Standalone Direct Acquisition of Weak Multi-Band Split-Spectrum Signals

Abstract

The binary offset carrier (BOC) modulation is used in many GNSS signals, not only for its facility for spectrum sharing but also for improvement of ranging performance due to a larger Gabor bandwidth. However, BOC signal acquisition and tracking encounters the ambiguity of multiple peaks and many methods have been proposed to address this problem. A simple method to avoid such an ambiguity is to extract the split spectra, treat them separately as BPSK signal each, and then combine the results. To recover the reduced sideband signal strength, the individual sideband correlations are combined non-coherently or coherently. Coherent combining is more gainful than the non-coherent counterpart particularly when the incoming signal is weak or when a jamming signal is present. Direct long coherent integration (LCI) requires a fine frequency search grid, which would be prohibitive if the frequency search were implemented by brute force. Alternatively, a two-stage approach seems efficient and practical. It starts with a first stage of short correlation for coarse search, followed by a second stage of coherent integration with refined search. In addition to dealing with unknown navigation data bits that change sign during the long integration interval, phase-alignment is required to compensate for code migration and frequency divergence, which are two detrimental effects unique to split-spectrum signals integrated over a long period of time. In this paper, a two-stage method for standalone direct acquisition of weak multi-band split-spectrum signals is described. By standalone, we mean absence of any external aiding, say, from a network or an inertial navigation system (INS). By direct, we mean the acquisition procedure without going through legacy codes. The proposed method makes judicious use of coherent and non-coherent integrations with time and frequency adjustments so as to maximize the combining of data-modulated and dataless (pilot) chips, upper and lower sideband signals, and signals on different frequencies. Issues and enabling techniques are described. Extensive computer simulation results are presented to illustrate the functionality and performance of the method.