First TerraSAR-X interferometry evaluation

The German radar satellite TerraSAR-X was launched in June 2007 [1] and is currently ending its commissioning phase. We anticipate quite different interferometric application scenarios compared to ERS- 1/2 and ASAR due to the X-band frequency, the short orbital repeat cycles of 11 days, the high range resolution and the spotlight mode of this sensor. During the commissioning phase we have scheduled a number of acquisitions over selected test sites with different characteristics to get an early quick look of TerraSAR-X's interferometric capabilities and to assess the phase quality of the sensor and DLR’s processor system [2]. Our first results are quite encouraging and the technical parameters of the system are as specified. Many spectacular image details let us expect that the high resolution will demand a different view on SAR interferometry and allow new applications in urban environments. In our paper we show interferograms and images of different test sites, coherence measurements and a first assessment of the interferometric properties. We will give hints to future scientific users on data selection and data processing. The results are of high relevance for the TanDEM-X mission scheduled for 2009, when a second compatible SAR-sensor will be launched for a joint 3 year bistatic interferometric formation flight

The 2-Look TOPS Mode: Design and Demonstration with TerraSAR-X

Burst-mode acquisition schemes achieve wide coverage at the expense of a degraded azimuth resolution, reducing therefore the performance on the retrieval of ground displacements in the azimuth direction, when interferometric acquisitions are combined. Moreover the azimuth varying line-of-sight can induce discontinuities in the interferometric phase when local azimuth displacements are present, e.g., due to ground deformation. In this contribution we propose the interferometric 2-look TOPS mode, a sustaining innovation, which records bursts of radar echoes of two separated slices of the Doppler spectrum. The spectral separation allows to exploit spectral diversity techniques, achieving sensitivities to azimuth displacements better than with StripMap, and eliminating discontinuities in the interferometric phase. Moreover some limitations of the TOPS mode to compensate ionospheric perturbations, in terms of data gaps or restricted sensitivity to azimuth shifts, are overcome. The design of 2-look TOPS acquisitions will be provided, taking the TerraSAR-X system as reference to derive achievable performances. The methodology for the retrieval of the azimuth displacement is exposed for the case of using pairs of images, as well as for the calculation of mean azimuth velocities when working with stacks. We include results with experimental TerraSAR-X acquisitions demonstrating its applicability for both scenarios

Burst-Mode Wide-Swath SAR Interferometry for Solid Earth Monitoring

This work addresses the generation of large area maps of ground deformation originated by dynamic processes of the Earth using spaceborne Synthetic Aperture Radar (SAR) systems. The main focus is put on the development of new interferometric processing techniques for SAR data and on the proposal, design, experimental implementation and demonstration with TerraSAR-X of an acquisition mode with wide-swath capabilities, able to improve the accuracy of the measurements. Geoscientists have traditionally exploited StripMap mode to generate 2-D deformation maps by combining interferometric and correlation techniques. Correlation techniques have been applied in the first contribution to map the Mw9.0 Tohoku-Oki Earthquake occurred in 2011 in Japan by employing TerraSAR-X pairs. The terrain observation by progressive scans (TOPS) mode has been recently implemented as the baseline acquisition mode of the European Sentinel-1 System, providing a swath of 250 km at approximately 20 m resolution. The TOPS mode requires special techniques for the interferometric processing of the data, especially regarding accurate coregistration. The interferometric processing algorithms for handling pairs of Sentinel-1 TOPS data are provided in the second contribution. The coregistration approach exploits the SAR data for the retrieval of the required fine azimuth shifts. Since the performance of the method depends on the coherence, it can become an issue when working with very long time series, as temporal decorrelation dominates. A method for coregistering time series of Sentinel-1 TOPS data exploiting all images of the stack is proposed in the third contribution. In the last contribution, the novel 2-look TOPS mode is proposed which achieves wide swath (employing bursts of data and a steering of the antenna in azimuth, as TOPS) but exploiting spectral diversity techniques, in order to mitigate the poor accuracy on the along-track deformation retrieval. Taking into account that Earth Observation satellites orbit in quasi-polar configurations, a high sensitivity to the ground deformation in the North-South direction can be obtained. A demonstration with experimental TerraSAR-X data for the mapping of slow azimuth displacements with time series covering two years of postseismic deformation over the Hoshab fault, Pakistan, is provided, making evident the high potential of this mode

Accurate Azimuth Ground Deformation Estimation From Sentinel-1 Time Series

This letter proposes the exploitation of the overlapping areas between bursts of Sentinel-1 interferometric wide (IW) swath data for the estimation of ground azimuth deformation velocities from time series, which provides high sensitivity to the North–South direction. The availability of two separated Doppler spectral looks over these areas allows one to obtain high accurate estimates, overcoming the limited performance of classical correlation techniques with coarse resolution modes. Since the estimation is restricted to the (sparsely distributed) burst overlaps, its applicability is limited to geophysical phenomena characterized by a slow spatial variability, e.g., seismic scenarios. We demonstrate the suitability of this approach to time series over a region in Pakistan affected by post-seismic deformation. The estimated mean azimuth velocity performance from real data indicates an accuracy better than 7 mm/year over high long-term coherent areas for a three-year data stack

Time-Series evaluation of azimuth displacements retrieval with the experimental TerraSAR-X 2-looks TOPS acquisition mode

This contribution will present the investigations carried out with stacked time-series of TerraSAR-X acquisitions made in the experimental 2-looks TOPS mode. The characteristics and benefits of this interferometric mode have been already presented in [1]. One of the advantages of the use of 2-looks TOPS mode images is the ability to estimate the scene motion in a repeat-pass configuration in the azimuth direction over non-stationary areas with a similar accuracy to the one given by the stripmap mode but providing wide coverage. The focus of this contribution is the application to time-series, where slow azimuthal motion is expected. Several aspects will be analyzed. In first place, the impact of the tropospheric delay on the azimuthal measurements is reviewed. The achievable accuracy employing a time series is presented for the along-track direction and compared to the accuracy achievable in the across-track direction. Moreover the achievable 3D performance when combining ascending and descending acquisitions will be exposed. A time-series over Mexico City with experimental TerraSAR-X acquistions has been started in March 2016 and we planned to make a validation of the achievable along-track accuracy and present results. [1] P. Prats-Iraola, N. Yague-Martinez, S. Wollstadt, T. Kraus, R. Scheiber. Demonstration of the Applicability of 2-Look Burst Modes in Non-Stationary Scenarios with TerraSAR-X. EUSAR 2016, Hamburg, Germany

Enhancing Interferometric SAR Performance over Sandy Areas: Experience from the TanDEM-X Mission

The TanDEM-X mission is served by two X-band synthetic aperture radar (SAR) satellites, which fly in close orbit formation acting as a large and flexible single-pass radar interferometer. The primary goal of the mission is the creation of a consistent and global digital elevation model (DEM). A very good and reproducible performance has been verified for most of the land masses. In this paper, a detailed performance analysis of TanDEM-X data over sandy desert areas is presented, which show a strong impact on the quality of spaceborne SAR surveys. The influence on SAR and interferometric (InSAR) performance of several acquisition parameters is evaluated by means of statistical analyses as well as long-term repeated acquisitions on defined test sites. Alternative processing approaches aiming at improving the quality of the interferometric products are presented, too. From the obtained analyses, a description of the scattering mechanisms occurring at X-band over sandy surfaces is derived, which allows to plan a dedicated reacquisition of such areas with optimized imaging geometry in order to improve the quality of the final TanDEM-X DEM