InSAR with Sentinel-1 (1)

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.

The attached PDF file contains more details and images.

Current spaceborne SAR missions steer the satellite to follow an orbital tube in order to guarantee interferometric compatibility between data takes. In this contribution it is shown that the size of the orbital tube can impact the interferometric performance in terms of azimuth spectral decorrelation and azimuth coregistration requirements under high Doppler centroids, which can be specially critical in burst modes, namely, ScanSAR and TOPS. The paper presents and analyses these aspects in the frame of the Sentinel-1 mission.

Please see attached PDF for a more detailed description.

>>---This is a text version of a PDF file. Formulas and Figures are missing. Please refer to the PDF document for a proper visualization ---<<

The European radar satellite Sentinel 1A was launched in April 2014 as the first of the Copernicus Space Component. A second satellite (B unit) will follow in 2015. Both carry an imaging C-band SAR instrument. The Sentinel 1 system has been conceived to provide repeat-pass interferometric capabilities with wide area coverage for medium resolution applications. Wide-swath is achieved by employing the novel TOPS (Terrain Observation by Progressive Scans) acquisition mode [1].
TOPS is a SAR acquisition mode that is capable of providing wide range swaths as with the ScanSAR technique but without the associated problems of scalloping and azimuth varying SCR, NESZ and azimuth ambiguities.
TOPS is the operating mode for the Sentinel-1 IW (Interferometric Wide swath) and EW (Extra-Wide swath) modes. The IW mode is the predefined mode over land and has been conceived to apply interferometric techniques, providing a range coverage of about 250 km at 5m x 20m resolution. The repeat cycle is 12 days.
The effect of squint on interferometry is critical for TOPS which relies on a systematically varying squint angle for its operation. The non-stationarity of the squint angle during the TOPS acquisition produces a linear variation of the Doppler Centroid frequency in the SAR data. It will be shown that considerable phase ramps in azimuth may appear if there are coregistration errors. Using a small-angle approximation, a misregistration of   seconds in azimuth between master and slave leads to an azimuth phase ramp over the burst of
      [cycles]

where    is the change in Doppler centroid over azimuth and   the change in squint angle,   is the effective sensor velocity and   the carrier wavelength.
For the Sentinel-1A parameters a coregistration accuracy in azimuth better than 1.3 cm (equivalent to 0.0006 resolution elements or 0.001 pixels) is necessary if a phase variation along the burst lower than 1/100 of a cycle is desired.
A description of the TOPS acquisition mode of Sentinel-1A will be presented, introducing the burst spectral properties. The necessity of burst synchronization for interferometric applications will be justified, and an assessment of the burst synchronization performance of Sentinel-1A with a set of products acquired during the Commissioning Phase will be included. A schematic processing chain (see Figure 1) will be presented emphasizing the measures that are needed to take into account for Sentinel-1A. Enhanced Spectral Diversity (ESD) [2] is used for coregistration.
Repeat-pass images acquired in TOPS mode have been processed using the operational Integrated Wide Area Processor (IWAP) at DLR [3], whose algorithm for burst-mode data are described in [4].

 
Figure 1: Burst level interferometric processing. Resampling, spectral shift filtering, in range and azimuth, and interferogram formation are computed at this level. A geometric coregistration is performed, which is corrected by using Enhanced Spectral Diversity (ESD).

The first IW InSAR pair was available in ascending geometry over Italy on August 19, 2014, 12 days after Sentinel-1A had reached its final orbit on August 7, 2014. This first TOPS pair could be processed with the IWAP [3] demonstrating the principal readiness of the Sentinel-1 system (both in-orbit instrument and on-ground commanding and processing) for interferometric applications.
Figure 2 shows the interferometric phase and SAR amplitude overlay image of the mosaic of the first two slices interferograms over Italy. The image is composed of three sub-swaths and has a ground range extension of 250 km. The effective baseline is about 120 m.  After flat Earth phase removal, one fringe colour cycle corresponds to a height of ambiguity of 128.82m.
 
Figure 2: Mosaic of first two Sentinel-1A TOPS Interferograms Over Italy

An Interferometric validation will be provided using a pair of Datatakes, with a temporal baseline of 1 cycle, which presents high coherence. The needed azimuth coregistration corrections will be quantified when using different Orbit products (Annotated in product, restituted, precise). We will analyse the variation of the azimuth shift for every overlapping area between adjacent bursts along azimuth (interburst-analysis). This is done for all three subswaths.

[1]    F. de Zan and A. Monti Guarnieri. TOPSAR: Terrain Observation by Progressive Scans, IEEE Transactions on Geoscience and Remote Sensing, Vol. 44,  No. 9, September 2006, pp 2352-2360.
[2]    P. Prats-Iraola, R. Scheiber, L. Marotti, S. Wollstadt and A. Reigber. TOPS Interferometry with TerraSAR-X, IEEE Transactions on Geoscience and Remote Sensing, Vol. 50, No. 8, August 2012, pp 3179-3188.
[3]    F. Rodriguez-Gonzalez, N. Adam, A. Parizzi, R. Brcic, The Integrated Wide Area Processor (IWAP): A Processor for Wide Area Persistent Scatterer Interferometry. ESA Living Planet Symposium, Edinburgh, September 2013.
[4]    N. Yague-Martinez, F. Rodriguez-Gonzalez, U. Balss, H. Breit, T. Fritz. TerraSAR-X TOPS, ScanSAR and WideScanSAR interferometric processing. EUSAR 2014 - 10th European Conference on Synthetic Aperture Radar, pp. 945-948. VDE Verlag. 03-05 June 2014, Berlin, Germany.

Although not a designated science mission, the European Commission's Sentinel-1 (S-1) constellation will be a game-changer for operational and scientific applications of Synthetic Aperture Radar Interferometry (InSAR). In particular, its unprecedented regular sampling with short repeat cycle will enable deformation monitoring over a much wider range of geophysical phenomena. Compared to earlier ESA SAR satellites, S-1 has a lower orbit, larger antenna, tighter orbital tube and shorter repeat cycle, which all play together to ensure much higher coherence. This is obviously very important for InSAR time series analysis. The operational acquisition scheme, coupled with the free and open data policy, makes planning of both operational monitoring and scientific experiments much more practical.

The primary objective of the INSARAP project is to develop the necessary tools to handle data from the novel Terrain Observation by Progressive Scans (TOPS) mode, and to validate the functionality in terms of geophysical applications by comparison to historical InSAR data and ground truth. Towards this end, number of pilot sites have been selected, covering the topics of tectonics, landslides, urban subsidence, ice motion, and system validation with corner reflectors.

The manifold increase in quality and availability of InSAR data provided by Sentinel-1 comes at a cost. The TOPS mode was necessary to achieve the specified repeat cycle of 12 days with one satellite. Processing algorithms for the traditional stripmap mode are generally mature in the community. However, for the new TOPS acquisition mode, several new aspects must be handled, including time varying doppler and multiple small images per product instead of one larger. Thus, common InSAR processing practices will need to be revised and extended. In particular, the TOPS mode requires an azimuth coregistration accuracy well below 1/100th of a pixel. No single known algorithm can consistently reach this accuracy, thus it is necessary with a multistage algorithm with successive refinement.

For time series analysis, algorithms for S-1 TOPS will in principle not be any different than for stripmap and/or spotlight data. However, due to the bursted nature of the TOPS mode, with multiple small images per product, there are multiple options for when and how to merge the results. It is important to keep in mind that nothing is lost with respect to the former situation, just more opportunities to improve results due to the diversity provided by the overlap zones between bursts.

In this contribution, we will discuss processing and algorithmic challenges related to TOPS acquisition mode and large data volumes, and provide a recommended workflow for InSAR processing of S-1 TOPS stacks. Some preliminary results based on the available Sentinel-1 TOPS data from the project test sites will be presented.

A new method is proposed to estimate components of the satellite orbits by exploiting multi-squint interferograms. The method is suited to exploit Sentinel.1 data in either STRIPMAP or TOPS modes. In this last case, an extraordinary accuracy is provided by the large squint difference made available in  bursts overlaps both inter and intra swaths. The phase shifts measured on the grid of overlaps are modelled in terms of orbit parameters. A linearized analysis is first performed to evaluate the parameters that are most likely to be estimated. Thereafter, a direct search in the parameter space is implemented by maximizing the coherence of the multi-squint interferogram. 
The accuracy of the parameters' estimate is evaluated on the basis of the geometry and the actual phase noise.
Results from data processed in S1 commissioning phase are shown.
In TOPS mode the method was able to assess a sub-ceninemtric accuracy provided by S1 "Precise Orbits". As a by product, a Multi-Squint Interferogram of Napa quake was able to monitor the N-S components of the deformation (see attached image)

Invited presentation