Editing Radio clock (section)

===GPS clocks===
{{Main|GPS disciplined oscillator}}
Many modern radio clocks use [[satellite navigation]] systems such as [[Global Positioning System]] to provide more accurate time than can be obtained from terrestrial radio stations. These ''GPS clocks'' combine time estimates from multiple satellite atomic clocks with error estimates maintained by a network of ground stations. Due to effects inherent in radio propagation and ionospheric spread and delay, GPS timing requires averaging of these phenomena over several periods. No GPS receiver directly computes time or frequency, rather they use GPS to discipline an oscillator that may range from a quartz crystal in a low-end navigation receiver, through oven-controlled [[crystal oscillators]] (OCXO) in specialized units, to atomic oscillators ([[rubidium]]) in some receivers used for [[synchronization in telecommunications]]. For this reason, these devices are technically referred to as [[GPSDO|GPS-disciplined oscillator]]s.

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. After approximately a day of operation, it will know its position to within a few meters. Once it has averaged its position, it can determine accurate time even if it can pick up signals from only one or two satellites.

GPS clocks provide the precise time needed for [[synchrophasor]] measurement of voltage and current on the commercial power grid to determine the health of the system.<ref>{{Cite journal
 | author = KEMA, Inc.
 | author-link = KEMA
 | title = Substation Communications: Enabler of Automation / An Assessment of Communications Technologies
 | publisher = UTC&nbsp;— United Telecom Council
 | date = November 2006
 | pages = 3}}</ref>