Interferometric Synthetic Aperture Radar (SAR) Missions Employing Formation Flying

Microwaves & Radar Inst., German Aerosp. Center, Wessling, Germany
Proceedings of the IEEE (Impact Factor: 5.47). 06/2010; 98(5):816 - 843. DOI: 10.1109/JPROC.2009.2038948
Source: IEEE Xplore

ABSTRACT This paper presents an overview of single-pass interferometric Synthetic Aperture Radar (SAR) missions employing two or more satellites flying in a close formation. The simultaneous reception of the scattered radar echoes from different viewing directions by multiple spatially distributed antennas enables the acquisition of unique Earth observation products for environmental and climate monitoring. After a short introduction to the basic principles and applications of SAR interferometry, designs for the twin satellite missions TanDEM-X and Tandem-L are presented. The primary objective of TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) is the generation of a global Digital Elevation Model (DEM) with unprecedented accuracy as the basis for a wide range of scientific research as well as for commercial DEM production. This goal is achieved by enhancing the TerraSAR-X mission with a second TerraSAR-X like satellite that will be launched in spring 2010. Both satellites act then as a large single-pass SAR interferometer with the opportunity for flexible baseline selection. Building upon the experience gathered with the TanDEM-X mission design, the fully polarimetric L-band twin satellite formation Tandem-L is proposed. Important objectives of this highly capable interferometric SAR mission are the global acquisition of three-dimensional forest structure and biomass inventories, large-scale measurements of millimetric displacements due to tectonic shifts, and systematic observations of glacier movements. The sophisticated mission concept and the high data-acquisition capacity of Tandem-L will moreover provide a unique data source to systematically observe, analyze, and quantify the dynamics of a wide range of additional 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. Enabling technologies and techniques are described in detail. An ou-
tlook on future interferometric and tomographic concepts and developments, including multistatic SAR systems with multiple receivers, is provided.

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Available from: Gerhard Krieger, Jul 11, 2015
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    • "The SAR data for this study were acquired during the TanDEM-X mission over Central Kalimantan on December 21, 2010. The mode used is called bistatic mode and is applied for the operational TanDEM-X mission acquisitions (Krieger et al., 2010). Both sensors are able to transmit the electromagnetic wave with a phased-array X-band antenna having a carrier frequency of 9.65 GHz (Pitz & Miller, 2010), corresponding to a wavelength of ~3.1 cm. "
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    ABSTRACT: Deforestation and forest degradation are one of the important sources for human induced carbon dioxide emissions and their rates are highest in tropical forests. Forman-kind, it is of great importance to track land-use conversions like deforestation, e.g. for sustainable forest management and land use planning, for carbon balancing and to support the implementation of international initiatives like REDD+(Reducing Emissions from Deforestation and Degradation). SAR (synthetic aperture radar) sensors are suitable to reliably and frequently monitor tropical forests due to their weather independence. The TanDEM-X mission (which is mainly aimed to create a unique global high resolution digital elevation model) currently operates two X-band SAR satellites, acquiring interferometric SAR data for the Earth's entire land surface multiple times. The operational mission provides interferometric data as well as mono- and bistatic scattering coefficients. These datasets are homogenous, globally consistent and are acquired in high spatial resolution. Hence, they may offer a unique basic dataset which could be useful in land cover monitoring. Based on first datasets available from the TanDEM-X mission, the main goal of this research is to investigate the information content of TanDEM-X data for mapping forests and other land cover classes in a tropical peatland area. More specifically, the study explores the utility of bistatic features for distinguishing between open and closed forest canopies, which is of relevance in the context of deforestation and forest degradation monitoring. To assess the predominant information content of TanDEM-X data, the importance of information derived from the bistatic system is compared against the monostatic case, usually available from SAR systems. The usefulness of the TanDEM-X mission data, i.e. scattering coefficients, derived textural information and interferometric coherence is investigated via a feature selection process. The resulting optimal feature sets representing a monostatic and a bistatic SAR dataset were used in a subsequent classification to assess the added value of the bistatic TanDEM-X features in the separability of land cover classes. The results obtained indicated that especially the interferometric coherence significantly improved the separability of thematic classes compared to a dataset of monostatic acquisition. The bistatic coherencewasmainly governed by volume decorrelation of forest canopy constituents and carries information about the canopy structurewhich is related to canopy cover. In contrast, the bistatic scattering coefficient had no significant contribution to class separability. The classification with coherence and textural information outperformed the classification with the monostatic scattering coefficient and texture by more than 10% and achieved an overall accuracy of 85%. These results indicate that TanDEM-X can serve as a valuable and consistent source for mapping and monitoring tropical forests.
    Remote Sensing of Environment 08/2014; 151:16-26. DOI:10.1016/j.rse.2013.08.024 · 6.39 Impact Factor
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    • "Fig. 2. A parabolic reflector focuses an arriving plane wave on one or a small subset of feed elements. As the swath echoes arrive as plane waves from increasing look angles, one needs hence to only read out one feed element after the other to steer a high-gain beam in concert with the arriving echoes [9], [21], [22], [29]. A drawback of the multibeam mode is the presence of blind ranges across the swath, as the radar cannot receive while it is transmitting. "
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    ABSTRACT: Synthetic aperture radar (SAR) is a remote sensing technique, capable of providing high-resolution images independent of weather conditions and sunlight illumination. This makes SAR very attractive for the systematic observation of dynamic processes on the Earth?s surface. However, conventional SAR systems are limited, in that a wide swath can only be achieved at the expense of a degraded azimuth resolution. This limitation can be overcome by using systems with multiple receive apertures, displaced in along-track, but a very long antenna is required to map a wide swath. If a relatively short antenna with a single aperture in along-track is available, it is still possible to map a wide area: Multiple swaths can be, in fact, simultaneously imaged using digital beamforming in elevation, but ?blind ranges? are present between adjacent swaths. This paper considers an innovative concept, Staggered SAR, where the pulse repetition interval (PRI) is continuously varied. This concept allows the imaging of a wide continuous swath without the need for a long antenna with multiple apertures. The choice of the sequence of PRIs and the pre-processing of the raw data are discussed in detail, showing how Staggered SAR is even less affected by ambiguities of point-like or extended targets with respect to a system with constant PRI, which simultaneously maps multiple swaths. Some system design examples are finally presented and compared.
    IEEE Transactions on Geoscience and Remote Sensing 07/2014; 52(7). DOI:10.1109/TGRS.2013.2282192 · 2.93 Impact Factor
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    • "ISTATIC and multistatic synthetic aperture radar (SAR) systems operate with distinct transmit and receive antennas that are mounted on separate platforms. The spatial separation enables new radar imaging modes and is well suited to increase the capability, flexibility, and performance of SAR systems and missions, thereby allowing the acquisition of novel information products [1]–[4]. A prominent example is the TanDEM-X mission, where a global digital elevation model (DEM) is acquired with two X-band SAR satellites flying in close formation [5]. "
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    ABSTRACT: This paper addresses relativistic effects in bistatic and multistatic SAR systems and missions. It is shown that the use of different reference frames for bistatic SAR processing and bistatic radar synchronization is prone to notable phase and time errors. These errors are a direct con-sequence of the relativity of simultaneity and can be explained in good approximation within the framework of Einstein’s special theory of relativity. Using the invariance of the spacetime interval, an analytic expression is derived which shows that the time and phase errors increase with increasing along-track distance between the satellites. The predicted errors are in excel-lent agreement with measurements from TanDEM-X and provide a satisfactory explanation for previously observed DEM height offsets that exceeded +- 10 m. Consideration of the unex-pected relativistic effects is essential for accurate DEM generation in TanDEM-X and has in the meantime been implemented in the operational processing chain.
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