High Resolution Imaging of Satellites and Objects in Space with IoSiS

Kurzfassung

Space debris nowadays is one of the main threats for satellite systems especially in low earth orbit (LEO). More than 700,000 debris objects with potential to destroy or damage a satellite are estimated. The effects of an impact often are not identifiable directly from ground. High-resolution radar images are helpful in analyzing a possible damage. Furthermore investigations on unknown space objects or satellites can be performed. Therefor DLR is currently developing a radar system called IoSiS (Imaging of Satellites in Space), being based on an existing steering antenna structure and our multi-purpose high-performance radar system GigaRad for experimental investigations. GigaRad is a multi-channel system operating at X band (8-12.4 GHz ) and using a bandwidth of up to 4.4 GHz in the IoSiS configuration. The radar utilizes the principle of spreading the energy over a longer pulse in frequency. The radar transmits with pulse length of 50 micro seconds. During this pulse the frequency is modulated over the entire 4.4 GHz. On receive the signals are digital cross correlated with the modulated transmitted signal resulting in a resolution of ca. 3.5 cm in the direction of the propagation direction. The GigaRad system provides fully separated transmit (TX) and receive (RX) channels. This enables a separation of the TX antenna and several RX antennas for research on new observation technics with distributed receivers. The goal is the processing of a 3D image of the observed objects for detailed analyses of the structure and materials. The third dimension gives additional information which is preferable for radar due to the different appearance and interpretation compared to optic. For the experimental phase IoSiS uses a 9 m cassegrain parabol as transmit antenna. The antenna is a reconfigured S band communication dish adapted to the special needs of radar system. As receive antenna two 1.8 m RX offset parabolic antenna are mounted main TX antenna for use of a common steerable positioner. For the observation of satellites a high-power traveling-wave-tube amplifier (TWTA) is mounted close to the TX antenna feed. This amplifier produces a peak output power of 8 kW for the duration of the pulse with a maximum repetition frequency of 1.2 kHz. Compared to an optical image were a two dimensional receiver array (e.g. CMOS Chip) behind a focusing aperture is used for the 2D generation, in radar the second dimension, the azimuth dimension is perpendicular to the propagation direction and is obtained by using Inverse Synthetic Aperture Radar (ISAR) techniques. With this technic high-resolution radar images are possible independent on the distance to the object. The technic is based on coherent observation of the object from different perspectives. This leads to the necessity to a guided tracking of the objects during orbit pass. Thereby a wide azimuth observation angle is performed, directly related to different perspectives. Thus high azimuth resolution comparable to the range resolution can be achieved. The main information that is necessary to form an image is the phase, not the amplitude or intensity, resulting in a strict phase coherent system for the whole image generation time. The intended spatial resolution of 3.5 cm yields to an observation angle of approximately 24° for one image. This high resolution is not standard for radar applications especially for remote sensing. Current resolutions of earth observation systems are in the range of several decimeters, that is one order of magnitude different. This results also in a higher amount of system and orbit correction. Especially an accurate calibration of the Radar system, the antennas and the feeding is essential. Another topic with a huge impact is the precise orbit determination of the observed objects. IoSiS is not a tracking system and has up to now no direct access to one. But for image formation a guided tracking is essential because the antenna beam width is approximately 0.2° for the TX antenna. The available Two Line Element TLE data are not precise enough to follow an object. Therefore a determination of the along track component is performed for objects with a high radar cross section by observing two consecutive overpasses, one with a fixed and the next with a corrected orbit determination. For smaller objects observation is only possible with precise orbit data collected from either an optical or radar system. This paper outlines technical characteristics of the IoSiS radar system. It shows the main error sources and solutions as well as the calibration effort to generate a centimeter-resolution radar image.