Microwaves and Radar Institute

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.

This paper introduces the innovative concept of multidimensional waveform encoding for spaceborne synthetic aperture radar (SAR). The combination of this technique with digital beamforming on receive enables a new generation of SAR systems with improved performance and flexible imaging capabilities. Examples are high-resolution wide-swath radar imaging with compact antennas, enhanced sensitivity for applications like alongtrack interferometry and moving object indication, and the implementation of hybrid SAR imaging modes that are well suited to satisfy hitherto incompatible user requirements. Implementationspecific issues are discussed and performance examples demonstrate the potential of the new technique for different remote sensing applications.

The displaced phase center (DPC) technique will enable a wide-swath synthetic aperture radar (SAR) with high azimuth resolution. In a classic DPC system, the pulse repetition frequency (PRF) has to be chosen such that the SAR carrier moves just one half of its antenna length between subsequent radar pulses. Any deviation from this PRF will result in a nonuniform sampling of the synthetic aperture. This letter derives an innovative reconstruction algorithm and shows that an unambiguous reconstruction of a SAR signal is possible for nonuniform sampling of the synthetic aperture. This algorithm will also have great potential for multistatic satellite constellations as well as the dual receive antenna mode in Radarsat 2 and TerraSAR-X.

Abstruct-This paper presents a generalized formulation of the extended chirp scaling (ECS) approach for high precision processing of air-and spaceborne SAR data. Based on the original chirp scaling function, the ECS algorithm incorporates a new azimuth scaling function and a subaperture approach, which allow an effective phase-preserving processing of ScanSAR data without interpolation for azimuth geometric correction. The azimuth scaling can also be used for automatic azimuth coregistration of interferometric image pairs which are acquired with different sampling distances. Additionally, a novel range scaling formulation is proposed for automatic range coregistration of interferometric image pairs or for improved robustness for the processing of highly squinted data. Several simulation and processing results of air-and spaceborne SAR data are presented to demonstrate the validity of the proposed algorithms.

TanDEM-X is a mission proposal for an innovative spaceborne radar interferometer which has been evaluated in a phase A study by a joint DLR and EADS/Astrium team. The mission concept is based on two TerraSAR-X radar satellites flying in close formation to achieve the desired interferometric baselines in a highly reconfigurable configuration. The main goal of the TanDEM-X mission is the generation of a world-wide, consistent, timely, and high-precision digital elevation model according to the HRTI/DTED-3 standard as the basis for a wide range of scientific research, as well as for operational, commercial DEM production. Secondary mission goals of TanDEM-X are moving target indication with a distributed four aperture displaced phase centre system, the measurement of ocean currents and the detection of ice drift by along-track interferometry, high resolution SAR imaging based on a baseline induced shift of the Doppler and range spectra (super-resolution), the derivation of vegetation parameters with polarimetric SAR interferometry, large baseline bistatic SAR imaging for improved scene classification, as well as regional very high resolution DEM generation based on spotlight and large baseline interferometry. The feasibility of these modes will be investigated and predictions about the achievable performance will be made.

The performance and capabilities of bi-and multistatic spaceborne synthetic aperture radar (SAR) are analyzed. Such systems can be optimized for a broad range of applications like frequent monitoring, wide swath imaging, single-pass cross-track interferometry, along-track interferometry, resolution enhancement or radar tomography. Further potentials arises from digital beamforming on receive, which allows to gather additional information about the direction of the scattered radar echoes. This directional information can be used to suppress interferences, to improve geometric and radiometric resolution, or to increase the unambiguous swath width. Furthermore, a coherent combination of multiple receiver signals will allow for a suppression of azimuth ambiguities. For this, a reconstruction algorithm is derived, which enables a recovery of the unambiguous Doppler spectrum also in case of non-optimum receiver aperture displacements leading to a non-uniform sampling of the SAR signal. This algorithm has also a great potential for systems relying on the displaced phase center (DPC) technique, like the high resolution wide swath (HRWS) SAR or the split antenna approach in the TerraSAR-X and Radarsat II satellites.

This paper discusses the capabilities of multistatic SAR satellite configurations for different applications like high resolution DEM generation using multibaseline single-pass crosstrack interferometry, 3-D vegetation mapping and layover solution with spaceborne SAR tomography, high resolution wide swath SAR imaging with sparse satellite arrays, multibaseline velocity estimation of moving objects and scatterer fields, spatial and radiometric resolution enhancement in SAR images, and multistatic imaging for improved detection and classification. Furthermore, some major challenges like phase and time synchronisation, multistatic SAR processing, satellite orbit selection, and relative position sensing will be addressed.

Bi-and multistatic synthetic aperture radar (SAR) operates with distinct transmit and receive antennas which are mounted on separate platforms. Such a spatial separation has several operational advantages which will increase the capability, reliability and flexibility of future SAR missions. We will introduce various spaceborne bi-and multistatic SAR configurations and compare their potentials for different applications like frequent monitoring, wide swath imaging, scene classification, single-pass cross-track interferometry or resolution enhancement.

This paper first summarizes the state of the art in spaceborne SAR systems and applications. The second part of this paper gives an overview of new concepts, techniques and technologies for future SAR systems, allowing an increase of flexibility in the SAR operation mode as well as a reduction in the overall system costs. Several innovative concepts and technologies as biand multi-static configurations, parasitic SAR, sparse aperture systems and digital beamforming will play an important role for future spaceborne SAR constellations.

This paper introduces and analyses the innovative paradigm of multidimensional waveform encoding for spaceborne synthetic aperture radar (SAR). The combination of this technique with digital beamforming on receive enables a new class of highly performant SAR systems employing novel and highly flexible radar imaging modes. Examples are adaptive high-resolution wide-swath SAR imaging with compact antennas, enhanced parameter estimation sensitivity for applications like along-track interferometry and moving object indication, and the implementation of hybrid SAR imaging modes that are well suited to satisfy the hitherto incompatible user requirements for frequent monitoring and detailed mapping. Implementation specific issues will be discussed and examples demonstrate the potential of the new technique for different remote sensing applications.

This paper introduces the innovative concept of multidimensional waveform encoding for spaceborne synthetic aperture radar (SAR). The combination of this technique with digital beamforming on receive enables a new generation of SAR systems with improved performance and flexible imaging capabilities. Examples are high-resolution wide-swath radar imaging with compact antennas, enhanced sensitivity for applications like along-track interferometry and moving object indication, or the implementation of hybrid SAR imaging modes which are well suited to satisfy hitherto incompatible user requirements. Implementation specific issues will be discussed and performance examples demonstrate the potential of the new technique for different remote sensing applications.

Modern synthetic aperture radar (SAR) systems are continually developing in the direction of higher spatial resolution. This requires the usage of high range bandwidths combined with long azimuth integration intervals. High-quality SAR processing methods, which are able to deal with such sensor parameters, are necessary for focusing the raw data of such sensors. Wavenumber-domain (v -k) processing is commonly accepted as the ideal solution to the SAR focusing problem. However, it is only applicable to spaceborne SAR data where a straight sensor trajectory is given. In the case of airborne data, wavenumber-domain processing is limited because of its inability to perform high-precision motion compensation. Here, the extended chirp scaling (ECS) algorithm has proven to be very powerful, although it has certain limitations concerning long aperture syntheses and highly squinted geometries. In the paper, a new stripmap SAR data-processing algorithm, called extended v -k (EOK), is analytically derived. The EOK algorithm aims to combine the high focusing accuracy of the wavenumber-domain algorithm with the high-precision motion compensation of the ECS algorithm. The new EOK algorithm integrates a three-step motion compensation correction in the general formulation of the wavenumber-domain algorithm, leading to a new airborne SAR processing scheme, which is also very robust in the cases of long synthetic apertures and high squint angles. As demonstrated, it offers the possibility of processing wide-band, low-frequency airborne SAR data up to nearwavelength resolution. The performance and accuracy of the new EOK SAR data-processing algorithm are demonstrated using simulated data in different data collection scenarios and geometries as well as using interferometric data acquired by the airborne experimental SAR system of DLR at L

This paper introduces innovative SAR system concepts for the acquisition of high resolution radar images with wide swath coverage from spaceborne platforms. The new concepts rely on the combination of advanced multi-channel SAR front-end architectures with novel operational modes. The architectures differ regarding their implementation complexity and it is shown that even a low number of channels is already well suited to significantly improve the imaging performance and to overcome fundamental limitations inherent to classical SAR systems. The more advanced concepts employ a multidimensional encoding of the transmitted waveforms to further improve the performance and to enable a new class of hybrid SAR imaging modes that are well suited to satisfy hitherto incompatible user requirements for frequent monitoring and detailed mapping. Implementation specific issues will be discussed and examples demonstrate the potential of the new techniques for different remote sensing applications.

Multi-aperture synthetic aperture radar (SAR) systems enable high performance SAR imaging thus meeting the rising demands of future remote sensing applications that conventional SAR cannot fulfill. To provide constant performance even in case of a non-uniformly sampled data array in azimuth caused by a non-optimum pulse repetition frequency (PRF), an appropriate coherent processing of the individual aperture signals is needed. An innovative multiaperture reconstruction algorithm for such a 'digital beamforming on receive' was presented and investigated in [1]-[6]. This paper continues with a system design example to demonstrate the algorithm's capability for high performance imaging. Further, innovative strategies and extended system architectures allowing for optimizing the system performance are introduced and their effectiveness is shown by simulation results. In this context, the theoretical characterization of the impact of the processing on the NESZ is extended to the new concepts.

Multi-aperture synthetic aperture radar (SAR) systems in combination with an appropriate coherent processing of the individual aperture signals enable high resolution wide swath (HRWS) SAR imaging [1]-[8]. An innovative reconstruction algorithm for such a digital beamforming on receive was presented in [9]-[12] that allows for HRWS even in case of a non-uniformly sampled data array in azimuth. This paper will compare this algorithm to different azimuth processing strategies regarding their performance in dependency of the overall sampling. Further, optimization strategies are discussed to maximize the system's performance by pattern tapering on transmit and "Pre-Beamshaping on Receive" networks that allow for pattern tapering on receive and adaptively adjust the virtual sample positions.