Precise observation of BOC modulated signals in the presence of noise and specular multipath

The introduction of M-Code to the GPS signal structure can redefine the accuracy of the broadcast ephemeris. Existing ephemeris generation systems use dual frequency observations, obtained through the tracking of existing precise codes on the LI and L2 frequencies. These codes are modulated using Binary Phase Shift Key (BPSK) modulation. The modernization signal M-Code is modulated using Binary Offset Carrier (BOC) modulation. In this study pseudorange observables derived from the tracking of M-Code are proven to have greater accuracy than those from existing precise codes, given equivalent receiver designs and operating conditions. In addition, the error due to specular multipath is derived. These general models of noise and multipath can be applied to any BOC modulated signals, including Galileo and QZSS. When applied to M-Code, the models predict that the maximum multipath error in the pseudorange is reduced in magnitude by 50% compared to the existing precise codes. However the range of multipath delays for which M-Code observables exhibit multipath is approximately twice that associated with existing precise BPSK codes. Much like other precise applications of GPS, some ephemeris generation processes use the ionosphere free combination and carrier phase smoothing of the pseudorange to form smoothed pseudoranges. The smoothed pseudoranges are then input as measurements to an ephemeris filter. The analytic models of multipath error in the pseudorange and carrier phase observables are applied to predict errors in the smoothed pseudorange. Multipath error, amplified by ionosphere free combination, causes a bias in the smoothed pseudorange when parameterized as a function of multipath delay. There are conditions under which the bias is zero mean, and in those conditions multipath is suppressed. The solution of carrier phase multipath for BOC modulated signals contains a feature not seen in the BPSK solution. There are multipath delays for which the carrier phase multipath is identically zero regardless of the multipath phase. The zero carrier phase multipath condition may be the most promising feature associated with observables derived from BOC modulated codes.