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Abstract

A direct sequence spread spectrum signal processing system permits the receiver to be turned off during most of the acquisition phase of reception to significantly reduce the on time of the associated receiver front end. Power requirements are further reduced by use of very low power high accuracy time and frequency circuits in GPS based tracking units. The microprocessor based GPS tracking system is shut down almost all the time, using extremely little electrical power, and is powered up for very short periods of time at scheduled intervals by an extremely low power clock circuit with accuracy that varies with temperature. Each time the microprocessor is powered up, system temperature is recorded in memory. At times when the microprocessor is powered up, the GPS system is accessed and the GPS standard time is read. The difference between the low power clock circuit based time and the GPS based time is correlated with the recorded temperature history. A correction table, built from this data, provides a temperature history based correction to the low power clock based circuit without reference to the GPS system, and provides a highly accurate time standard. An additional table constructed in the microprocessor temperature compensates the local frequency standard used for frequency synthesis in satellite communication channels. This table is constructed by measuring offset between the calculated and actual synthesized frequency required to lock onto the satellite transmitted reference (pilot tone), and correlating this measured offset with the system ambient temperature.

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First Claim