When the GPS (Global Positioning System) is subjected to interference, the system performance gradually deteriorates as the interfering levels increase. Two modes of interference are discussed in detail, namely, that from transmissions at frequencies close to the GPS frequencies and that from transmissions with a harmonic in the GPS band. It is argued that the former requires RF filtering in the receiver with a quality better than that generally specified. The latter cannot be dealt with in such a way. Measurements carried out on the harmonic levels transmitted by one UK TV transmitter and several hundred aircraft VHF transmitters are reported. The measurements show there is a measurable level of harmonics in the GPS band. The UK TV transmitter does not, however, represent a threat to aviation unless the aircraft is so close as to represent a physical danger. The probability that one aircraft's VHF transmitter will interfere with the GPS receiver on another aircraft is tolerably small, but there is a significant probability that a GPS receiver can suffer when there is a VHF transmission from the same aircraft. Several recommendations are made, including an international effort to ensure that spurious emissions are both quantified and kept at a level significantly lower than that achieved today.
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