Precise positioning with AM radio stations

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Palmer, Ronald J.
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Precise low-cost navigation is required for many industries such as farming and mining. Such a system is proposed that utilizes the existing carrier from AM radio stations. A single stationary reference station is used to monitor the phase of the carrier from many AM stations and to broadcast this information to passive mobiles. A digital technique is developed that uses direct conversion wide-band receivers and a Discrete Fourier Transform to measure the phase of the signal very accurately. Sources of error in measuring phase with this technique are investigated. They include quantization error, DC offset error, Direct Digital Synthesis round-off error as well as something called 'end-effect'. The technique was capable of measuring the phase of the carrier to 0.01 to 0.1 degrees even under full modulation. Field tests were done to determine the amount of range error that would result from propagation differences in the paths from the AM transmitter to the reference and mobile stations. The results from these preliminary tests would suggest that the technique is capable of sub-meter accuracy.