Preliminary Test Results of Variable IF Tracking Loop (VITAL) for GNSS Signals

Correlation at an intermediate frequency (IF) produces a composite correlation function made of an envelope being that of the underlying code and an internal structure being that of the IF carrier. The IF carrier correlation introduces zero-crossings to the code correlation as if it pinches the code correlation envelope at the zero-crossing points, having the effect of creating a main peak and several secondary peaks. The quick decay towards zero-crossings sharpens the main peak, which is desired for accurate timing of the direct signal while isolating close-in multipath and spoofing signals. The functionality of variable IF correlation (VIC) in acquisition and its multipath performance are analyzed in a recent paper, together with its lock onto the main peak among secondary peaks without ambiguity and false lock. In this paper, the implementation of VIC into a variable IF tracking loop (VITAL) is presented. A publically available open-source software GPS receiver is modified into VITAL by simply replacing the conventional correlators using a half-chip spacing with the VIC using a spacing of one eighth cycle of IF. The modified software receiver is then used to demonstrate VITAL on real-world GPS C/A-code signals. The preliminary testing results show an increased code tracking accuracy as well as an increased performance positioning results.