Hardware-in-the-Loop Simulation of GPS L1 C/A, Galileo E1B and BeiDou B1 Weak Signal Tracking in Highly Elliptical Orbits

Earlier research has clearly demonstrated the value of both multi-constellation GNSS and weak signal tracking for above-the-constellation users. Further research in this area has been fundamentally limited by the availability of multi-constellation, multi-frequency receivers capable of operating in the HEO environment, and second by the available information to reliably simulate all the GNSS signals and systems. In order to study the advantages and disadvantages of the various GNSS systems, a hardware-in-the-loop simulation was set up to simulate the GPS L1 C/A, Galileo E1b and BeiDou B1 signals for a PROBA-3 type mission in highly elliptical orbit. The simulation realistically handles both the link budgets for the various signals and the most relevant GNSS error sources. Further, various weak signal acquisition and tracking algorithms were implemented into the GSNRx(TM) software receiver in order to better understand the performance of each of the three different signals under the high dynamic, weak signal conditions encountered in highly elliptical orbit. The processed measurements demonstrate the value of the wider bandwidth signals such as E1b, which have lower measurement noise. They also demonstrates the value of the higher geosynchronous navigation satellites from the BeiDou system, which provide longer arcs of continuous tracking data when the receiver spacecraft is opposite China and able to track them over the Earth's limb. Finally, a significant advantage can be seen when using fine search grid pull-in trackers in conjunction with a tight bandwidth Kalman filter weak signal tracking loop, both in terms of an increased number of available measurements and in terms of measurement noise, as compared to a standard tracking strategy. These improvements come at the cost of biases under high dynamics and rapidly changing ionospheric conditions. Further receiver development work is required to bring the performance of the software receiver closer to the state of the art GPS performance of existing space mission.