A review of satellite time transfer technology: Accomplishments and future applications

Abstract

A brief review of the research accomplishments by NASA in meeting the needs of the space program for precise time in satellite tracking is presented. As a major user of precise time signals for clock synchronization of NASA's worldwide satellite tracking networks, the agency provided much of the necessary impetus for the development of stable frequency sources and time synchronization technology. The precision in time required for both satellite tracking and space science experiments has increased at a rate of about 1 order of magnitude per decade from 1 ms in the 1950's to 100 μs during the Apollo era in the 1960's to 10 μs in the 1970's. In the 1980's, when the Tracking and Data Relay Satellite System (TDRSS) comes into operation, satellite timing requirements will be extended to 1 μs and below. These requirements are needed for spacecraft autonomy and data packeting which are now in active planning stages.

Wireless Picosecond Time Synchronization for Distributed Antenna Arrays

“…The difficulty of this process is that either the receiver and transmitter's positions must both be known, or multiple sources with known positions are required to solve for the propagation delay. An alternative approach is two-way time synchronization, which inherently solves for both the time of flight and clock offset, assuming a quasi-static channel during the synchronization epoch; the two-way time transfer technique has been used for satellite time transfer for many decades to synchronize satellites to ground clocks and with other satellites [17]- [19]. A more recent protocol which uses two-way time transfer is precision time protocol (PTP) which achieves timing precision on the order of 1 µs; PTP is also the foundation for the White Rabbit protocol, a popular industrial synchronous Ethernet protocol which acts as a refinement on the PTP estimation which uses the carrier phase of Ethernet over fiber to determine residual time delay with high precision, typically on the order of 10 ps [20].…”