High-resolution radar imaging of space objects

Abstract

In view of the increasing number of space objects, comprehensive high-quality space surveillance becomes ever more important. Together with the enormous amount of space debris, satellites are at risk of being destroyed or at least severely damaged. In addition to detecting this damage to satellites, more and more unknown satellites are orbiting the earth for which image-based reconnaissance is mandatory. Radar is a powerful tool that, in addition to detection and tracking of objects, also enables spatially high-resolution imaging independent of daylight and most weather conditions. Together with the technique of Inverse Synthetic Aperture Radar (ISAR), very high-resolution and distance-independent two-dimensional images can be obtained. In cooperation with optical sensors radar provides a powerful tool for proper space surveillance. The main advantage of using the electromagnetic spectrum at longer wavelengths compared to optical systems, apart from the range-independent spatial resolution of radar, is that the effects of the atmosphere or weather conditions are reduced or even negligible. However, advanced high-performance radar imaging of space objects is a complex and demanding task, touching many technological and signal processing issues. Therefore, besides theoretical work, the Microwaves and Radar Institute of German Aerospace Center (DLR) has developed and constructed an experimental radar system called IoSiS (Imaging of Satellites in Space) for basic research on new concepts for the acquisition of advanced high-resolution radar image products of objects in a low earth orbit. Based on pulse radar technology, which enables precise calibration and error correction, IoSiS has imaged space objects with a spatial resolution in the centimeter range, being novel in public perception and accessible literature. Compared to existing radar-based space surveillance systems, which has monostatic antenna configurations, in future IoSiS will use not one but more spatially distributed antennas in order to handle the upcoming amount of orbiting satellites and more important to realize bi-static and even multi-static imaging geometries. The latter will improve the information content of the images compared to existing monostatic radar images from satellites by providing true three-dimensional (3D) images of space objects. Besides the experimental measurements of IoSiS a software tool was implemented for performance estimations. Using that tool the hardware setup was optimized in parallel during development and construction. In addition, the software tool enables the visualization of the performance of the planned future imaging system using the aforementioned multi-static imaging geometries. This paper provides a brief introduction to high-resolution imaging of satellites using ground-based inverse synthetic aperture radar (ISAR), and also gives a brief overview of the experimental radar system IoSiS (Imaging of Satellites in Space). Achieving the desired spatial resolution in the centimeter range poses several challenges and sources of error, which can be overcome through careful consideration of the imaging system and its components. The main challenges and sources are briefly described. Latest measurement results of real space targets demonstrate the capability of the system and the potential for future radar-based space surveillance using centimeter-resolution imaging radars. The results of the measurements are used as an example to show how an accurate evaluation of a space object can be carried out. New to the field of radar-based imaging of space objects, comprehensive simulation results show how accurately a space target can be imaged in three dimensions using the new intended imaging concept realized using multi-static imaging geometries.