|Table of contents|
2 GPS clocks
3 External links
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:
- the WWV, WWVB and WWVH radio stations in the United States
- the CHU radio station in Canada
- the DCF77 radio station in Germany
- the MSF radio station in the United Kingdom
- the JJY radio station in Japan
- they refer their broadcast frequency to the frequency standard
- they broadcast seconds 'pips' to identify the start of second intervals
- also broadcast time codes as a way of identifying seconds intervals
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.
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.