GPS Time Transfer with Implementation of Selective Availability
NATIONAL INST OF STANDARDS AND TECHNOLOGY BOULDER CO
Pagination or Media Count:
The international community of time metrology is facing a major challenge with the Selective Availability SA degradation of GPS satellite signals. At present there are 6 Block I satellites and 8 Block II satellites operating. According to the policy of the U.S. Department of Defence the Block I satellite signals will not be degraded, but these satellites are old with a finite life. The Block II satellites, which have all been launched since 1988, were subject to Selective Availability from March 25, 1990. The effect of SA should be to limit precision to about 100 meters for navigation and 167 ns for timing. A study has been conducted in order to understand the nature of the actual introduced degradation, and to elaborate the means of removing the effects of this degradation on time transfer. This study concerns the time extraction from GPS satellites at NIST USNO and Paris Observatory, and the comparison of atomic clocks between these laboratories by common view approach. The results show that when using the data taken over several days the time extraction can be achieved with uncertainty of a few tens of nanoseconds, while strict common-view has removed entirely the effects of SA during the periods under study.
- Navigation and Guidance