Correlation at an intermediate frequency (IF) produces a composite correlation function of an envelope being that of the underlying code and an internal structure being that of the IF carrier. By varying the IF, one can introduce zero-crossings to the envelope so as to shape and sharpen the main peak. Such a variable peak is desired for accurate timing of the direct signal while isolating close-in multipath and spoofing signals. The functionality of variable IF correlation in acquisition and its multipath performance are analyzed in a recent paper published in this journal. In this paper, the details of an implementation of variable IF tracking loop are presented with a theoretical background analysis. A publically available open-source software GPS receiver (SoftGNSS V3.0) is modified into variable IF tracking loop and tested on real-world GPS C/A-code signals. The preliminary test results demonstrate increased code tracking accuracy as well as increased positioning performance.     «

Correlation at an intermediate frequency (IF) produces a composite correlation function of an envelope being that of the underlying code and an internal structure being that of the IF carrier. By varying the IF, one can introduce zero-crossings to the envelope so as to shape and sharpen the main peak. Such a variable peak is desired for accurate timing of the direct signal while isolating close-in multipath and spoofing signals. The functionality of variable IF correlation in acquisition and its...    »