Design and Test of an Algorithm for Satellite-to-Satellite Time Transfer

Dolan E. Highsmith, Patrick W. Binning and Penina Axelrad

Abstract:	An algorithm which solves for accurate relative GPS receiver clock bias and frequency in near real time is presented and tested. Single frequency P-code (P1) double difference observations are used to solve for pre- cise relative position and velocity states. The precise positions are then used as known parameters in a sec- ond pass through a three-state clock filter. This filter uses P1 single differences and differences in time of single frequency carrier phase (L1) single differences to solve for relative receiver clock bias, frequency, and frequency drift. The algorithm is tested using hardware-in-the-loop simulation, with the truth clock biases measured with a universal time interval counter (UTIC). The UTIC measures the difference between the 1 pulse per sec- ond output of the receiver and a reference pulse gen- erated at the simulated GPS system time. Simulated GPS signal errors include ionosphere, GPS constella- tion, and multipath. With an along-track separation of 50 km at an altitude of 1300 kin, relative position 3- D RSS error is less than 15 cm. At 100 km separation, the 3-D RSS error is less than 24 cm. For both 50 and 100 km separations at 10-second measurement update intervals, relative clock bias RMS error is 0.6 ns or less. Relative frequency RMS errors are less than 0.03 ns/s (3 x 10 -11) for 10-second averages. These results represent the estimation accuracy of the relative clock filter, one portion of the time trans- fer error budget. Actual on-orbit time transfer per- formaace would also include effects due to calibration uncertainties, thermal variations, and other environ- mental disturbances. Under the proper conditions, however, the results indicate that this algorithm can be used by spacecraft in low Earth orbit with sepa- rations of up to 100 km to perform nanosecond-level time transfer on orbit in near real time.
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:	1582 - 1594
Cite this article:	Highsmith, Dolan E., Binning, Patrick W., Axelrad, Penina, "Design and Test of an Algorithm for Satellite-to-Satellite Time Transfer," 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. 1582-1594.
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