Marie Lachaise

TerraSAR-X SAR Processing and Products

The TerraSAR-X mission was launched in June 2007. After successful completion of the commissioning phase, the mission entered its operational phase in January 2008. Since that time, TerraSAR-X provides the scientific remote sensing community and commercial customers with high-quality spaceborne synthetic aperture radar (SAR) data products. The intention of this paper is to present the SAR data processing concept and the comprehensive portfolio of products reflecting the instruments diverse imaging capabilities together with options of processing and achieved product quality as well as the essentials of SAR processing. Furthermore, it shall also provide details on how to fully exploit the precision of the TerraSAR-X products.

Interferometric processing of TanDEM-X data

Since July 2010, TerraSAR-X and TanDEM-X jointly acquire interferometric data. Starting their common commissioning phase with a so called pursuit monostatic configuration with 3 seconds time lag between the two passes, they were later put in a close formation in October 2010, acting since then as the first freely configurable bistatic SAR interferometer in space. All operational acquisitions were processed from instrument raw data to DEMs from day one of the data taking on by one single processing system: the Integrated TanDEM-X Processor (ITP) (see [1],[2]). Data take analysis, common parameter calculation, synchronization, bistatic focusing, filtering, co-registration, phase unwrapping and geocoding are all performed in one sequence inside this processor. This approach allows a high precision processing by passing all applied corrections and determined parameters from one step to the next. Specifically the geometric & phase accuracy and stability of the instruments, the processor and the auxiliary data (i.e. the millimetric precision of the baseline products) provide an unprecedented level of relative and absolute geometric accuracy in the bistatic operation. While many challenges of bistatic processing of the TanDEM-X data are encountered, the benefits of this single pass acquisition mode can be used to derive additional information from the data itself for further processing and calibration. In this paper, we will outline the bistatic interferometric processing steps of the ITP and focus on the aspects of geometric accuracy in DEM generation.

The dual-baseline interferometric processing chain for the TanDEM-X mission

During the second operational year of the TanDEM-X mission, a second coverage of the whole land mass is acquired in order to produce a high accurate and high resolution DEM from a combination of both data sets. This paper presents the dual-baseline interferometric processing chain. Its main steps consist of coregistering the different interferograms (having different baselines), of unwrapping the phases and of comparing them to eliminate the possible unwrapping errors.

Interferometric processing and products of the TanDEM-X mission

Started in June 2010, the TanDEM-X satellite joined the TerraSAR-X satellite in space to perform the conjoint interferometric TanDEM-X mission to acquire a truly global Digital Elevation Model (DEM) of unprecedented accuracy [1]. Since the very first interferometric acquisitions, the Integrated TanDEM-X Processor (ITP) delivered operationally “Raw”-DEMs and complex products of mono- and bistatic data. The RawDEMs are scenes of about 50 km × 30 km, generated for a dedicated DEM Mosaicking and Calibration Processor (MCP) which produces the final DEM. The so-called Coregistered Single-look Slant-range Complex (CoSSC) products are provided for each of these scenes in different flavors for production internal purposes and system performance monitoring as well as for scientific use. The capabilities of the ITP go far beyond the primary mission objective of DEM generation alone: it also provides the operationally available end-user products from different experimental modes as e.g. pursuit monostatic, dual polarization bistatic data, alternating bistatic in single and dual polarization and different bistatic and alternating bistatic spotlight modes. This paper focuses on the accuracy of the generated products, the ITPs contribution to the achieved accuracy of the data and the direct effect of it on the use and interpretation of RawDEM heights for temporal change detection. Also the basic characteristics of the operational experimental products are introduced..

TerraSAR-X payload data processing — First Experiences

In February 2007 the German TerraSAR-X satellite will be launched and the TerraSAR-X mission will enter its approximately 5 months commissioning phase. At that time, the challenging developments on both sides, the advanced high- resolution multi-mode SAR instrument on the one hand and the corresponding sophisticated TerraSAR-X ground segment on the other hand will prove correct interaction and functioning. Screening and processing of the SAR data is the task of the DLR developed TerraSAR Multi Mode SAR Processor TMSP. Preceded by data reception, transcription including decryption and followed by archiving, cataloguing and product delivery, processing of the data by the TMSP is the central part of the SAR data workflow implemented in the Payload Ground Segment PGS. Space and ground segment have been subject to intense complete system testing on ground. Here, the compatibility of SAR instrument commanding, SAR instrument operations and subsequent SAR data processing has been successfully proven for the various acquisition modes of the sensor. Compliance of specified and measured product performance has been investigated as far as possible utilizing simulated point target SAR data. However, the real challenge will be the screening and SAR processing of TerraSAR-X data acquired in orbit and linked down to the receiving station. Therefore, the complete reception and processing chain will be properly tuned and adjusted to the properties of the received TerraSAR-X payload data. The TMSP algorithms have to be configured, e.g. thresholds for calibration pulse analysis, estimation window sizes for SAR data analysis, parameterization of estimation algorithms. Also the configuration of product variants with respect to resolution and radiometric quality will be checked and refined. This paper presents the very first experiences in reception, transcription, screening and processing of TerraSAR-X data with respect to performance, throughput and quality. During the TMSP checkout phase the compatibility of instrument commanding and SAR processing have to be verified and the accordance of SAR performance prediction and the obtained product performance and quality have to be investigated. First characteristics of the SAR data with respect to raw data statistics, calibration pulse analysis and Doppler centroid measurements will be shown. As far as available examples of SAR image products featuring the different image modes, Stripmap, ScanSAR and Spotlight at different incidence angles and polarizations will be displayed and a first estimate of product performance parameters will be given.

Multi baseline SAR acquisition concepts and phase unwrapping algorithms for the TanDEM-X mission

The TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) mission will start in 2009 with the aim of generating a global Digital Elevation Model with high accuracy corresponding to HRTI-3 specifications (12 m posting, 2 m relative point-to-point height accuracy for flat terrain). To achieve this goal, a second satellite similar to TerraSAR-X will fly close to TerraSAR-X in a controlled Helix configuration for 3 years to jointly acquire interferometric SAR data in bistatic mode. According to the current mission concept, there will be at least two complete coverages of the global land surface, each one running one year. The different coverages will have different heights of ambiguity to allow multi-baseline phase unwrapping. For the sake of a homogenous data quality the second acquisition will be shifted by half the swath width with respect to the first coverage. Finally difficult terrain will be covered two more times with different acquisition geometries (i.e. different look direction and/or incidence angles). This paper presents first study results of phase unwrapping algorithms foreseen to process SAR data from the bistatic TanDEM-X configuration.

Multibaseline gradient ambiguity resolution to support Minimum Cost Flow Phase Unwrapping

The TanDEM-X Mission has as primary objective to generate a high resolution global Digital Elevation Model. This paper proposes a new method for multibaseline Phase Unwrapping which is the critical point of this generation. We propose to combine both Minimum Cost Flow (MCF) and Maximum a Posteriori (MAP) estimation. The latter is used to solve phase gradient ambiguities. The problem is posed as an energy minimization one and solved using Belief Propagation (BP) which is an iterative process. Nevertheless, although very good results are obtained on loopy graphs, it is not guaranteed to converge. Thus, phase unwrapping of the most accurate interferogram is finally performed with the MCF algorithm and takes as input the unwrapped gradients.

Interferometric processing algorithms of TanDEM-X data

The purpose of this paper is to provide an algorithmic overview of the interferometric processing embedded in the Integrated TanDEM-X Processor (ITP), settled to the generation of the raw digital elevation model (DEM). The main processing blocks are described, with a focus on the spectral matching of the azimuth spectra, the high-precision coregistration, the dual-baseline phase unwrapping and the geocoding of the products. The robustness of the algorithms is demonstrated through a dual-pass TerraSAR-X scenario.

Fusion of ascending and descending pass raw TanDEM-X DEM

This paper deals with the fusion of TanDEM-X raw DEMs in ascending and descending pass over Mumbai test area and enhance its quality. Before applying fusion method, a robust layover and shadow map has been calculated in ITP using TanDEM-X DEM and the corresponding slant range image. The selection of optimum weights for fusion has been based on height error map calculated from interferometric coherence. Results show a substantial reduction in number of invalid pixels after fusion. In the fused DEM, invalid is only 1.2%, while ascending and descending pass DEMs have 6.7% and 5.7% respectively. The improvement in accuracy of the DEM is very slight in this case which is due to the coarse resolution of the SRTM DEM used as reference.

Beyond the 12m TanDEM-X DEM

The standard TanDEM-X product meats HRTI-3 DEM specification and comes with a sample spacing of 12 m. We apply non-local means (NL) interferogram filtering to the TanDEM-X data. In this paper, we present modifications of the original NL filter which render it more appropriate and efficient for massive processing of TanDEM-X data. Further, we investigate the noise reduction properties as well as the resolution and the coherence estimation accuracy of the new NL filter. Simulations and tests with TanDEM-X data hint that the improved DEMs possess a quality close to the HRTI-4 standard. Also future global InSAR missions like Tandem-L will greatly benefit from this type of filters.