A GPS L5 Software Defined Vector Tracking Receiver

Abstract

As the world continues towards autonomy, the need for continuous, precise, and robust navigation is becoming increasingly important. A new wave of modernized Global Navigation Satellite Systems (GNSS) signals are currently being implemented on the latest satellite blocks. The focus of this work is on the Global Positioning System (GPS) L5 signal, and while it is not yet fully available there are now windows of time where experimental data can be used to test modified algorithms exploiting the nature of the new signal structures. The GPS L5 signal is a Quadrature Phase-Shift keying (QPSK) signal that has both an in-phase (data) and quadrature (pilot) arm in the signal that are transmitted together. The pilot channel is not modulated with a data message which historically limits the length of coherent integration during tracking. This work adapts a vector tracking algorithm, which has been shown in prior works to provide a more robust navigation solution than scalar tracking architectures, to the GPS L5 signal structure. The traditional vector tracking architecture for both GPS L1 and L5 quadrature independently are compared to adapted methods. For the first method, the L5 in-phase and L5 quadrature discriminator measurements are combined before being added to the Kalman filter to update the Position, Velocity, and Timing (PVT) solution and Numerically Controlled Oscillators (NCOs). For the second method, L5 in-phase and L5 quadrature are tracked independently and both provide measurements to the same filter for PVT and NCO updates. This second method removes the limit on integration lengths for the L5 quadrature channel allowing for varying extended integration lengths on this channel. All methods are post processed on live sky data from only Block IIF and III satellites and simulated data from a Spirent GNSS simulator and evaluated. A covariance analysis is also performed comparing the methods. Lastly an ionospheric free vector tracking measurement is developed and applied to a GPS L1 GPS L5 dual frequency vector tracking receiver. This receivers results are compared against a single frequency receiver with an ionospheric model for corrections. The GPS L5 receiver with extended integration periods provided an improvement to the stability of the receiver in degraded environments and provided better filtering of measurements and states. The GPS L5 combined receiver was able to increase the tracking threshold of the receiver by 1-3 dB-Hz over receivers with equivalent integration periods. The ionospheric free combination receiver was able to provide equivalent results to the ionospheric model in simulation, but the live sky results were not as stable as the simulated results. The linear combination of the two vector tracking measurements greatly increases the variance of the measurements.