Smoother-Based GPS Signal Tracking in a Software Receiver

Global Positioning System (GPS) signal tracking algorithms have been developed using the concepts of Kalman filtering and smoothing. The goal is to improve phase estimation accuracy for non-real-time applications. A bit-grabber/software-receiver has been developed for the GPS L1 coarse/acquisition signal. The bit grabber down-converts, digitizes, and stores the raw RF signal. The software receiver tracks each signal using a 2-step process. The first step uses phase-locked and delay-locked loops. The second step refines the tracking accuracy through the use of linear smoothing techniques. These techniques make optimal use of after-the-fact data. Introduction A GPS user receiver needs to track the spread-spectrum signals that it receives from the GPS constellation. Almost all receivers track the phase of the pseudo-random number (PRN) code that is used to spread the signal's spectrum, and many receivers also track the phase of the underlying carrier signal. The phase of the PRN code is used to infer the pseudo range from the GPS satellite to the user, and the accuracy with which this phase can be tracked constitutes a fundamental limit to the achievable accuracy of the receiver's determination of absolute position and time. The phase of the carrier signal is used to determine the Doppler shift and the accumulated delta range. Accurate carrier phase tracking is necessary for precise differential GPS measurements and for precise velocity determination. ∗ Associate Professor, Sibley School of Mechanical and Aerospace Engineering.