Search

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. Two special aspects are pointed out which are the relative phase referencing and the mutual interference issue.

This work presents an analysis of the attainable performance of a bistatic SAR for oceanographic applications, such as the determination of sea state parameters and the reconstruction of the sea wave spectrum. The analysis has been developed with regard to the spaceborne bistatic SAR of the BISSAT mission. It is based on a receiving-only antenna, flying on board a small satellite, that gathers echoes from the area illuminated by an active SAR operating on an independent large platform. It is shown that in the case of bistatic acquisitions the region of the sea wave spectrum that can be retrieved through linear inversion is wider than the one relevant to a monostatic survey. Specifically, the improvement consists of higher sea states (up to 15-20%) and a wider range of azimuth angles of propagation of the sea waves, increasing for larger antenna baselines. Also, the image degradation due to acceleration smearing is reduced. Improvements can achieve 20% for larger azimuth angles of propagation and baselines.

For its intrinsic properties, bistatic SAR processing cannot be approached in the same manner monostatic SAR process- ing is approached. In general, a bistatic SAR processor is more than a powerful focussing algorithm. Because of the lack of precise time (range) and phase (Doppler) references in non-cooperative (and even cooperative) bistatic systems, a prior software synchronisation step is used to correct the residual errors caused by the use of different clocks. Moreover, depending on the bistatic configuration, different kinds of SAR focussing algorithms can be used. Azimuth-invariant configurations may benefit from the convolution property of the Fourier transform, whereas more general configurations are better suited for efficient time-domain techniques. A discussion on the performance of three Fourier-domain focussing algorithms, range-Doppler, chirp scaling, and range migration, is included in the paper.

This paper introduces a new fractionated radar concept that offers the potential to significantly reduce the costs of future multistatic SAR missions. The key idea is based on a radical simplification of the receiver satellites by limiting their main functionality to a kind of microwave mirror (or space transponder) which redirects the incoming radar echoes to one or more transmitters. There, the routed signals are coherently demodulated using the same local oscillator that was used before to generate the radar pulses. This MirrorSAR concept not only saves the hardware for coherent demodulation, data storage, downlink and digital control within the receiver satellite, but also avoids the need for a bidirectional phase synchronization link between the transmitter and receiver, as it is currently employed in TanDEM-X. As the requirements for the receiver satellites are significantly reduced in this approach, it becomes possible to use low-cost platforms such as those currently being developed in the NewSpace context and to scale their number without any cost explosion. This will ultimately pave the way for novel remote sensing applications like multi-baseline SAR interferometry and single-pass tomography.

Bistatic and multistatic SAR constellations offer increased performance at the expense of increased operational complexity. Due to geometric or cost constraints, multistatic SAR constellations might be forced to operate in a partially cooperative manner, i.e., without a direct synchronisation link. In demanding scenarios, like high-resolution bistatic SAR imaging or cross-platform SAR interferometry or tomography, the data need undergo a calibration step to compensate the lack of synchronisation between transmitter and receiver master clocks. Autonomous synchronisation, based on the inversion of the phase and positioning errors of the bistatic SAR images caused by the lack of synchronisation, is used to calibrate the time and phase references of the system with the sole help of the received radar data, which drastically reduces the requirements on the hardware of the system.

Due to the lack of an appropriate symmetry in the acquisition geometry, generalised bistatic SAR cannot benefit from the two main properties of low-to-moderate resolution monostatic SAR: azimuth-invariance and topography-insensitivity. Precise accommodation of azimuth-variance and topography is a real challenge for efficent algorithms working in the Fourier-domain, but can be quite naturally handled by time-domain approaches. We report about the first efficient and practical implementation of a generalised bistatic SAR imaging algorithm with an accurate accommodation of these two effects. The algorithm has a common structure with the monostatic fast factorised backprojection (FFBP), and is therefore based on subaperture processing. The algorithm is tested with both simulated and actual bistatic SAR data. The actual data correspond to the spaceborne-airborne experiment between TerraSAR-X and F-SAR performed in 2007. The presented approach proves its suitability for precise imaging of any given scene acquired by any given bistatic SAR configuration.

Bistatic configuration is an attractive concept for spaceborne and airborne SAR missions when distributed radars are necessary as for example in the case of interferometric applications. The first reason is the important cost reduction achieved over the multiple radar elements, by having only one transmitter (expensive part) and multiple receivers. The most promising applications are single-pass interferometry with a large baseline and target or surface characterisation from bistatic scattering signature analysis. In a defence context, the improved stealth associated with the receive-only component can provide a wider operational capability. In order to explore the potentials and technical challenges associated with bistatic radar, DLR and ONERA have conducted a joint bistatic airborne radar experiment involving both their SAR systems E-SAR and RAMSES between October 2002 and February 2003. Two main geometrical configurations were flown to explore different scientific and technical objectives. In the first geometrical configuration, the quasi-monostatic mode, the two planes were flying very close to each other to acquire interferometric data in a single-pass cross-platform configuration with large interferometric baselines. The second geometrical configuration was designed to acquire images with a large bistatic angle. The two planes were flying on parallel tracks around 2 km apart, at about the same altitude, with the antennas pointing at the same area on the ground. The authors describe this research programme, including the preparation phase, the analysis of the technological challenges that had to be solved before the acquisition, the strategy adopted for bistatic image processing, the first results and a preliminary analysis of the acquired images.