5.1 Overview

In practice, a GPS receiver must first replicate the PRN code that is transmitted by the SV being acquired by the receiver; then it must shift the phase of the replica code until it correlates with the SV PRN code. When cross-correlating the transmitted PRN code with a replica code, the same correlation properties occur that occurs for the mathematical autocorrelation process for a given PRN code. As will be seen in this chapter, the mechanics of the receiver correlation process are very different from the autocorrelation process because only selected points of the correlation envelope are found and examined by the receiver. When the phase of the GPS receiver replica code matches the phase of the incoming SV code, there is maximum correlation. When the phase of the replica code is offset by more than 1 chip on either side of the incoming SV code, there is minimum correlation. This is indeed the manner in which a GPS receiver detects the SV signal when acquiring or tracking the SV signal in the code phase dimension. It is important to understand that the GPS receiver must also detect the SV in the carrier phase dimension by replicating the carrier frequency plus Doppler (and usually eventually obtains carrier phase lock with the SV signal by this means). Thus, the GPS signal acquisition and tracking process is a two-dimensional (code and carrier) signal replication...