prats-Iraola, R. Scheiber, L. Marotti, S. Wollstadt, and A. Reigber, “TOPS interferometry with TerraSAR-X,” IEEE Trans.
Geosci. Remote Sens., vol. 50, no. 8, pp. 3179–3188, Aug. 2012.
[4] F. De Zan, P. Prats-Iraola, R. Scheiber, A. Rucci, “TOPS Interferometry: coregistration and azimuth shifts”, in Proc. European
Conference on Synthetic Aperture Radar (EUSAR), Berlin, Germany, 2014