Jamming of Aviation GPS Receivers: Investigation of Field Trials Performed with Civil and Military Aircraft

Abstract

GPS becomes more and more important for approach and departure procedures. Aircraft rely on GPS on and close to the ground, where intentional or unintentional jamming signals may occur. Because of the low power, the received GPS signal is susceptible to RF interference, even for low power jammers. While theoretical calculations and laboratory experiments are essential for the understanding of jamming effects, they cannot reflect reality in all the details. From the known transmit power of a jammer the calculation of the field strength at the aircraft in distance d is straightforward. The directional gain of the GPS antenna with its low noise amplifier and the cable that is connected to the GPS receiver are specified. However, the total receiver gain, which includes the influence of the fuselage of the aircraft, is unknown. This is because the GPS antenna is normally mounted on top of the aircraft, whereas the interference is transmitted from the ground. Without the total receiver gain, the calculation of the interference power at the GPS receiver, and thus the interference-to-signal ratio (J/S) or the carrier-to-noise ratio (C/N0) is not feasible. By laboratory experiments, the critical thresholds of J/S or C/N0, where the tracking of the C/A code is lost, has been determined for different types of interference signals for the aviation receiver CMA-5024. Comparing those results to the ones of field trials reveal critical distances to the interference source and thus reliable values for the total receiver gain. This paper describes tests of civil and military aircraft performed in live jamming scenarios. The analysis of the recorded aviation GPS and flight management system (FMS) data reveal the effects of the jamming signals on different aviation receivers and aircraft types. An equivalent isotropically radiated power of 200 mW has been chosen for the jamming signal. This is comparable to low power jamming devices. Four different types of interference signals were radiated from a biconical broad bandwidth antenna during predefined times, namely a pseudo random noise (PRN) sequence, a continuous wave (CW), a frequency hopping (FH) and a radar like signal with high pulse repetition frequency (PRF). The bandwidth of the interference signals was limited to 2 MHz around the center frequency of the GPS L1 band. Civil and military organizations participated with two fighters, four helicopters, two business aircraft and one flight calibration aircraft. Three helicopters and one of the business aviation aircraft are equipped with specific data recorder units that collect data from the aviation GPS receiver, the FMS and attitude data of the aircraft. In addition, independent multiband GNSS receivers record reference tracks in the GPS and GLONASS bands. Several flights have been conducted while the four interference signals were successively radiated. As results, most of the GPS L1 receivers were susceptible to three of the four transmitted jamming signals, namely the PRN sequence, the CW and the FH. The high PRF signal seemed not to impact the GPS reception. Data of the four aircraft equipped with additional data recorder units is analyzed in the position and range domain. GPS tracks are compared to the FMS position solution and the track retrieved from an independent multiband GNSS receiver. Performance parameters like the horizontal integrity level (HIL), number of used satellites and position differences are discussed. The evaluation of the critical tracking threshold C/N0 with respect to the distance to the jammer yields reliable values for the total receiver gain for one specific receiver type. Analyses of further recorded data show that the detection of jammer events by monitoring C/N0 is generally reliable. The field trials complement findings of theoretical calculations and laboratory experiments and support the understanding of realistic jamming scenarios.