A GPS and GLONASS L1 Vector Tracking Software-Defined Receiver

Abstract

There has been significant work on vector tracking loops for the GPS L1 C/A signal. The GLONASS L1 signal is very similar in structure to GPS L1 C/A, and the same vector algorithms can be applied to this signal. The advantages of vector tracking over traditional scalar tracking are briefly introduced along with the advantages of combining GPS and GLONASS in the receiver’s navigation estimator. The implementation of a vector software receiver is discussed where the code is tracked by a Vector Delay Lock Loop (VDLL), and the carrier is tracked by a Vector Frequency Lock Loop (VFLL) with Phase Lock Loop (PLL) aiding for each satellite channel. The PLL aiding allows the channel replicas to stay carrier phase locked. An Extended Kalman Filter (EKF) is used to operate the vector tracking algorithm. A Common Transmission Time (CTT) navigation implementation of GPS and GLONASS in the vector algorithm is discussed. Fault Detection and Exclusion (FDE) is implemented into the EKF to mitigate vector tracking’s noise sharing issues. A constellation signal blockage experiment is described, and results are shown. From the results, the GLONASS constellation can keep lock on the GPS constellation during a GPS outage when the combined GPS & GLONASS vector algorithm with FDE is applied. The same is true when there is a GLONASS constellation outage and GPS is available. Additional performance results are shown to compare GPS, GLONASS, and GPS & GLONASS vector software receivers in the presence of multipath and signal attenuation. It is concluded that combining GPS and GLONASS can aid each other’s tracking in challenging signal conditions, but precision can be lost in the tracking and navigation estimations of both constellations.