Calibration of a Low Cost Clock Using Wide Area Augmentation System (WAAS) (Preprint)
AIR FORCE RESEARCH LAB WRIGHT-PATTERSON AFB OH SENSORS DIRECTORATE
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The WAAS uses geo-stationary satellites to receive data measured from many ground stations and transmits information to GPS users for position correction. Since the WAAS satellites are geo-stationary, the Doppler frequency caused by their motion is very small, typically, in the order of a few tens of Hz. Thus, the signal transmitted by the WAAS can be used to calibrate the sampling frequency in a GPS receiver. The WAAS signal frequency is at 1575.42 MHz. The sampling frequency of a CA code GPS receiver is in the neighborhood of 5 MHz. The ratio of these two frequencies is about 300, thus, 10 Hz inaccuracy in the WAAS frequency will be translated to about 0.03 Hz 10300. The accuracy of the sampling frequency measured through this approach should be less than 1 Hz. The clock in a low cost Motorola GPS receiver Model M12 Oncore is used in this study. The WAAS signal can be rather weak for users in certain areas. Two seconds of data collected by a software receiver using the Motorola front end are used in this study. The result using this approach has been compared with an high accurate RF Radio Frequency frequency counter 1 Hz accuracy. There is no difference in result. The detail of the algorithms and the clock calibration is reported.
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