Influence of GPS Satellites Cross-Correlation on the TDOA Measurements within the GNSS Environmental Monitoring System (GEMS)

As the importance of GNSS to civilian infrastructure increases, the ability to detect and localise sources of interference that disrupts its operation becomes increasingly important. In order to calculate the time-delay measurements in a Time Difference Of Arrival (TDOA) based localisation system for localising interference to GPS within the GNSS Environmental Monitoring System (GEMS), data collected at different sensor stations need to be cross-correlated together. However, along with the interference signals, the collected sets of data contain the GPS signals themselves, which create undesired correlation peaks. These correlation peaks could be mistaken for weak interferers during the TDOA processing. This work investigates and reports on the behaviour of these correlation peaks and the impact they have on using TDOA techniques for localising interference. If the sensor stations are close together, the time-delays of the satellites will be close together. This causes a large cross-correlation peak near the zero delay point as the satellite signals arrive at the sensor stations at a similar time. As distance between the stations increases the cross-correlation peaks spread out, but there will be situations where the time-delays for individual satellites will overlap, again causing a larger central cross-correlation peak. This results in an increase of the interferer effective Signal-to-Noise Ratio (SNR) required for its cross-correlation peak to be above the GPS crosscorrelation peaks which affects the detection and estimation threshold. This work is part of an ARC Linkage grant with the University of New South Wales, University of Adelaide and GPSat Systems, which is looking into building a network of sensor stations that can quickly detect and geo-locate interference to GPS within a given area.