Proposal of Acceptable Means of BOC(1,1) False Locks Mitigation for DFMC SBAS Equipment

The next generation of GNSS airborne equipment will use dual-frequency multi-constellation (DFMC) signals augmented by modernized SBAS L5 signals. For Galileo and BeiDou constellations, it means that the L1 channels will have to process signals that are mainly composed of a BOC(1,1) signal, instead of BPSK(1) for legacy L1 GPS and SBAS signals. Although such signals present better performance than BPSK(1) in terms of reduction of the multipath error envelope, the counterpart is the risk of false lock on the side peaks of the correlation function, leading to a pseudorange bias of around ±150 m. Obviously, this bias exceeds the allocation for receiver noise and introduces hazardous misleading information if undetected by the equipment. Different approaches have been proposed in the literature to deal with these false locks (such as virtual sub-carrier tracking [1] or unambiguous correlation function [2]), but standards for future DFMC SBAS require that code tracking loops use “Early minus Late” discriminators only, with tight bounds on correlator spacing. Mitigation means such as RAIM/FDE or ARAIM/FDE are not designed to mitigate the risk of false lock, for which the probability is not known. The estimation of this probability necessitates the fine assessment of all conditions possibly leading to a false lock, like ground multipath, masking of the direct LOS signal, receiver clock anomaly, interference, etc. which make such analysis very difficult to carry out. Instead, this paper proposes and evaluates a solution for continuous monitoring, assuming false lock can occur even once in nominal tracking due to undetected disturbance. This solution is based on the “standard” and well-known monitoring implementing Very Early and Very Late correlators: these correlators, whose positions are selected to be close to the side peaks, have unbalanced powers when the code tracking loop is not locked on the main peak of the BOC(1,1) correlation function, and trigger the exclusion of the faulty measurement. The paper derives then the main characteristics of the proposed detector, and analyzes the probabilities of false alerts and missed detection with respect to airborne multipath scenarios.