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.

Radio clocks depend on time signal radio stations, which usually have the following attributes:

Terrestrial time signals
Radio clocks synchronized to terrestrial time signals can usually achieve an accuracy of around 1 millisecond relative to the time standard, generally limited by uncertainties and variability in radio propagation.

Time signals that can be used as references for radio clocks include:

Loran Clocks
Loran-C time signals may also be used for radio clock synchronization, by augmenting their highly accurate frequency transmissions with external measurements of the offsets of LORAN navigation signals against time standards.

GPS clocks
Many modern radio clocks use the GPS satellite positioning system to provide more accurate time than can be obtained from these terrestrial radio stations. These GPS clocks combine time estimates from multiple satellite atomic clocks with error estimates maintained by a network of ground stations. Because they compute the time and position simultaneously from readings from several sources, GPS clocks can automatically compensate for line-of-sight delay and many radio propagation defects, and can achieve sub-microsecond accuracy under ideal conditions. GPS units intended primarily for time measurement as opposed to navigation can be set to assume the antenna position is fixed; in this mode the device will average its position fixes so that after a day or so of operation it will know its position to within a few meters. Once it has averaged its position, it can then determine accurate time even if it can only pick up signals from one or two satellites. The highest-quality GPS clocks have their own precision clocks -- either an atomic clock or a temperature-controlled crystal clock -- so they can maintain accurate time during any interruption of GPS signals.

Note that although any GPS receiver that is performing its primary navigational function must have an internal time reference accurate to a small fraction of a second, the displayed time on most consumer GPS units may not be as exact. This is because an inexpensive GPS unit typically has one CPU that is multitasking; the highest-priority task for the CPU is maintaining satellite lock, while updating the display gets the lowest priority. Therefore, the displayed time of most consumer handheld GPS units will be accurate to around half a second.

GPS, Galileo and GLONASS: These satellite navigation systems, have caesium or rubidium atomic clocks on each satellite, rated from clocks on the ground. Some navigation units can serve as local time standards, with an accuracy of about one microsecond.

However, GPS clocks are dependent on the goodwill of the United States for the operation of the GPS satellite constellation. This is not acceptable for many critical non-US 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 also equipped to receive and decode the Galileo signals.

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