Simon Anger, German Aerospace Center (DLR); Matthias Jirousek, German Aerospace Center (DLR); Stephan Dill, German Aerospace Center (DLR); Markus Peichl, German Aerospace Center (DLR)
Keywords: Radar imaging, Inverse synthetic aperture radar, ISAR, IoSiS, Space situational awareness, high-resolution, radar system
Abstract:
There are more than 6000 operational satellites as well as nearly 3000 out-of-service satellites which are still orbiting earth. From the observed tendency it has to be expected, that the total number of satellites, and especially the rate of increase per year, both will keep increasing. According to the European Space Agency (ESA) there are in addition tens of thousands particles classified as space debris. Considering the potential hazard of damage and destruction by space debris, which is massively present in the low earth orbit (LEO), research on high-performance radar concepts was initiated for the purpose of doing a precise evaluation of the satellite structure based on high resolved radar images. Radar techniques allow the acquisition of very high resolved images of satellites measured from ground and constitutes therefore in cooperation with optical sensors a powerful tool for proper space surveillance.
Therefor 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), being presently still in the commissioning phase. The overall goal of that system is research on concepts for advanced acquisition of high-resolution radar images of objects in a low earth orbit (LEO). Compared to existing radar based space surveillance systems, which has monostatic antenna configurations, in future IoSiS will use not one but more spatial distributed antennas in order to handle the upcoming amount of orbiting satellites and more important to realize bi-static imaging geometries. The latter allows the enhancement of the image information content compared to existing monostatic radar based images of satellites.
Using the electromagnetic spectrum in the region of longer wavelengths compared to optical systems has actually the main advantage that influences caused by the atmosphere or the weather conditions are smaller or negligible, respectively. Nevertheless considering the radar based acquisition of images of satellites with a spatial resolution of 10 cm and even better, influences of the atmosphere like the ionospheric delay as well as tropospheric range delay cant be neglected any more. Both lead to a massive phase error and hence to severe distortion in the processed radar image. Hereby the frequency dependent ionospheric range delay is caused by free electrons in the atmosphere whereas the tropospheric range delay depends on the refractive index of this part of the atmosphere.
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. Further on the software tool allows the visualization of the mentioned and expected influences of the atmospheric distortions on a radar image.
The paper outlines first a short description of the novel IoSiS system as well as the crucial steps to obtain a high-resolved radar image of a satellite in LEO. Further on the basic theory of radar-based satellite imaging using an inverse synthetic aperture radar (ISAR) is addressed. The main part constitutes the description of atmospheric effects on radar signals, the illustration based on simulated imaging results as well as the method of atmospheric correction. A measurement result obtained by the IoSiS system of a real space object with spatial resolution in the centimeter region show the successively implemented error correction strategies.
Date of Conference: September 27-20, 2022
Track: SSA/SDA