Radio frequency interference affects the integrity, continuity and availability of the GPS system. These are the highly important parameters for critical applications like aviation. Interference also has an adverse impact on accuracy. Continuously operating reference stations (CORS) networks are employed to improve the accuracy of GPS position measurements used by surveyors. Interference can affect the normal operation of these stations. The goal of this work was to find if there is an interference source for GPS L1 in Sydney. It was shown that the third harmonic emission from a TV station lies within the L1 bandwidth. This source of interference is characterized in terms of frequency and power and the effect that it can have on the RF front-end of the receiver.