Testing the susceptibility of GNSS receivers to radio frequency interference

This paper describes an algorithm for interference or jamming detection as part of the GAARDIAN system, based on signal to noise measurements from a single-frequency GPS receiver, and gives some preliminary results from the testing of the system. An RF network to facilitate the injection of jamming signals into the GPS signal path a RF signal path was constructed, using an amplifier and attenuator chain and a vector signal generator. The combined signal was then fed into the GPS receiver, and with the receiver locked onto a stable position CW signals at L1 frequency were introduced into the receiver. To determine if the jamming signals could be detected from this data, a mask for signal to noise from the receiver compared to satellite elevation, and lower threshold for signal to noise, was calculated using data from a period where no jamming was being applied Once a mask and thresholds were determined, possible jamming and interference events were located by looking at each epoch in turn to determine how many satellites had reduced signal to noise or may have lost lock due to interference. Timing of jamming or interference was located by looking for those epochs where four or more satellites suffered reduced signal to noise or loss of lock and this state continued for 30 seconds or more. In this test, CW jamming of -100dBm was required before the number of satellites with carrier lock dropped below four even for a single epoch, and -90dBm was required to cause a sustained loss of lock, but jamming of -105dBm was still detectable with no false positives returned. Further tests will be conducted to test the performance of the algorithm in real time and with different jamming scenarios, as well as with multiple receivers.