Time transfer

Time transfer describes methods for transferring reference clock synchronization from one point to another, often over long distances. 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 time synchronization only determined retrospectively.

In particular, time synchronization has been accomplished by using pairs of radio telescopes to listen to a pulsar, with the time transfer accomplished by comparing time offsets of the received pulsar signal.

In a one-way time transfer system, one end transmits its current time over some communication channel to one or more receivers. [cite book | last=Jones | first=T | title=Splitting the second | pages=116 | publisher=Institute of Physical Publishing | date=2000] 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. A drawback of the 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; 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.

In a two-way time transfer system, the two peers will both transmit, and will also receive each others messages, thus performing two one-way time transfer measurements where the difference of the remote clock compared to the local clock is being noted. [cite book | last=Jones | first=T | title=Splitting the second | pages=118 | publisher=Institute of Physical Publishing | date=2000] The sum of these time differences will become the sum of the delay between the two nodes. It is often assumed that this delay is evenly distributed between the directions between the peers. Under this assumption, the difference between the measured delays divided by two will provide the time difference between the clocks without being affected by propagation delay. The advantage of the two-way time transfer system is that propagation delays gets compensated, but the drawback is that there needs to be means for bi-directional communication (of preferably symmetrical propagation delay) between the directions. Another drawback is that the source of reference needs to be actively measuring slaves, which can become a scale issue. The Two-Way Satellite Time and Frequency Transfer (TWSTFT) system being used in comparison among some time laboratories uses a satellite for a common link between the laboratories. The Network Time Protocol uses packet based messages over an IP network.

ee also

* Global Positioning System
* LORAN
* Radio clock
* radio-controlled watch
* Atomic clocks
* International Earth Rotation and Reference Systems Service
* Network Time Protocol
* Two-Way Satellite Time and Frequency Transfer
* Time signal

References