Unambiguous tracking of high-order BOC signals in urban environments: Channel considerations

The usage of BOC modulations in GNSS provides a better code tracking accuracy than BPSK modulation at the cost of new correlation peaks appearing in the autocorrelation function. For high-order BOC modulations side peaks might be only few meters away from the main correlation peak to be tracked, so there is a risk to lock into one of those peaks, inducing therefore a bias in the pseudorange estimation. The probability of false lock can be in particular important in harsh channel conditions, where the unambiguous tracking can be difficult due to the low C/No and the presence of multipath, which impact can be relevant during fading periods. This paper presents typical channel conditions that should be considered in the design and assessment of unambiguous tracking techniques of high-order BOC signals, focusing in urban environments where the receiver might need to track the signal even at low C/No. A preliminary assessment of some state-of-the-art techniques in urban environments is presented, showing the limitations of some of the techniques, which have been designed considering very mild propagation conditions typically present in open sky environments. The multi-correlator approach is then presented as a possible way to overcome the limitations of techniques based on legacy GNSS receiver architectures. In this case the unambiguous tracking problem is presented as the minimization of a non-linear Least Squares cost function subject to restrictions and some design alternatives are discussed.