Marian Werner

TanDEM-X: A Satellite Formation for High-Resolution SAR Interferometry

TanDEM-X (TerraSAR-X add-on for digital elevation measurements) is an innovative spaceborne radar interferometer that is based on two TerraSAR-X radar satellites flying in close formation. The primary objective of the TanDEM-X mission is the generation of a consistent global digital elevation model (DEM) with an unprecedented accuracy, which is equaling or surpassing the HRTI-3 specification. Beyond that, TanDEM-X provides a highly reconfigurable platform for the demonstration of new radar imaging techniques and applications. This paper gives a detailed overview of the TanDEM-X mission concept which is based on the systematic combination of several innovative technologies. The key elements are the bistatic data acquisition employing an innovative phase synchronization link, a novel satellite formation flying concept allowing for the collection of bistatic data with short along-track baselines, as well as the use of new interferometric modes for system verification and DEM calibration. The interferometric performance is analyzed in detail, taking into account the peculiarities of the bistatic operation. Based on this analysis, an optimized DEM data acquisition plan is derived which employs the combination of multiple data takes with different baselines. Finally, a collection of instructive examples illustrates the capabilities of TanDEM-X for the development and demonstration of new remote sensing applications.

TanDEM-X: a TerraSAR-X add-on satellite for single-pass SAR interferometry

TanDEM-X is a mission proposal for a TerraSAR-X add-on satellite for high-resolution single-pass SAR interferometry. This mission proposal has been selected for a Phase A study within the scope of a Call for Proposals for a next German Earth Observation Mission to be launched in 2008/2009. The mission has the goal of generating a global Digital Elevation Model (DEM) with an accuracy corresponding to the DTED-3 specifications (12 m posting, 2 m relative height accuracy for flat terrain). This goal will be achieved by means of a second, TerraSAR-X like satellite (TanDEM-X) flying in a close orbit configuration with TerraSAR-X. This paper describes the mission concept and requirements, including several innovative aspects like operation modes, orbit selection and maintenance as well as PRF and phase synchronization. Results from a detailed performance estimation show the achievable DEM accuracy. Finally, an overview of the potential of the TanDEM-X mission for several scientific applications is presented.

The tandem-L mission proposal: Monitoring earth's dynamics with high resolution SAR interferometry

Tandem-L is a proposal for an innovative interferometric and polarimetric radar mission that enables the systematic monitoring of dynamic processes on the Earth surface. Important mission objectives are global forest height and biomass inventories, large scale measurements of millimetric displacements due to tectonic shifts, and systematic observations of glacier movements. The innovative mission concept and the high data acquisition capacity of Tandem-L provide a unique data source to observe, analyze and quantify the dynamics of a wide range of mutually interacting processes in the bio-, litho-, hydro- and cryosphere. By this, Tandem-L will be an essential step to advance our understanding of the Earth system and its intricate dynamics. This paper provides an overview of the Tandem-L mission concept and its main application areas. Performance predictions show the great potential of Tandem-L to acquire a wide range of bio- and geophysical parameters with high accuracy on a global scale. Innovative aspects like the employment of advanced digital beamforming techniques to improve performance and coverage are discussed in detail.

The TanDEM-X mission: A satellite formation for high-resolution SAR interferometry

TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurements) is an innovative spaceborne radar interferometer mission that was approved for full implementation by the German Space Agency in spring 2006. This paper gives an overview of the TanDEM-X mission concept, summarizes the basic products, illustrates the achievable performance, and provides some examples for new imaging modes and applications.

SRTM-Mission - cross comparison of X and C band data properties

In February 2000 the Shuttle Radar Topography Mission (SRTM) mapped large areas of the global landmass using two radar systems operating simultaneously in X- and C-band. The radar mapping instrument consisted of modified versions of the SIR-C C-band and X-band radars flown on the shuttle in 1994. Modifications included a 60 m retractable: boom, with C-band and X-band receive-only antennas attached to the booms end. High accuracy metrology systems were added to measure the shuttle position and attitude, and the position of the boom antennas. The dual apertures at each band form radar interferometers suitable for making high accuracy topographic maps of the Earth. The C-band data set is being processed by JPL for the archives of the US National Imaging and Mapping Agency (NIMA) and the National Aeronautics and Space Administration (NASA). The X-band data set is processed and distributed at DLR Germany. This paper compares the specific properties of the X- and C-band data sets with respect to global coverage, height accuracy, sensor specific errors, product definition, product format and availability.

TanDEM-X: mission concept and performance analysis

This paper analyses the potential of TanDEM-X to acquire highly accurate digital elevation models (DEMs) on a global scale. For this, an appropriate mission concept will be introduced. This concept is based on an add-on satellite almost identical to the TerraSAR-X satellite, orbiting in close formation with it. Radar images of these two satellites allow for the generation of a world-wide DEM according to the emerging HRTI-3 standard within less than three years. The achievable height accuracy will be derived from a detailed performance analysis taking into account major system and scene parameters. Critical issues will be identified on both the system and mission level. Furthermore, additional applications as demanded by user requirements will include regional DEMs according to HRTI-4 standard or new techniques such as digital beamforming, bi-static observations or polarimetric interferometric SAR. A mission scenario is presented, which fulfils these requirements within the three year mission time.

Shuttle radar topography mapper (SRTM)

The use of interferometric SAR (IFSAR) to measure elevation is one of the most powerful and promising capabilities of radar. A properly equipped spaceborne IFSAR system can produce a highly accurate global digital elevation map, including cloud-covered areas, in significantly less time and at significantly lower cost than with other systems. For accurate topography, the interferometric measurements must be performed simultaneously in physically sperate receive system, since measurements made at different times with the same system suffer significant decorrelation. The US/German/Italian spaceborne imaging radar C/X-band SAR (SIR-C/X-SAR), successfully flown twice in 1994 aboard the Space Shuttle Endeavor, offers a unique opportunity for global multifrequency elevation mapping by the year 2000. With appropriate augmentation, SIR-C/X-SAR is capable of producing an accurate elevation map covering 80 percent of the Earths land surface in a single 10-day Shuttle flight. The existing US SIR-C SCANSAR mode provides a 225-km swath at C-band, which makes this coverage possible. Addition of a C-band receive antenna, extended from the Shuttle bay on a mast and operating in concert with the existing SIR-C antenna, produces an interferometric pair. Accuracy is enhanced by utilizing the SIR-C dual polarizations simultaneously to form separate SCANSAR beams. Due to the practical limitation of approximately 60 meters for the mast length, the longer SIR-C L-band wavelength does not produce useful elevation measurement accuracy. IFSAR measurements can also be obtained by the German/Italian X-SAR, simultaneously with SIR-C, by utilizing an added outboard antenna at X-band to produce a swath coverage of about 50 km. Accuracy can be enhanced at both frequencies by processing both ascending and descending data takes. It is estimated that the 90 percent linear absolute elevation error achievable is less that 16 meters for elevation postings of 30 meters. This will be the first use of spaceborne IFSAR to acquire accurate topographic data on a global scale.

Tandem-L: And innovative interferometric and polarimetric SAR mission to monitor earth system dynamics with high resolution

Tandem-L is a proposal for an innovative interferometric and polarimetric radar mission that enables the systematic monitoring of dynamic processes on the Earth surface. Important mission objectives are global forest height and biomass inventories, large scale measurements of millimetric displacements due to tectonic shifts, and systematic observations of glacier movements. The innovative mission concept and the high data acquisition capacity of Tandem-L provide a unique data source to observe, analyze and quantify the dynamics of a wide range of mutually interacting processes in the bio-, litho-, hydro- and cryosphere. By this, Tandem-L will be an essential step to advance our understanding of the Earth system and its intricate dynamics.

The TanDEM-X Mission Concept

TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) is an innovative radar interferometry mission to generate a global, consistent and highly accurate digital elevation model (DEM) and to provide a configurable SAR interferometry platform for demonstrating new SAR techniques and applications. This paper summarizes the mission concept starting from the user requirements, the HELIX orbit and TanDEM-X operational modes to the expected height performance. Examples of new SAR techniques are presented.

X-SAR/SRTM instrument phase error calibration

Before starting the operational processing of all the acquired X-SAR interferometric SAR data from the SRTM mission, the calibration of deterministic phase errors as far as possible is necessary. Besides the phase errors caused by the shuttle and mast dynamic variations and the position and attitude inaccuracy, there are phase errors originating from the radar instrument itself that may be compensated. Temperature variations and radar parameter changes like gain settings are the major contributors to these phase errors. During the mission we monitored and recorded telemetry data indicating periodic temperature variation along with the orbit, during a data-take and from mission day one to the end. Temperature variations in microwave devices and cables cause a phase variation, which contributes to the overall phase error and degrades the accuracy of the interferometric height measurement. The phase errors have been measured directly, like for the long mast cables, or derived from the calibration tone phase analysis or simply by correcting the known phase to temperature characterization values of the microwave parts with the measured mission temperature data. The lessons learned from that mission and the results of the analysis and the improvement of the height accuracy is presented in this paper.