Investigations of the DINSAR Performance in the TOPS Mode Using Experimental TerraSAR-X Data
Nannini, Matteo; Prats-Iraola, Pau; De Zan, Francesco
German Aerospace Center (DLR), GERMANY

The TOPS (Terrain Observation by Progressive Scan) [DeZan] mode has been selected as the default mode of operation for the Sentinel-1 satellite. This new mode provides a wide coverage and circumvents the limitations of ScanSAR, namely a strong azimuth dependence of the SNR due to the ScanSAR burst operation and the antenna pattern, by introducing a steering of the antenna in the along-track direction of the sensor from backward to forward.

In this paper we will focus on urban subsidence estimation. In particular, the case of Mexico City will be presented and analyzed, by processing a stack of more than 20 TOPS images acquired by TerraSAR-X. In this context, results obtained by means of persistent scatterers interferometry (PSI) will be shown. Such technique exploits the interferometric phase information within the image stack in order to separate the topographic information, the atmospheric phase screen and the motion occurring in the scene [Ferretti]. In the recent years another possibility to select valid pixels with which initialize the DInSAR processing chain has been proposed and the so-called coherent pixels (CP) have been defined [Berardino,Mora]. A point will be a CP if its averaged interferometric coherence is higher than a threshold within all the considered interferograms. Performing the selection directly at the interferogram stage allows one to freely combine the images and, in principle, to estimate the deformation with fewer acquisitions compared to PSI. On the other hand, CPs are subject to a resolution loss when compared to PS, due to the coherence computation.

The goal of this paper is to analyze how PSI, as well as CP DInSAR, behaves when applied to data acquired in the TOPS mode and if the estimated deformation is consistent to the one obtained for the stripmap case. To do such a comparison, a stack of more than 30 stripmap images acquired over Mexico City has been processed as well.

Such a comparison allows one to detect and understand special aspects of image stacks processing for DInSAR with the TOPS mode. In particular, it will be analyzed if additional phase or amplitude signal contributions are present and could influence the DInSAR performance , for example, effects due to the antenna steering, to different Doppler centroid occurring because of timing errors in the data take synchronization, or the higher coregistration requirements due to the higher Doppler centroids [Prats]. For example, in the TOPS mode the residual scalloping due to the electronic steering needs to be considered, since it translates automatically into a lower signal-to-noise ratio (SNR) at burst edges of about 0.15-0.2 dB in the TerraSAR-X case. Note in any case that this amount of scalloping is smaller than in the ScanSAR case, which is about 3 dB. In this context, the influence of the TOPS residual scalloping in the selection of valid pixels and DInSAR performance will be also addressed for both PS and CP.

In addition, to increase the temporal sampling of the image stack, the possibility to combine TOPS and stripmap will be analyzed. It will be observed if the common PS (or CP) between those two acquisition modes, tend to concentrate at the center of the burst rather than at the edge of it, because of the expected directivity of the PS (or CP). Figure 1 shows the mean deformation velocity and the DEM error over Mexico City obtained with an image stack of 36 TerraSAR-X stripmap images using the PS selection criterion. The TOPS results will be compared with these.

Figure 1: (left) Mean deformation velocity estimated over Mexico City using the PS technique. (right) Zoom over the city of the refined DEM retrieved as an additional product to the deformation velocity, where the individual buildings can be observed. The size of the PSs has been enlarged for visualization purposes. The scene size is approximately 8km x 8km. A stack of 36 TerraSAR-X stripmap images was used.

[DeZan] De Zan, Francesco, and A. Monti Guarnieri. "TOPSAR: Terrain observation by progressive scans." Geoscience and Remote Sensing, IEEE Transactions on 44.9 (2006): 2352-2360.

[Ferretti] Ferretti, Alessandro, Claudio Prati, and Fabio Rocca. "Permanent scatterers in SAR interferometry." Geoscience and Remote Sensing, IEEE Transactions on 39.1 (2001): 8-20.

[Berardino] P. Berardino, G. Fornaro, R. Lanari, and E. Sansosti, ''A new algorithm for surface deformation monitoring based on small baseline differential SAR interferograms,'' IEEE Trans. Geosci. Remote Sens., vol. 40, no. 11, pp. 2375-2383, Nov. 2002.

[Mora] Mora, Oscar, Jordi J. Mallorqui, and Antoni Broquetas. ''Linear and nonlinear terrain deformation maps from a reduced set of interferometric SAR images.'' Geoscience and Remote Sensing, IEEE Transactions on 41.10 (2003): 2243-2253.

[Prats] 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, 2012.