Weak GPS signal detection in the presence of strong signals with varying relative Doppler and long integration gain

© 2016 IEEE. The detection of a desired weak GPS signal in the presence of other strong GPS signals is often problematic because the strong signal is not adequately attenuated by the receiver processing. The situation becomes worse in the case where multiple strong signals compete with the desired weak signal. This is a further complexity of the near-far problem, since the presence of strong signals that are not sufficiently orthogonal to the weak signal hinders detection of the weak signal due to cross-correlation of the receiver matched filter and the strong signals. The consistent Doppler offset during the integration period can result in an aliasing problem. It has been shown that even for a long integration period the cross-correlation from strong interference can remain problematic, and depends on the relative Doppler offsets between the strong and weak signals. In the case of critical Doppler offsets, defined as integer multiples of cycle offsets per integration period, longer integration does not by itself reduce the cross-correlation interference. This is because the weak desired and strong interfering signals have almost constant Doppler offsets over the observation period. In this work we investigate cross-correlation behavior when the Doppler offset between the desired and interfering signal is large and non-linear, in the integration period. Due to this non-linearity, the receiver experiences different relative carrier phases for each observation interval, so the previously reported persistence of cross-correlation interference does not occur. The main contribution of this paper is to show that, on the order of minutes, signals with non-linear and large relative Doppler offsets are uncorrelated. Thus, for a receiver that preserves and tracks the phase of one signal during an integration period, the other unwanted signals for which the phases haven't been tracked are attenuated by the normal factor expected of un-correlated signals. The simulation results confirm that the residual part of the phase after a complete cycle changes in each observation interval and allows the correlation gain to be cancelled out for a longer coherent period of integration. Hence, the effect of interfering signals upon tracking the desired signal is no greater than a noise signal of the same level. The major application of this study is to investigate the feasibility of using Global Navigation Satellite System (GNSS) signals to track 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 Offset). In this case very long coherent integration with phase preservation can detect the target signal in spite of the presence of much stronger direct arrival GNSS signals.