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Original Publication Date: 1991-Jul-01
Included in the Prior Art Database: 2001-Dec-18
Document File: 2 page(s) / 103K

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H. L. Kennedy: AUTHOR [+2]


This publication describes high accuracy oscilla- tors for use in Global Positioning System receivers.

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MOTOROLA INC. Technical Developments Volume 13 July 1991


by H. L. Kennedy, T. M. King and I. N. Durboraw

This publication describes high accuracy oscilla- tors for use in Global Positioning System receivers.

  A constellation of satellites has been placed in orbit around the earth to implement a global position- ing system (GPS). Interchange of coded radio signals between a ground control station, the satellites, and a remote portable GPS receiver allows the physical location of the receiver to be calculated. GPS receivers require stable local oscillator frequencies in order to accurately determine receiver location. The local oscillator acts as a "clock" which the GPS receiver uses to determine time differences needed for the calculation of its position.

  Oscillator 27 comprises voltage-controlled crystal oscillator (VCO) 40, digital to analog (D/A) convertor 46 and temperature sensor 50. Signals from an exter- nal satellite-borne reference oscillator are delivered by means of a downconverter and connection 31 to digi- tal correlator 70 and then by connection 76 to micro- processor 68 where they are compared to local oscilla- tor signals from VCO 40 coming via interconnection
44. Microprocessor 68 derives a frequency correction signal, which is a measure of the difference between the target frequency at which oscillator 27 is desired to be operating and the actual frequency of oscillator 27. The target frequency is determined from the satellite reference oscillator. The frequency correction signal is sent via interconnection 36 to converter 46, which derives an analog output voltage in response to data from microprocessor 68. This analog output voltage is sent via interconnection 48 to VCO 40. D/A converter 46 may be separate as shown or may be included as a part of microprocessor 68, or contained within vco 40.

  Digital signal processor 72 compares the signal from VCO 40 and signals from one or more external reference oscillators, and automatically generates an

output signal at either predetermined intervals or when the oscillator error equals or exceeds a predetermined value. This is desirably done in conjunction with aging d...