Design and Performance of Code Tracking for the GPS M Code Signal

The binary offset carrier (BOC) modulation of the new GPS military ranging signal, the M code signal, provides essential benefits in many respects. Because of this signal's differences from conventional ranging signals, M code receiver performance also benefits from some changes to conventional designs. Extensive analysis, simulation, and hardware experimentation have yielded useful insights in receiver design; some of the key insights are provided in this paper. A discriminator design approach is described based on theoretically developed S curves and predictions of code tracking accuracy, and used to configure the experimental hardware. It is seen that the design approach must be somewhat different from that used for receivers of C/A code and Y code signals, in order to take advantage of the unique characteristics of BOC modulations. Theoretical expressions are presented that describe performance of despreading and code tracking the M code signal's BOC(10,5) modulation using a delay-locked loop with noncoherent early-late discriminator. Simple algebraic approximations are also provided. Theoretical predictions of signal-to-noise ratio and code tracking accuracy in white noise are compared with measured results, demonstrating the utility of the design approach and close comparison between theoretical predictions and measurements. Finally, the effect of front-end bandwidth on receiver performance is assessed.