Two-Way Satellite Time Transfer to a Moving Platform

Edward Powers, James Romberg, Roger Griffin, Angela McKinley, David Diener, Harold A. Klotz Jr. and David Wolters

Abstract:	Precision time transfer is a key requirement for many applications. Three methods of precision time transfer that have typically been used are GPS Stand-alone, GPS Common View, and Two–Way Satellite Time Transfer (TWSTT). Only the GPS Stand-alone approach has been regularly used on a moving platform and this is the least accurate of these three time transfer methods. Analyses show that the standalone method is inadequate to meet the time accuracy requirement specified for some military applications. Boeing and the US Naval Observatory (USNO) have investigated and tested a Two-Way Satellite Time Transfer (TWSTT) method that has the potential of providing excellent time transfer accuracy to aircraft in flight. This approach employs a TWSTT system that communicates between a time reference center (such as the USNO) and an aircraft, via satellite, to provide excellent absolute time transfer. Equipment used in the TWSTT test consisted of a TWSTT time transfer modem and a GPS time transfer receiver furnished by the USNO, and a Boeing Connexion Ku-band aircraft transceiver and phased array antennas. The Connexion transceivers provided the satellite communication link between the moving platform and a Geostar-1 satellite that was used to relay information between USNO headquarters and the Boeing Seattle test site. Special test software, including a Kalman filter that processed raw pseudorange and pseudorange rate outputs from the GPS time transfer receiver, was also developed for the test. Data collected during the tests are presented in this paper along with an analysis of the results. Low nanosecond time transfer accuracy is indicated after applying user platform motion corrections and filtering. Scatter plots show that the precision of the time transfer readings were nominally within ± 0.5 nanoseconds. Today, accurate two-way satellite time transfer to a platform is based on the assumption that the forward and return RF transmission equipment and path time delays are the same. That is, the time of transmission for the forward path from the USNO time source to the geostationary satellite to user platform is the same as the return path from the user platform to satellite to USNO. The path lengths are the same for a stationary user platform but are not the same for a moving user platform. Therefore, correction algorithms were developed during this demonstration to account for motion of the user platform during the time transfer. The two-way link (TWSTT) offers the benefit of canceling all time delay errors in the time transfer channel.
Published in:	Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002) September 24 - 27, 2002 Oregon Convention Center Portland, OR
Pages:	1175 - 1184
Cite this article:	Powers, Edward, Romberg, James, Griffin, Roger, McKinley, Angela, Diener, David, Klotz, Harold A., Jr., Wolters, David, "Two-Way Satellite Time Transfer to a Moving Platform," Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002), Portland, OR, September 2002, pp. 1175-1184.
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