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Multistatic or distributed satellite systems offer new and unique capabilities necessary for Earth observation with high spatial and temporal resolution. This letter describes a multistatic synthetic aperture radar (SAR) experiment employing the satellites TerraSAR-X and TanDEM-X. In order to demonstrate distributed SAR imaging from space, special data acquisitions with a dedicated geometry were performed. The data evaluation approach is outlined and azimuth profiles over a region with high-contrast backscatter are used to evaluate the azimuth signal reconstruction performance. The results are verified with simulations performed with a flexible SAR simulator reproducing the acquired scene. Finally, the effect of target motion on the reconstruction is analyzed and discussed based on the experimental data. more...

In the context of spaceborne synthetic aperture radar (SAR) imaging, high resolution and wide swath are inherently conflicting requirements. These may, however, be simultaneously satisfied by advanced imaging modes with multichannel architectures in elevation and/or azimuth. This article elaborates on a new mode based on multiple elevation beams and a simple pulse repetition interval (PRI) variation scheme which allows high-resolution wide-swath imaging. It is shown to use the illumination time more efficiently than ScanSAR and yet to be simpler than staggered SAR. Good SAR imaging performance is achieved with a rather compact antenna design. The proposed imaging mode is suitable for spaceborne SAR systems with planar and reflector antennas. In order to improve the imaging performance, a reflector antenna architecture with a multichannel feed in both elevation and azimuth is considered. more...

ESA’s observation program has a consolidated heritage lasting for more than two decades with C-band Synthetic Aperture Radar Systems (SAR). In this framework, the current C-band Sentinel-1 observations constitute an essential component of Copernicus program for the monitoring of the Earth characterized by wide-swath systematic global observations of the Earth to build-up a systematic, long-term archive of globally consistent data. The next generation of C-Band Sentinel-1 mission represents the next logical step in ESA’s C-band line, conceived to serve more applications and services through increased capabilities, first among the others, the improvement of the mapping capability with wide swath, high resolutions and full-polarimetry. The possibility of providing simultaneously 400 km wide swath and high resolution of 5 x 5 m2 single-look resolution represents a factor 5 to 6 improvement with respect to Sentinel-1’s IWS mode. A 280 km swath width, with the same single-look resolution but with full-polarimetry enabled, could be thought as even more attractive and highly beneficial for services and scientific applications. It is well known, however, that wide unambiguous swath coverage and high azimuth resolution pose contradicting requirements on the design of spaceborne SAR systems. Nevertheless, Digital Beam Forming (DBF) techniques, such as Scan-on-Receive (SCORE), and multiple azimuth channels, allow to overcome these fundamental limitations of conventional SAR systems. A multichannel system based on this technique was proposed for a follow-on of the Sentinel-1 system[1, 2]. However, new operational modes allow to reach the same imaging capabilities, both with planar and with reflector-based antennas [3]. The next generation of C-Band Sentinel-1 mission, would ensure a revisit time better than the current one of 6 days for the constellation of Sentinel-1 flying over a 12 days Sun-synchronous orbit at an obit height of 693 km. For the next generation of Sentinel-1 satellites, a mission repeat cycle of 4 days is required, which is achievable when three satellites fly over the same orbit of Sentinel-1 and displaced of 120 degrees. Furthermore, with this configuration and enabling acquisitions with 400m km swath width, daily acquisitions are ensured for higher latitudes (i.e., starting from 45 degrees), as required by several applications, especially those dedicated to the maritime surveillance and sea ice monitoring. Further solutions in terms of orbits are available, however at the cost of breaking the continuity with Sentinel-1. A first possibility is to deploy two satellites, displaced of 180 degrees, over a 8 days – orbit that with 400 km swath width are able to map the global Earth surface, as in the previous case, in 4 days. In this case however, areas at 45° latitude are observed every 2 days benefiting from swath overlap at higher latitudes. An additional option would be to deploy a constellation of satellites over a higher orbit (in the order of 1000 km - 1300 km), also in this case over 12 days or 8 days sunsynchrous repeat orbits. The great advantage for a higher orbit is the possibility of enabling even a wider swath of 600 km, that allows to fully cover the North Pole, keeping the access range limited, otherwise prohibitive for lower orbits. This means in other words that the instrument is less complex in this case than for lower orbits. In the final paper a more detailed assessment of the mission performance would be provided. more...

High Resolution Wide Swath (HRWS) is an ambitious Synthetic Aperture Radar (SAR) mission proposed by Airbus Defence and Space, which will potentially exploit formation flight and the novel MirrorSAR concept to achieve unprecedented imaging characteristics by means of fractionated radar architecture, Ref. [1] and [2]. Originally planned as a follow-on of the extremely successful TerraSAR-X project, the HRWS mission and the satellite itself gradually took on a very different form, as the scientific goals grew more and more challenging. Now to be possibly equipped with electric propulsion and more than 3 times heavier than TerraSAR-X, the HRWS satellite is to be maintained within the control tube of merely 100 m (desired minimum) to maximum 250 m radius around the repeat ground-track reference orbit. On top of that, it is planned to augment the mission by 3 to 4 low-cost companion satellites flying in close proximity (down to ~100 m) to one another. Altogether, these challenges call for a very careful consideration of the absolute and relative orbit control concepts in order to develop a safe and precise maneuver strategy. more...

State-of-the-art and future spaceborne SAR systems increasingly often face the requirement of providing high-resolution images with reduced revisit times, requiring coverage of wide swaths. Since these are contradicting drivers in terms of system design, different alternatives for High-Resolution Wide Swath (HRWS) SAR imaging have been investigated, relying on digital beamforming (DBF) and the use of multiple receiver channels, both in elevation and azimuth dimensions. In this context, Staggered SAR, which operates with a pulse repetition frequency variation, using a single channel in azimuth proves itself as a promising alternative for covering wide continuous swaths with moderate azimuth resolution, whereas the use of multiple azimuth receiver channels bears the potential of improving the azimuth resolution over a given swath, but has yet only been applied to systems with a fixed pulse repetition frequency. This paper introduces and analyzes in detail processing techniques suitable for the combination of these techniques, leading to novel multichannel Staggered SAR imaging modes with the potential for very fine azimuth resolution over ultra-wide swaths. A system concept with 2.0 m azimuth resolution over a 400 km swath in quad-pol is provided as an example. more...

The potential of differential SAR interferometry (D-InSAR) techniques for the study of the 3-D deformation phenomena has been extensively demonstrated. In particular, two different performance approaches (one analytical and one numerical solutions) have been implemented and analyzed. The primary objective is to investigate the capabilities of the Sentinel-1 follow-on SAR mission (named here HRWS) and its preferred acquisition modes for this particular application domain, which might help to the mission optimization. more...

In this paper the influence of the antenna pattern on azimuth performance for High Resolution Wide Swath system is analysed and different approaches of its compensation described. Both transmit and receive antenna patterns affect the signals and hence have influence on performance of the whole system. Consideration of antenna pattern in the processing should help to achieve better performance, since more information about influences on a signal is provided. Different approaches of compensating antenna pattern in HRWS are analysed and compared with the focus on azimuth performance (Azimuth-Ambiguity-to-Signal Ratio – AASR and azimuth resolution). Additionally the influence of antenna pattern compensation on the signal-to-noise ratio scaling is examined. A novel extension of reconstruction algorithm about consideration of antenna pattern directly inside the reconstruction is proposed and compared to other approaches. more...