GPS Jamming … the Enemy Inside!

Marino Phocas, Jacqueline Bickerstaff and Tony Haddrell

Abstract:	The low level of interference which is required in order to jam GPS is well known. The Volpe report identified real world problems such as in-specification harmonic emissions from TV transmitters as well as the much reported Russian GPS jammers. Until recently the focus for jamming investigations, and mitigation, has been for outdoor signal levels, and the effects of relatively high power but distant emissions. With the advent of indoor location, emissions much closer to the receiver have become a problem. Indoor GPS requires sensitivities 20dB below that of older, conventional, receivers in order to utilise signals transmitted through, or reflected off walls etc.. At these levels, jamming may come from low power sources inside the room, inside the equipment, or inside the chipset. The paper reviews the r.f. integrity required of an indoor GPS receiver. Because of the co-location of antenna, r.f., baseband, and host electronics, interference levels are easily exceeded in real implementations. The paper will address these ‘inside’ sources, together with some suggested design considerations including: clock and bus related harmonics, frequency plans, serial comms and peripherals, proximity issues – obtaining enough r.f. isolation, power supplies, cellphone transmission bursts, blanking the receiver. The authors have discovered during much of this activity that industry outside of GPS itself is not familiar with dealing with signals in the 10E-18 watts region, and really has no desire to anticipate the finer integration details. Thus it is left to IP, chip and module providers to seek out and fix problems. Test equipment such as Spectrum Analysers and Oscilloscopes have difficulty providing useful help unless the jammers are so large that they obliterate the GPS signal altogether. The paper provides plots and traces obtained from test gear, as an illustration. In the modern world, IT equipment is now all-pervading. It is demonstrated that even compliance with EMC regulations is not sufficient to prevent interference with indoor GPS, so that these present an additional forms of ‘inside’ enemy – inside the building. Even with the ‘cleanest’ design and layout, the hardware designer is unlikely to be able to combat these interferences entirely successfully. The paper will discuss possible approaches for mitigating the effects through signal processing: • interference detection, • filters, • pre- or post-correlation compensation. The paper concludes with some speculation on areas for future developments.
Published in:	Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004) September 21 - 24, 2004 Long Beach Convention Center Long Beach, CA
Pages:	156 - 165
Cite this article:	Phocas, Marino, Bickerstaff, Jacqueline, Haddrell, Tony, "GPS Jamming … the Enemy Inside!," Proceedings of the 17th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2004), Long Beach, CA, September 2004, pp. 156-165.
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