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Abstract
There are currently two navigation satellite systems employing clock-carrying satellites that are useable for the dissemination of Precise Time and Time Interval (PTTI). These are the Navy Navigation Satellite System (NNSS or Transit), which now has five satellites in low altitude (1000 km) polar orbits, and the Global Positioning System (GPS), which currently has six satellites in circular (20,000 km) twelve-hour orbits. The US Naval Observatory (USNO) is engaged in an ongoing program to improve the performance of these systems and to make them operational time dissemination systems as soon as possible.
Transit has a demonstrated time-transfer capability in the 25 microsecond region. GPS is capable of time transfers of better than 100 nanoseconds. This paper describes equipment available for recovering time from the satellites, current efforts aimed at improving both satellite system and receiver performance, and the data collection and reporting schemes used at the USNO to provide data for publication. Following this, a description and discussion of tests and test results obtained over the past several months are presented. Finally, a means of gaining near real-time access to USNO PTTI data is presented.