System Concepts for Bi- and Multi-Static SAR Missions

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

A Versatile Processing Chain for Experimental TanDEM-X Product Evaluation

TanDEM-X is a high-resolution interferometric mission with the main goal of providing a global digital elevation model (DEM) of the Earth surface by means of single-pass X-band SAR interferometry. It is, moreover, the first genuinely bistatic spaceborne SAR mission, and, independently of its usual quasi-monostatic configuration, includes many of the peculiarities of bistatic SAR. An experimental, versatile, and flexible interferometric chain has been developed at DLR Microwaves and Radar Institute for the scientific exploitation of TanDEM-X data acquired in non-standard configurations. The paper describes the structure of the processing chain and focusses on some essential aspects of its bistatic part

Analysis of a POD-based approach for phase and time synchronization of bistatic and multistatic SAR systems

The feasibility of multistatic SAR mission concepts largely relies on the ability to achieve the matching of the carrier phase of the different elements of the constellations to a few degrees. This paper analyses the accuracy of phase synchronization schemes based on precise orbit determination (POD) and GNSS raw data, in which the GNSS receiver and the radar payload share the same oscillator. Performance expressions are contrasted with results from the simulations. The results suggest the proposed approach is capable of delivering reliable estimates of carrier frequency and phase errors in the absence of strong baseline velocity deviations

EXPERIMENTAL EVALUATION OF GNSS-BASED FREQUENCY SYNCHRONIZATION FOR SAR APPLICATIONS

Phase synchronization is a crucial requirement for bistatic and multistatic SAR missions. One possible solution for this problem could be to use the same master oscillator for the radar payload and the GNSS receiver, which could allow the recovery of the oscillator phase noise from the carrier phase measurements. This solution would be cost-effective and straightforward, but its effectiveness depends on whether the GNSS receiver would maintain the spectral purity of the master oscillator internally. We executed zero and short baseline experiments to evaluate this technique with a commercial receiver. Despite some unexpected signatures on the carrier phase single-difference measurements, we achieved frequency synchronization of less than 10 ppm and phase synchronization of less than one degree at the L1 carrier frequency after applying a moving average filter of one second at a 10 Hz sampling rate. The results suggest the feasibility of the GNSS-based phase synchronization concept, but further experiments are necessary to evaluate it thoroughly

The theoretical performance of different algorithms for shift estimation between SAR images

This paper compares several algorithms for shift estimation between SAR images, presenting their performance along with their respective merits and demerits. It shows that there is a parallelism between correlation-based estimators (coherent and incoherent) and two different implementations of split-spectrum methods (Delta-k), both in terms of robustness to interferometric phase variations within the estimation window and in terms of performance. The character of Delta-k estimators is regulated by the amount of multi-looking at interferogram level and is related to the statistical efficiency of the standard estimator for the interferometric phase in case of Gaussian speckle

On the Equivalence of LEO-SAR Constellations and Complex High-Orbit SAR Systems for the Monitoring of Large-Scale Processes

High Earth orbit Synthetic Aperture Radar (SAR) systems offer high temporal sampling and moderate spatial resolution on a global scale, potentially outperforming conventional Low Earth Orbit (LEO) systems in revisit times. However, this requires complex system architectures such as burst operation modes with multiple subswaths, large antennas, and digital beamforming. Similar temporal sampling and coverage enhancements can be realized with constellations of classical monostatic SAR instruments in LEO. This letter compares the complexity of such equivalent monostatic LEO-SAR constellations to complex high-altitude SAR systems and provides design numbers for two Medium Earth Orbit (MEO)-SAR mission examples and their LEO counterparts