Abstract
On June 21, 2010, the TanDEM-X mission was launched and opened a new era in spaceborne radar remote sensing. The first formation flying radar system was built by extending the TerraSAR-X synthetic aperture radar (SAR) mission by a second, TerraSAR-X-like satellite TanDEM-X. The resulting large single-pass SAR interferometer features flexible baseline selection, enabling the acquisition of highly accurate cross-track interferograms not impacted by temporal decorrelation and atmospheric disturbances. The primary objective of the mission was the generation of a global Digital Elevation Model (DEM) with unprecedented accuracy (12-m horizontal resolution and 2-m relative height accuracy). The main mission phase for DEM data acquisition has been finished in 2014; the processing of the global TanDEM-X DEM was concluded in September 2016. The final DEM product is well within specifications and features an extremely low percentage of void areas. It is of fundamental importance for a wide range of commercial and scientific applications. But the scientific exploitation of TanDEM-X is not limited to the DEM. TanDEM-X has unique capabilities, including along-track interferometry, and new bistatic and multistatic SAR techniques, that support numerous secondary mission objectives. Indeed, some of these experiments were directly performed during the DEM acquisition phase, when suitable satellite formation geometries were available. Moreover, regular acquisitions over selected super test sites enabled multitemporal analyses. A dedicated science phase after the DEM acquisitions included up to 4 km cross-track baselines, operation in the so-called Dual-Receive Antenna mode, as well as a period in pursuit monostatic flight formation. Comparisons of the TanDEM-X DEM with that of SRTM, or among multitemporal TanDEM-X data, revealed dramatic, ongoing, changes in Earth's topography, especially over ice and forests. In the last 3.5 years the mission has further acquired data for a global change layer showing the height changes relative to the first global DEM dataset. The so-called “Change DEM” is planned for release in 2021. Despite being well beyond their design lifetime, both satellites are still fully functional and have enough consumables for several additional years. Therefore, bistatic operations continue with a focus on changes in the cryosphere, biosphere, and densely populated urban areas.
Highlights
S PACEBORNE synthetic aperture radar (SAR) interferometry is a key technique for topographic mapping
The acquisition strategy to generate the global Digital Elevation Model (DEM) has been driven by the product specification for the relative height accuracy, which is in turn directly related to the phase noise that can be quantified through the interferometric coherence, and the perpendicular baseline or, equivalently, the height of ambiguity [13]
A global map of the tree line boundary altitude has been derived by combining information from the TanDEM-X global DEM and the GlobCover classification map provided by the European Space Agency [84]
Summary
S PACEBORNE SAR interferometry is a key technique for topographic mapping. Early demonstrations have been performed with single satellites acquiring image pairs at time intervals given by the orbit repeat cycle. Beyond the primary mission goal of generating the global DEM, TanDEM-X supports applications based on along-track interferometry and the demonstration and application of new SAR techniques, with a focus on multistatic SAR, polarimetric SAR interferometry, digital beamforming, and super resolution Both radar instruments feature a very flexible commanding scheme, which in combination with the adjustable baseline geometries enables hitherto unprecedented capabilities for experimental modes and new techniques. The second satellite (TDX, launched on June 21, 2010) is mostly a rebuild of TSX with only minor modifications like an additional cold gas propulsion system for formation fine tuning, double-sized on-board solid-state memory for increased data recording capacity, and an additional S-band receiver to enable the reception of telemetry and GPS position information broadcast by TSX The instruments on both satellites are advanced high-resolution X-band (center frequency 9.65 GHz, chirp bandwidth up to 300 MHz) synthetic aperture radars, which can be operated in Spotlight, Stripmap, and ScanSAR mode with full polarization capability [7]. For TanDEM-X, it was necessary to design the observation campaigns with values of hamb that would balance height acuity with phase disambiguation
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