A radio clock is a clock that is synchronized by a time code transmitted by a radio transmitter connected to a time standard such as an atomic clock.
Sources of time for radio clocks include:
- the WWV radio station in the United States
- the CHU radio station in Canada
- the DCF77 radio station in Germany
- the MSF radio station in the United Kingdom
Many modern radio clocks use the GPS satellite constellation to provide more accurate time than can be obtained from these terrestrial radio stations. However, this makes them dependent on the goodwill of the United States for the operation of their systems. This is not acceptable for many critical civilian and military systems, although it may be acceptable for many civilian purposes, as it is assumed by most users that the civilian GPS signal would not be switched off except in the event of a global crisis of unprecedented proportions.
The planned establishment of the Galileo positioning system by the EU (expected to be fully operational in 2008) is intended to provide a second source of time for GPS-compatible clocks that are equipped to receive and decode the Galileo signals.