Method and appartus for accurate frequency synthesis using global

Abstract

A method and apparatus for synthesizing a stable reference signal of a desired frequency within a spread spectrum receiver is disclosed herein. The spread spectrum receiver is designed for use in conjunction with a global positioning system (GPS) receiver, and operates to receive broadcast differential GPS correction information. The present frequency synthesis technique contemplates generating a sequence of timing signals within the GPS receiver on the basis of GPS satellite signals received thereby, and providing the timing signals to the signal receiver. Within the signal receiver, the signal cycles of a local oscillator occurring between ones of the timing signals are counted. The frequency of the local oscillator is then determined by dividing the counted cycles of the local oscillator by one of the known time intervals. The determined frequency of output signals produced by the local oscillator is then scaled so as necessary to produce the reference signal of desired frequency. This allows precisely controlled reference frequencies to be obtained irrespective of the existence of frequency instability within the local oscillator. In a preferred implementation, the stable reference signals are employed during acquisition of differential GPS correction signals received by the spread spectrum receiver. In particular, the desired reference frequency is incrementally adjusted during the process of searching for and acquiring the exact frequency of the incident differential GPS correction signals. The spread spectrum receiver is disposed to provide differential GPS correction information extracted from the acquired differential GPS correction signals to the GPS receiver.