Time and frequency transfer
Time and frequency transfer is a scheme where multiple sites share a precise reference time or frequency. The technique is commonly used for creating and distributing standard time scales such as International Atomic Time (TAI). Time transfer solves problems such as astronomical observatories correlating observed flashes or other phenomena with each other, as well as
Multiple techniques have been developed, often transferring reference clock synchronization from one point to another, often over long distances. Accuracy approaching one nanosecond worldwide is economically practical for many applications. Radio-based navigation systems are frequently used as time transfer systems.
In some cases, multiple measurements are made over a period of time, and exact
Examples
Examples of time and frequency transfer techniques include:
- Simultaneous observation methods:
- Simultaneous
- Time transfer by simultaneous observation of TV transmissions[3]
- Time transfer by simultaneous observation of radio astronomy signals
- Two-way transfer methods:
- Network methods:
One-way
In a one-way time transfer system, one end transmits its current time over some communication channel to one or more receivers.[4]: 116 The receivers will, at reception, decode the message, and either just report the time, or adjust a local clock which can provide hold-over time reports in between the reception of messages. The advantage of one-way systems is that they can be technically simple and serve many receivers, as the transmitter is unaware of the receivers.
The principal drawback of the one-way time transfer system is that propagation delays of the communication channel remain uncompensated except in some advanced systems. Examples of a one-way time transfer system are the clock on a church or town building and the ringing of their time-indication bells; time balls, radio clock signals such as LORAN, DCF77 and MSF; and finally the Global Positioning System which uses multiple one-way time transfers from different satellites, with positional information and other advanced means of delay compensations to allow receiver compensation of time and position information in real time.
Two-way
In a two-way time transfer system, the two peers will both transmit, and will also receive each other's messages, thus performing two one-way time transfers to determine the difference between the remote clock and the local clock.
Common view
The time difference between two clocks may be determined by simultaneously comparing each clock to a common reference signal that may be received at both sites.[7] As long as both end stations receive the same satellite signal at the same time, the accuracy of the signal source is not important. The nature of the received signal is not important, although widely available timing and navigation systems such as GPS or LORAN are convenient.
The accuracy of time transferred in this way is typically 1–10 ns.[8]
GNSS
Since the advent of GPS and other
See also
- International Earth Rotation and Reference Systems Service
- Precision Time Protocol
- Synchronization
- Time and frequency metrology
- Time signal
- Time synchronization in North America
References
- ^ Global Positioning System Carrier-Phase
- ^ Time and Frequency Transfer using the phase of the GPS Carrier
- ^ GPS and TV Time Comparison Techniques
- ^ a b Jones, T (2000). Splitting the second. Institute of Physical Publishing.
- ^ "Set your computer clock via the Internet using tools built into the operating system". National Institute of Standards and Technology. Retrieved 2012-12-22.
- ^ Novick, Andrew N.; et al. Time Distribution Using the World Wide Web (PDF). Archived from the original (PDF) on 2016-03-03.
- ^ Allan, David W.; Weiss, Marc A. (May 1980), "Accurate Time and Frequency Transfer During Common-View of a GPS Satellite" (PDF), Proceedings of the 34th Annual Frequency Control Symposium USAERADCOM, Ft. Monmouth, NJ: 334–346
- ^ Marc Weiss, Common View GPS Time Transfer, NIST Time and Frequency Division, archived from the original on 2012-10-28, retrieved 2011-11-22
- ^ Department of Defense and Department of Transportation (1994). "USNO NAVSTAR Global Positioning System". Federal Radionavigation Plan. US Navy. Retrieved 2008-11-13.
- ^ "Global Positioning System Timing". U.S. Coast Guard Navigation Center. Retrieved 2008-11-13.
- ^ "GPS and UTC Time Transfer". RoyalTek. Archived from the original on 2010-03-23. Retrieved 2009-12-18.