Evaluation of Relative GPS Timing Under Jamming Conditions

In this paper the performance of GPS timing under jamming conditions is evaluated in the context of a GPS jammer localization system that uses the TimeDifference-Of-Arrival (TDOA) technique. To investigate the performance of GPS timing receivers under jamming conditions, jammer signals of different types and powers were combined with real GPS signals and input into two timing receivers and the response of the PPS and frequency outputs measured. It was found that for at least 3 seconds after the jammer was turned on there was no noticeable drift. This is long enough to switch to a backup timing source or to perform a number of TDOA measurements. The implementation of post-processed time synchronization based on common-view satellite time transfer was also explored to examine the performance that can be expected when using the GPS signals embedded in the data captured for TDOA localization. The relative clock drift between two GPS receivers was then measured for 11 hours to see how Temperature Compensated Crystal Oscillators (TCXOs) behave over time. Using a linear fit to the relative clock drift values before simulated GPS outages, it was found that in the worst case a similar amount of holdover could be achieved as the timing receivers before the drift became significant. The performance of the timing receivers during a jammer localization field-trial was then analyzed. It was found that the TDOA localization could be improved significantly by correcting for timing offsets detected using the post-processing technique.