Design Approach for a Microprocessor-Based GPS Time Transfer Receiver

P.C. Ould, R.J. Van Wechel

Abstract:	This paper describes tile design concept and characteristics of a self-contained microprocessor-based GPS time transfer receiver. A prototype of this unit is currently in the test phase. It employs two-bit digital baseband correlation rather than analog IF correlation of the signals with the reference code. The correlator, numerically controlled oscillators (NCO) and code generator are implemented in a special-purpose digital signal processor. The time is recovered in the digital code. tracking loop, and final corrections are applied in the control processor. By means of asynchronous sampling techniques for the digital correlator and NCO, the time transfer resolution limit is 2-N part of a code chip period, where N is 32 for a 32-bit accumulator in the NCO. Other features of this design are: drift-free digital mechanization, high reliability of digital circuits, flexible control capability of the microprocessor, and potential for a high degree of digital VLSI chip development leading to compact, low-cost units. A description is given of the process by which the precise measurement is made between user clock and received signal code. The user-clock-derived sample times in the code tracking loop yield high-resolution uncorrected time sample words. These uncorrected time words are corrected for range to the satellite, satellite clock error, ionospheric error, and relativistic errors. They are then differenced with the user clock to yield user-clock error estimates. User-clock outputs consist of 1-pps, time readout, and direct digital outputs of time and time error of the user clock. Test data on random error of the approach are also presented.
Published in:	Proceedings of the 13th Annual Precise Time and Time Interval Systems and Applications Meeting December 1 - 3, 1981 Naval Research Laboratory Washington, DC
Pages:	373 - 388
Cite this article:	Ould, P.C., Van Wechel, R.J., "Design Approach for a Microprocessor-Based GPS Time Transfer Receiver," Proceedings of the 13th Annual Precise Time and Time Interval Systems and Applications Meeting, Washington, DC, December 1981, pp. 373-388.
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