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 transmitts its current time over some communication channel to one or more receivers. 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 inbetween the reception of messages. The benefit of one-way system is that they can be technically simple and can serve many receivers as the transmitter is unaware of the receivers. A drawback of a one-way system is that propagation delays of the communication channel remains uncompensated except for some advanced systems. Examples of a one-way Time transfer system is the clock on a church or town building and the ringing of their time-indication bells, Radio clock such as LORAN, DCF77 and MSF provides uncompensated radio transmission and finally Global Positioning System uses multiple one-way Time transfer from different satellites, 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 transmitt and they will receive each others messages and thus perform two one way time transfers mesurements where the difference of the remote clock compared to the local clock is being noted. The sum of these time time differances 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 benefits of the two-way Time transfer system is that propagation delays gets compensated. The drawback is that there need 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 comparision 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.