Very low power high accuracy time and frequency circuits in GPS based tracking units
First Claim
1. A low power, high accuracy clock circuit for a GPS tracking system comprising:
- a microprocessor controlled by a control program and normally being in a low power "sleep" mode;
a local clock providing local time data to the microprocessor;
a memory for storing data for later access by the microprocessor and for preserving data while the microprocessor is in the "sleep" mode;
temperature sensing means controlled by the microprocessor for sampling system temperature, the microprocessor being adapted to periodically store time and system temperature data in the memory; and
a GPS receiver controlled by the microprocessor to periodically turn on and supply data to the microprocessor, the microprocessor being controlled by the control program to detect a difference between the local clock time and a GPS based time and to construct a correction table of correlated temperatures and time differences for storage in the memory, the microprocessor being adapted to access the correction table to provide a temperature history based correction to the local clock.
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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.
174 Citations
12 Claims
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1. A low power, high accuracy clock circuit for a GPS tracking system comprising:
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a microprocessor controlled by a control program and normally being in a low power "sleep" mode; a local clock providing local time data to the microprocessor; a memory for storing data for later access by the microprocessor and for preserving data while the microprocessor is in the "sleep" mode; temperature sensing means controlled by the microprocessor for sampling system temperature, the microprocessor being adapted to periodically store time and system temperature data in the memory; and a GPS receiver controlled by the microprocessor to periodically turn on and supply data to the microprocessor, the microprocessor being controlled by the control program to detect a difference between the local clock time and a GPS based time and to construct a correction table of correlated temperatures and time differences for storage in the memory, the microprocessor being adapted to access the correction table to provide a temperature history based correction to the local clock. - View Dependent Claims (2, 3, 4)
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5. A low power, high accuracy clock circuit comprising:
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a microprocessor controlled by a control program and normally being in a low power "sleep" mode; a local clock providing local time data to the microprocessor; a memory for storing data for later access by the microprocessor and for preserving data while the microprocessor is in the "sleep" mode; temperature sensing means controlled by the microprocessor for sampling system temperature, the microprocessor being adapted to periodically store time and system temperature data in the memory; and an external source of accurate time periodically supplying accurate time data to the microprocessor, the microprocessor being controlled by the control program to detect a difference between the local clock time and the accurate time data and to construct a correction table of correlated temperatures and time differences for storage in the memory, the microprocessor being adapted to access the correction table to provide a temperature history based correction to the local clock. - View Dependent Claims (6)
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7. A method of generating highly accurate local time data using a clock circuit comprising the steps of:
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periodically recording, at regular intervals, local time and temperature data; at intervals less frequent than said regular intervals, periodically accessing an external accurate time source; building and storing a table of clock errors versus a history of temperature; and accessing the table to correct the local time as a function of present temperature. - View Dependent Claims (8, 9, 10)
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11. A low power, high accuracy clock circuit for a GPS tracking system comprising:
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a microprocessor controlled by a control program and normally being in a low power "sleep" mode; a local clock providing local time data to the microprocessor; a memory for storing data for later access by the microprocessor and for preserving data while the microprocessor is in the "sleep" mode; voltage sensing means controlled by the microprocessor for sampling system voltage, the microprocessor being adapted to periodically store time and system voltage data in the memory; a local frequency synthesizer for generating a signal used to acquire a satellite pilot tone; and a GPS receiver controlled by the microprocessor to periodically turn on and supply data to the microprocessor, the microprocessor being controlled by the control program to construct, and store in memory, a correction table of correlated voltages and frequency differences obtained by measuring an offset between a calculated and an actual synthesized frequency required to lock onto the satellite pilot tone, the microprocessor being adapted to access the table to provide a voltage history based correction to the frequency synthesizer.
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12. A method of generating highly accurate local time data using a clock circuit comprising the steps of:
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generating a signal used to acquire a satellite pilot tone; periodically sampling and recording system voltage; constructing and storing a table of correlated system voltages and frequency differences based on measured offsets between a calculated and an actual synthesized frequency required to lock onto the satellite pilot tone; and accessing the table to provide system voltage history based frequency correction to the generated signal.
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Specification