Precise Ephemerides for GPS Time Transfer

Abstract : The present technology of atomic clocks motivates time transfer techniques with nanosecond accuracy. Global Positioning System (GPS), the most common means for international time comparisons could achieve such accuracy over short distances (up to 1000 km). Over intercontinental distances the accuracy of the GPS time transfer ranges between 20 and 30 ns. Some of the principal error sources are the broadcast ephemerides, the broadcast ionospheric model, and the local antenna coordinates. This study investigates the quality of broadcast ephemerides by comparing them with precise ephemerides and by using precise ephemerides for time transfer. Another aspect of this work is to suggest a strategy to overcome the planned degradation of GPS satellite messages via Selective Availability (SA).