Interference detection and localization in the GPS L1 frequency band

Abstract: The usage of Global Navigation Satellite Systems (GNSS) in general and the American GPS in particular increases everyday and so does the number of applications where it is used. The GNSS receivers relies on receiving signals from satellites orbiting the earth at an altitude of about 20 000 km and the signals received by the receiver are signicantly weaker then the background noise. Due to the weak signals it is fairly easy to intentionally or accidentally make it very hard or even impossible for a receiver to detect and track the satellites.With this in mind there is a need to develop cost eective methods to detect and localize interference so that appropriate counter measures can be taken. A number of methods have been proposed to detect and localize these sources. The complexity of these methods ranges from requiring future cellphones to contain software to monitor the GNSS environment to dedicated systems with multiple antennas and complicated hardware. In this thesis, two complementary methods will be presented which can detect and also localize interference in the GNSS bands using minimum amount of equipment. The equipment is based around a type of GNSS "receiver" that only samples the GNSS frequency so that it can be processed using a software dened GNSS eceiver. It will be shown that it is capable of detecting and localizing interference sources while also be cost eective and easily deployable. The rst technique is based on measuring the received power level. Since the GNSS signals are below the noise oor, the number of visible satellites will not aect the received power level. Instead the received power level will be aected by changes in the spectrum and changes in the receiver hardware. The GNSS signal is fairly robust against interference so an interferer usually has to have a signicantly higher signal power than the received power from the satellites in order to cause problems for the receiver. By monitoring the received signal power using multiple receivers it is possible to both detect interference and estimate the position of the transmitter. This method requires very little bandwidth but since the signal is measured in the analog domain it is sensitive to process variations between dierent receivers. Also, the nonlinear behaviour of the analog components in the receiver limits the accuracy of the position estimations. To improve the accuracy of the interference localization, a second method has been evaluated. In this method the GNSS samples recorded by dierent receivers at different locations is compared. When a GNSS receiver calculates a position it is actually calculating the time it takes for the signals to travel from the satellite to the receiver. This made it possible to synchronize data from multiple independent receivers both in time and frequency and then estimate the time dierence of arrival of the interfering signal between the dierent receivers. Both localization methods were evaluated during experiments done with assistance from the Swedish armed forces research agency (FOI). It will be shown that the signal power measurement can be used as a detector for interference and that the GPS signal can be used to synchronize data from independent stations so that the dierence in distance to a wideband transmitter can be estimated. To determine the amount of interference in the GPS L1 band two measurement campaigns were made. The rst campaign, measured where interference might be present in an urban area using a car mounted receiver. The other campaign took place at two airports in the summer and fall of 2011 and measured the interference level from xed antennas over an extended period of time.All research was done using the GPS L1 signal but the methods can easily be applied to other GNSS signals as well.