Calibrated real time clock for acquisition of GPS signals during low power operation

Abstract

Power is conserved in a Global Positioning System (GPS) receiver (100) by shutting down selected components during periods when the GPS receiver (100) is not actively calculating the GPS receiver location. A low power time keeping circuit (200) accurately preserves GPS time when the selected components are deactivated. When the selected components are turned on in response to a wake-up command, time provided from the low power time keeping circuit (200), corrected for actual operating temperatures, and data from the GPS clock temperature / frequency table (224), are used to recalibrate time from a GPS oscillator (204). Positions of the GPS satellites are then estimated such that the real GPS time is quickly determined from the received satellite signals. Once real GPS time is determined from the detected satellite signals, the selected components are deactivated. The process described above is repeated such that accurate GPS time is maintained by the low power time keeping circuit (200).

Description

technical field [0001] The present invention relates generally to Global Positioning System (GPS) receivers. Specifically, it relates to maintaining an accurate GPS receiver clock circuit during low power operation. Background technique [0002] The Global Positioning System (GPS) is a collection of 24 Earth-orbiting satellites. Each GPS satellite follows a precise orbit approximately 11,000 miles above the Earth's surface. A GPS receiver automatically tracks at least three satellites to determine its precise location. Each satellite transmits a signal modulated with a unique pseudonoise (PN) code. Each PN code is a sequence that repeats 1023 chips per millisecond consistent with a chip rate of 1.023 megahertz (MHz). Each satellite transmits on the same frequency. For civilian use, this frequency is called L1 and is 1575.42MHz. The GPS receiver receives the signal as a mixture of the transmitted signals of the satellites visible to the receiver. The receiver detects t...

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