Two-way time transfer via satellite is one of the most precise methods of time synchronization. This method makes it possible to obtain 0,1 nanosecond precision and decrease systematic error to the nanosecond level. The two-way time transfer system designed in Russia and the results of experiments are described. The experiments consist of two-way time transfer via satellite, ground station delay measurements and a relativity effect calculation. The results of two-way time transfer are compared with GLONASS data.
Experiments conducted at ground stations of the Quasar satellite communication network using both a null base as well as a base of around 2000 km is described. In experiments with a null base, the time scale transmission error was 1 nsec. Comparison of the results of collocation with data cbtained by the GLONASS system in the course of experiments using a 2100-km base demonstrated a divergence of the results of from 5 to 25 nsec.The modern understanding of problems of synchronization support and methods of maintaining time synchronization impose new requirements on the process of synchronizing remote atomic clocks. These requirements mainly entail collocating all the synchronization equipment at the main ground stations of a synchronization support system. It turns out that the existence of an independent means of comparing clocks with error of 0.1 nsec is not essential for achieving the theoretical precision of the synchronization equipment.In general, there are three areas in which synchronization of clocks with subnanosecond precision is of essential importance.Because of the existence of an independent means of synchronizing clocks, in radio interferometry with ultra-long bases it becomes possible to increase the precision of synchronized measurements [I] and to extend the basic reference system associated with these types of radio interferometry systems to remote regions [2].Through independent synchronization of the clocks at ground stations of the GLONASS and GPS satellite-based navigation systems, it is possible to determine the coordinates of these stations without having exact knowledge of the orbits and without any information concerning the course of the space clocks.A third area is the need to extend the State Unified Time System and System of Frequency Standards. The need to provide users with a state time scale standard that does not involve any loss of precision also entails reducing the synchronization error down to fractions of a nanosecond.A method of reciprocal exchange of synchronizing signals through geostationary communcations satellites, called the "duplex method," would, we believe, be the best means of achieving this high a degree of synchronization precision.Since 1989 research has been conducted at the Institute of Applied Astronomy of the Russian Academy of Sciences to create and investigate a system for synchronizing remote time scales using as reference the KVAZAR radio interferometric complex. The design of the system is based on simultaneous reciprocal exchange of time signals through a satellite retranslator, i.e., application of the duplex method. A detailed description of the duplex method is presented in [3][4][5].The experimental system for merging together different time scales incorporated into the KVAZAR complex ( Fig. 1) includes standard equipment (satellite communications stations, modems for transmission of digital signals and measurement equipment) as well as specially designed devices for generating and processing synchronization signals, a system for automatic data...
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