Christian Minet

Imaging Geodesy—Toward Centimeter-Level Ranging Accuracy With TerraSAR-X

In this paper, we report on experiments to measure large-scale Earth surface displacements, such as those caused by solid Earth tides, with centimeter-level accuracy using TerraSAR-X radar images. With two totally different approaches, corner reflectors and image correlation techniques, we show the clear interrelation between the radar range measurements and the projection of the solid Earth tide motion vector onto the radar line of sight. Pixel location accuracies of up to 2.6-cm standard deviation can be achieved after a single calibration. We further demonstrate that solid Earth tides and tropospheric water vapor variations are the largest sources of ranging error if not compensated for. Alternatively, tropospheric water vapor can be estimated with centimeter accuracy using our proposed technique of synthetic aperture radar (SAR) image correlation and solid Earth motion compensation by the existing models. We also consider ionospheric delays which improve the results marginally in the X-band. Our results show the best ranging accuracies so far reported for spaceborne radar amplitude images and make TerraSAR-X-together with our simple compensation methodology-suitable for the imaging of centimeter-level Earth displacements. Absolute measurements of volcanoes or glaciers are possible without the use of ground equipment and without the use of SAR interferometry, thus avoiding the associated problems of phase ambiguity, phase unwrapping, and reference points.

Pre-survey suitability evaluation of the differential synthetic aperture radar interferometry method for landslide monitoring

The active remote-sensing technique differential radar interferometry (D-InSAR) is a powerful method for detection and deformation monitoring of landslides. But the radar-specific imaging geometry causes specific spatial distortions in radar images (as e.g. the layover and shadowing effect), which have a negative impact on the suitability of these images for D-InSAR applications. To address this issue, we present a geographical information system (GIS) procedure to accurately predict the areas in which layover and shadowing will occur, before the area of interest is recorded by radar. Additionally, the percentage of measurability of movement of a potential landslide can be ascertained. In the third part of the GIS procedure, the main types of land cover are classified in regard to their influence on applicability of the D-InSAR technique, depending on the characteristics of the sensor used. The results of the analyses are objective pre-survey estimation of the potential applicability of the D-InSAR technique for landslide monitoring prior to the costly investment of a radar survey.

Ground Displacement Measurement by TerraSAR-X Image Correlation: The 2011 Tohoku-Oki Earthquake

Japan was struck by an M9.0 megathrust earthquake on March 11, 2011. Synthetic aperture radar (SAR) images from the TerraSAR-X satellite have been used to generate a ground motion map by means of correlation techniques. Geophysical corrections due to solid Earth tide and atmospheric path delay effects have been applied. These corrections can reach up to 20 cm in the radar line of sight. Using this approach, absolute displacements in the radar line of sight and in the satellite flight direction are determined. This letter shows the potential of correlation techniques for ground motion monitoring and a comparison with the interferometric technique. Using multiple scenes, a wide area displacement map is generated and quantitatively compared with GPS data, showing a divergence of about 15 cm.

High precision measurement on the absolute localization accuracy of TerraSAR-X

The German SAR (synthetic aperture radar) satellites TerraSAR-X (TSX-1) and TanDEM-X (TDX-1), launched in June 2007 and June 2010 respectively, provide an unprecedented geometric accuracy. Previous studies showed an absolute pixel localization for both sensors at the centimeter level [4] [5] [6]. However, recent measurements show that in range, under extraordinary good conditions, a location accuracy of even a few millimeters seems to be attainable. While on a long-term scale, we observed a slow variation of subsequent measurements; on a short-term scale, they coincided to within a few millimeters. The measurement series will be continued. The cause of the long-term variation is the subject of current investigation.

Techniques for high accuracy relative and absolute localization of TerraSAR-X / TanDEM-X data

The German SAR (synthetic aperture radar) satellites TSX-1 and TDX-1 outperform former civilian space-borne radar sensors in terms of geometric accuracy. Recently, both satellites constitute the first bistatic SAR system in space: TanDEM-X. With the high geometric accuracy of both satellites, the geolocation of ground targets has to accurately consider signal propagation effects and geodynamic effects which formerly were negligible. The prediction of expected radar positions is additionally complicated by the bistatic acquisition geometry of TanDEM-X. Here, we present our approach to cope with these demanding requirements. With the application of these techniques, measurements of the geometric accuracy of TSX-1 and TDX-1 reveal that their pixel localization accuracy is significantly better than in previous studies.

Scientific requirements and feasibility on an L-band mission dedicated to measure surface deformation

DLR is currently studying a space borne mission based on two L-band satellites to map Earth surface deformation and vegetation structure from space. In this study the scientific requirements for deformation measurements are collected, traded off versus technical feasibility and a mission concept is investigated that provides a global monitoring capability of geo-tectonic threats [6].

Techniques and examples for the 3D reconstruction of complex scattering situations using TerraSAR-X

The German radar satellite TerraSAR-X was launched in June 2007. Since then, it is continuously providing high resolution space-borne radar data which are perfectly suitable for sophisticated interferometric applications. I.e. the mission concept and the SAR sensor support the coherent stacking of radar scenes which is the basis for advanced processing techniques e.g. Persistent Scatterer Interferometry (PSI) and SAR tomography. In particular, the short repeat cycle of eleven days and the highly reproducible scene repetition of the spotlight acquisitions support the stacking and consequently the time series analysis of the radar data. Furthermore, the sensors orbital tube is precisely controlled to be in the order of 200 m which basically allows to utilize the baseline spread of the stacked acquisitions. However, this small spread is actually limiting the resolution in the SAR tomography. Interferometric applications could be demonstrated already in a very early stage of the TerraSAR-X mission. Because the resolution is 0.6 m in slant range and 1.1 m in azimuth in the high resolution spotlight mode the PSI and the SAR tomography processing results were impressive. Urban areas and single buildings could be mapped from space in three dimensions. Even the structural stress of single buildings caused by thermal dilation could be demonstrated. However, extended layover areas are caused by typical buildings and as a consequence complicated scattering situations need to be resolved. DLRs operational In-SAR processing system GENESIS had already been adapted to cope with the new sensor modes of TerraSAR-X and their new specific spectral characteristics. Now, the new image characteristics e.g. the extended layover areas and the long time coherent distributed scatterer need better to be supported. Subject is to optimally exploit the available information e.g. the radar reflectivity. Several algorithms of the processing system can take advantage of this, e.g. the scatterer configuration detection. As a matter of fact, the scatterer configuration has now become a very important characteristic for each resolution cell. It influences e.g. the estimation data extraction, the estimation of the 3D location and basically the estimation precision. A typical resolution cell can be composed of a single dominant point scatterer surrounded by clutter, two or more dominant point scatterers in clutter and of distributed scatterers with a specific phase stability over time. The paper provides technical details and a processing example of a newly developed algorithm to retrieve the 3D location of point scatterers from the scenes intensity which finally also provides the information on the scatterer configuration in a resolution cell.

Requirements for an L-band SAR-mission for global monitoring of tectonic activities

SAR interferometry is a well established technology for mapping tectonic activities from space. In this study a mission concept is investigated that provides a global monitoring capability of geo-tectonic threads. Due to the requirement of long-time interferometric coherence of the mapped surfaces an L-band system is proposed. Four scenarios are defined and the respective product requirements are given for each of them: tectonics/earthquakes, tectonics/volcanoes, landslides, and urban subsidence.

TanDEM-X for mass balance of glaciers and subglacial volcanic activities

The TanDEM-X satellite mission through its bistatic formation provides high horizontal and vertical resolution topographic data. Multi temporal TanDEM-X acquisitions allow short term analysis of variations of the earth surface elevation which in turn can be converted to volume changes. We are investigating the capabilities of the TanDEM-X data to detect and map short term changes at glacier surfaces in two different situations. One study case is Jorge Montt outlet glacier of the South Patagonia Icefield known for its accelerated mass depletion and front retreat. The second example is located on the Vatnajökull Ice Cap in Iceland and is related to the activity of the Bárǒarbunga volcanic system in summer 2014 as a consequence of climate driven depletion as well as due to volcanic activities below the ice surface.

Haiti earthquake (12.01.2010) surface shift estimation using TerraSAR-X data

On the 12th of January 2010, Haiti was struck by a M7.0 earthquake; the hypocenter (18.443°N, 72.571°W, Depth 13 km) was located very close to the capital Port-au-Prince, causing considerable damage to buildings and infrastructure in the city and its conurbation and a high number of human fatalities. For evaluation of the co-seismic surface shift caused by this earthquake, especially at the Enriquillo-Plantain Garden fault zone (EPGF), TerraSAR-X Stripmap data from ascending and descending orbits were acquired and processed using InSAR and incoherent cross-correlation techniques.