Spaceborne synthetic aperture radar systems continuously evolve because of technological progress and increasing user demands. The System Performance Group covers an end-to-end SAR expertise from system design and development to in-orbit operation, verifying the performance of the delivered SAR data products. Our asset is the combined system knowledge of SAR theory, simulation, and analysis, based on over ten years of experience from on-going satellite missions. Major activities and projects are shown in the following.
SAR data quality is influenced by various system acquisition parameters, such as sensor orbit and attitude, underlying topography, antenna patterns, transmit bandwidth, and pulse repetition frequency (PRF). The optimization of such parameters by simulation represents the first step to design a SAR acquisition scenario. Our group is specialized in the design and optimization of the acquisitions parameters as well as the definition of effective strategies to achieve a high quality of the delivered products. Once the operational mission has started, the received SAR raw data is analyzed for detecting saturations or noise effects while the quality of the focused SAR images is further assessed by evaluating specific parameters, such as scene size, resolution, signal-to-noise ratio, ambiguities, or side lobe suppression. The obtained results serve as basis for further refining the acquisition strategy. An example is presented in Figure 1, where the suppression of ambiguities in TerraSAR-X quad polarization products by PRF optimization is depicted. For the TerraSAR-X and TanDEM-X satellite SAR missions, we are responsible for the definition and characterization of all SAR modes and products, up to Level 1b SAR image quality specification.
We also concentrate on the development and optimization of new algorithms to support on-going and future SAR missions, such as the study of innovative data reduction approaches and the investigation of full polarimetric data.
Excellent system performance and advanced data processing are necessary basis for deriving high-quality products by using SAR interferometry. Since 2010 the TerraSAR-X and TanDEM-X satellites have been flying in a close-orbit formation, acting as single-pass interferometer. Such a configuration allows for the derivation of a high-resolution digital elevation model (DEM), by exploiting the phase difference between a co-registered pair of bistatic SAR images. The complex correlation coefficient of this data pair is called interferometric coherence and gives valuable information about the amount of noise in the interferogram and, therefore, about the final DEM quality (the higher the coherence, the better the final DEM performance will be). The expected coherence is predicted from the combination of several decorrelation sources which are due to the limited signal-to-noise ratio, quantization noise, ambiguities, range and azimuth shifts, temporal and volume decorrelation. We designed a global strategy for regularly acquiring data over dedicated test sites and investigating the interferometric coherence behavior with respect to the acquisition geometry and the land-cover type, as presented in Figure 2 (a). The result is a substantial support for the development of an optimized overall acquisition strategy within the TanDEM-X mission, leading to a significant coherence improvement by optimizing the acquisition geometry, e.g. over forested areas and sandy deserts (Figure 2 (b) and (c)).
Furthermore, a large-scale outlook on the final DEM performance is an essential feature for understanding systematic and global effects of the mission performance. For this purpose, we can combine quicklook images depicting several parameters, such as SAR amplitude, coherence, and relative height error, which represent a random noise contribution on the estimated height. An example is presented in Figure 3, where a global mosaic of the relative height error, derived from the coherence, is depicted. Such an error describes a random noise contribution on the estimated height.
Since the start of the generation of the global TanDEM-X DEM, we have been actively involved in the definition of the mosaicking algorithm and in the evaluation of the resulting quality. We are responsible for the constant monitoring and verification of the global TanDEM-X DEM final performance. As an example, a global map of the vertical absolute height error 90% confidence level of each TanDEM-X DEM tile (1°x1° in latitude/longitude) is presented in Figure 4 (Status: March 15th, 2016).
For each TanDEM-X bistatic scene, the operational TanDEM-X processor (ITP) produces several quicklook images as by-products from the interferometric processing chain, depicting several quantities, such as amplitude and interferometric coherence, at a ground pixels spacing of 50x50 m2. This unique global dataset represents the ideal starting point for the generation of global geo-maps and for the investigation of remote sensing related topics, aiming at delivering useful data for the international scientific community. We are currently focusing on the generation of a global X-Band backscatter map, the analysis of Greenland ice sheet snow facies, and the development of efficient algorithms for the generation of forest/non-forest classification maps using interferometric TanDEM-X data. Some examples are presented in Figure 5.
The German TerraSAR-X mission involves two highly versatile SAR satellites, TerraSAR-X and TanDEM-X, acquiring X-band high-resolution SAR images of the Earth since 2007. The Group has been involved over the whole development and operational phase and is responsible for SAR quality parameter analysis and optimization.
The TanDEM-X mission derives a global digital elevation model (DEM) and exploits the bistatic satellite formation for scientific experiments with focus on SAR interferometry (InSAR). The Group has a strong expertise of the unique TanDEM-X capabilities and has been a major driver for successful interferometric SAR and DEM quality improvements.
PAZ is a Spanish SAR satellite mission based on the same technology as TerraSAR-X. The Group is responsible for the operational radar tools such as instrument operations, verification, and calibration which are integrated in the PAZ ground segment. In close cooperation with INTA Spain, we have been conducting the PAZ project from the design phase until the commissioning phase after launch.