Code smoothing for BOC ambiguity mitigation

The most recent generation of Global Navigation Satellite Systems (GNSS) are implementing Binary Offset Carrier (BOC) modulation. These signals are expected to provide not only better precision in the estimation of the signal's delay and phase but also more robustness to multipath effects. The advantage of BOC signals is that the main lobe of the correlation is very narrow, but on the other hand they present side lobes. For high-order signals, the amplitude of the side lobes can be similar to the amplitude of the main one or even exceed it under specific scenarios. Some techniques to mitigate the code ambiguity exploit the fact that BOC signals can be understood as the sum of two BPSK signals. Even though these techniques achieve their objective, they lose the robustness against multipath and increase the tracking noise. This paper presents a new combination between the time delay estimated by these kind of techniques and the time delay estimated using the full BOC. The idea of the combination is the same as the carrier smoothing but instead of using the carrier measurement, two code measurements are combined. Since the delay introduced by the ionosphere is the same, or very close, using the Full-BOC and the two-BPSK techniques, as it will be shown in this paper, the smoothing time can be large values, compared with the common carrier smoothing time. Several simulations of the new code smoothing strategy for different scenarios are presented in this paper.