Self adjustment of a frequency offset in a GPS receiver

This invention relates in general to location determining apparatus using satellite signals, and more particularly to satellite positioning receivers which have frequency errors introduced during manufacture of the receiver, and where such errors change over a period of time after manufacturing the receiver. A global positioning system (GPS) receiver () has a reference oscillator () for use in synthesizing signals for use in receiving and acquiring GPS satellite signals by a GPS downconverter (). Upon manufacture of the receiver (), the receiver is tested and a factory frequency offset of the oscillator is determined (). It has been found that the heat used in reflowing the solder used to attach receiver components causes the operating frequency of oscillator devices, such as crystals, to shift considerably. In addition, during a brief period after manufacture of the receiver, the frequency of the oscillator shifts back towards its original operating frequency, and so much so that the narrow search window used to search for a GPS carrier is inadequate, and it is likely that the receiver would not locate the desired carrier signal. The invention takes advantage of the time period after manufacture, when the frequency of the oscillator settles, and the first time unit is turned on, the receiver is used proactively with a wide search window () to determine an operating frequency offset. Then, for subsequent location determination operations, a narrow search window is used ().