Tandem-L is a highly innovative SAR satellite mission for the global observation of dynamic processes on the Earth's surface with hitherto unknown quality and resolution. Thanks to its novel imaging techniques and its unprecedented acquisition capacity, Tandem-L will deliver urgently needed information for the solution of pressing scientific questions in the areas of the biosphere, geosphere, cryosphere and hydrosphere. The feasibility of Tandem-L has been analyzed and confirmed in the scope of a phase A study, which has been conducted in close cooperation between the German Aerospace Center (DLR) and the German space industry. This paper provides an overview of the Tandem-L mission concept and summarizes the actual development status.

The German Tandem-L mission is devoted to provide an overall observation of the dynamic processes within the Eart system at different spatial and temporal scales. Among the mission objectives, the measurement of the 3-D fores structure by means of multi-pass coherence SAR tomography (TomoSAR) has particular relevance. Up to now, th potential of TomoSAR for the characterization of the forest volume has been proved with airborne data, but Tandem-will allow for the first time to map the vertical forest structure and monitor seasonal and annual variations globall with a high spatial resolution. Recently, a performance model for the characterization of the structure information o multibaseline (MB) data stacks has been presented, and the primarily objective of this paper is to examine in dept such model for global performance analysis and investigate the required acquisition trade-offs in the framework of th Tandem-L mission design and acquisition plan.

It has often been claimed that spaceborne single-pass cross-track SAR interferometry is - up to an overall range shift - not affected by atmospheric effects. A closer analysis reveals, however, that both the troposphere and the ionosphere may cause notable phase, range and height offsets even in a single-pass interferometer. One such offset is due to slightly different incident angles which cause different path lengths through both the troposphere and the ionosphere. Another subtle effect arises from the wavenumber shift in combination with a dispersive medium like the ionosphere, which may induce differential range offsets in coherent radargrammetry. The analyses in this paper are not only of importance for future low-frequency bistatic SAR missions like Tandem-L, but they may also shed some new light on how the ionosphere can influence repeat-pass interferometry and radargrammetry in case of non-vanishing cross-track baselines.

TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurements) is an innovative formation-flying radar mission that opens a new era in spaceborne radar remote sensing. The primary objective is the acquisition of a global digital elevation model (DEM) with unprecedented accuracy (12 m horizontal resolution and 2 m relative height accuracy). This goal is achieved by extending the TerraSAR-X synthetic aperture radar (SAR) mission by a second, TerraSAR-X like satellite (TDX) flying in close formation with TerraSAR-X (TSX). Both satellites form together a large single-pass SAR interferometer with the opportunity for flexible baseline selection. This enables the acquisition of highly accurate cross-track interferograms without the inherent accuracy limitations imposed by repeat-pass interferometry due to temporal decorrelation and atmospheric disturbances. Besides the primary goal of the mission, several secondary mission objectives based on along-track interferometry as well as new bistatic and multistatic SAR techniques have been defined, representing an important and innovative asset of the TanDEM-X mission. TanDEM-X is implemented in the framework of a public-private partnership between the German Aerospace Center (DLR) and EADS Astrium GmbH. The TanDEM-X satellite was successfully launched in June 2010 and the mission started its operational data acquisition in December 2010. This paper provides an overview of the TanDEM-X mission and summarizes its actual status and performance. Furthermore, results from several scientific radar experiments are presented that show the great potential of future formation-flying interferometric SAR missions to serve novel remote sensing applications.

This paper reports on several experiments performed with the TerraSAR-X (TSX) and the TanDEM-X (TDX) satellites. The experiments consist in the generation of improved image products by coherently combining the images acquired individually by both satellites, hence the name distributed imaging. In the literature it has already been suggested the use of two or more satellites in close formation not only to have interferometric capabilities, but also to improve the azimuthal or range resolutions by performing the acquisitions simultaneously (no temporal decorrelation). This idea can be carried out by acquiring different portions of the spectrum and adding them coherently afterwards. Another possibility is to synthesize quad-pol acquisitions using dual-pol ones, an idea already carried out with airborne systems. Such experiments have been performed with TSX and TDX and are described in this paper.