Subcarrier slip detection for high-order BOC signals

The most recent generation of Global Navigation Satellite Systems (GNSS) are implementing Binary Offset Carrier (BOC) modulation. The advantage of BOC signals is that the main lobe of the correlation is very narrow, but on the other hand, they present side peaks, i.e. multiple stable tracking points. For high-order BOC signals, the amplitude of the side peaks 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. Therefore, BOC signals are composed by three components: the code, the subcarrier and the carrier. Code gives unambiguous but noisy measures, whereas that the subcarrier tracking gives ambiguous but precise measurements. The technique presented in this paper solves the ambiguity combining these two delays: the same principle as the carrier smoothing techniques is exploited. Additional, a new subcarrier slip detector is presented in order to detect the subcarrier slips and reset the code smoothing. This paper is focused on high-order BOC signals as the BOC(15,2.5) present in the European Galileo System. The performance of the new algorithm has been evaluated though several simulations using the DLR channel model and other synthetic scenarios.