Low-cost, High Accuracy GPS Timing

Thomas A. Clark, Richard M. Hambly and Reza Abtahi

Abstract:	This paper describes our long-term efforts to develop low- cost (under $1,000) timing systems for use at isolated locations that are accurate at levels of <50 nanoseconds. The initial application was at radio telescopes around the world used for geodetic and astronomical Very Long Baseline Interferometry (VLBI) measurements. In VLBI, wide bandwidth microwave noise signals from extragalac- tic radio sources are recorded on magnetic tape. The tapes are brought together days later and correlated coherently. To achieve coherence at microwave frequencies for hundreds of seconds, each VLBI station uses a Hydrogen Maser frequency standard. In order for the correlation process to proceed, the relative timing of the station clocks all around the world must be known at levels <100 nsec and rates <100 nsec/day (i.e. <1:1012). To meet the needs of VLBI, in 1994 the “Totally Accurate Clock” (TAC) was developed as a low-cost GPS-based solution based on off-the-shelf OEM GPS receivers. After evaluating the receivers from several manufacturers, we selected the Motorola PVT-6 and its successor, the ONCORE VP. With the VP ONCORE, we have been able to demonstrate ~20 nsec RMS timing on time scales from minutes to days. The situation has recently changed. Motorola announced the discontinuance of the VP ONCORE receiver in 1999 because of component shortages. This left us with the lower cost (and reduced capability) UT+ receiver. This prompted us to begin reviewing an alternative prototype based on the SiRF chipset. And, to everyone’s joy, in May 2000, the DoD removed the S/A (Selective Avail- ability) clock “dither” making a significant improvement in timing performance of the GPS system. Since S/A was turned off, data taken with ONCOREs show an improvement of short-term (time scales of minutes) to ~4 nsec RMS, but shows “glitches” that were formerly masked by S/A. This is compared to the SiRF- based receiver, which shows short-term noise at the ~660 psec level with very few “glitches”. The perform- ance of the best of the single frequency receivers (not surprisingly) shows uncalibrated ionospheric biases with diurnal signatures at the 10-20 nsec level. After filtering the ionosphere, the residuals agree to a few nsec with the UTC(USNO) minus UTC(GPS) offset values published daily on the USNO Web site.
Published in:	Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000) September 19 - 22, 2000 Salt Palace Convention Center Salt Lake City, UT
Pages:	905 - 913
Cite this article:	Clark, Thomas A., Hambly, Richard M., Abtahi, Reza, "Low-cost, High Accuracy GPS Timing," Proceedings of the 13th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2000), Salt Lake City, UT, September 2000, pp. 905-913.
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