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Abstract
Space debris is an increasing hazard to space assets, and accurate, affordable tracking of all space objects is necessary to maintain efficient and safe space operations. There is a very large quantity of debris objects 10cm or smaller which pose a hazard to spacecraft, but are difficult to maintain an accurate trajectory for. There is growing interest in the use of existing radio signals as the transmitter segment of bi-static or multi-static radars. In this work, we focus on GNSS as an emitter of opportunity to enhance Space Situational Awareness (SSA) by tracking small items of space debris using bistatic radar. However, the scattered GNSS signal levels from small items of space debris are incredibly low and the detection of a desired weak GNSS signal in the presence of other strong signals is often problematic. We investigate cross-correlation behaviour when the Doppler offset between the desired and interfering signal is large and non-linear during the integration period. Due to this non-linearity, the receiver experiences different relative carrier phases throughout the observation period, so the cross-correlation interference is not persistent. We have shown that on the order of minutes, signals with non-linear and large relative Doppler offsets are uncorrelated. We simulated a scenario representative to the case discussed above and it confirms that the weak GNSS signal reflected by a space object can be detected in the presence of direct GNSS signals. The major application of this study is to investigate the feasibility of using GNSS signals to detect small space debris in Low Earth Orbit (LEO). The LEO debris moves at approximately double the speed of the GPS satellite, and with much greater angular rate from the receiver. Hence the illumination and reflected signal are expected to differ in both phase and phase rate (Doppler). Detection of such weak signals in the presence of relatively strong direct-arrival signals and noise requires extremely processing gain through an extended duration. We show that sufficient processing gain, including the necessary cross-correlation protection against the direct-arrival signals is possible. We explore techniques that show how the computational cost can be reduced further by exploiting the non-uniform distribution of the Doppler residual over time. We conclude with an outline of a system using these techniques that could provide centimetre level tracking of small orbital objects at a modest processing cost.