Editing Radio clock (section)

==Multiple transmitters==
A radio clock receiver may combine multiple time sources to improve its accuracy. This is what is done in [[Global Navigation Satellite System|satellite navigation systems]] such as the [[Global Positioning System]], [[Galileo positioning system|Galileo]], and [[GLONASS]]. [[Satellite navigation system]]s have one or more caesium, rubidium or hydrogen maser atomic clocks on each satellite, referenced to a clock or clocks on the ground. Dedicated timing receivers can serve as local time standards, with a precision better than 50&nbsp;ns.<ref>{{cite web|url=http://www.ilotus.com.sg/sites/all/themes/zeropoint/pdf/tx/TX%20Oncore%20-%20TDS%20(Ver%203.5.0).pdf|title=datasheet i-Lotus TX Oncore|access-date=2014-01-22|archive-date=2015-10-16|archive-url=https://web.archive.org/web/20151016222646/http://www.ilotus.com.sg/sites/all/themes/zeropoint/pdf/tx/TX%20Oncore%20-%20TDS%20(Ver%203.5.0).pdf|url-status=live}}</ref><ref>{{cite web|url=http://www.symmetricom.com/products/time-frequency-distribution/gps-instruments/xl-gps/|title=Symmetricom XL-GPS|access-date=2014-01-22|archive-date=2014-02-01|archive-url=https://web.archive.org/web/20140201153456/http://www.symmetricom.com/products/time-frequency-distribution/gps-instruments/xl-gps/|url-status=live}}</ref><ref>{{cite web|url=http://www.trimble.com/timing/pdf/022542-039A_Resolution_SMT_GG_DS_0412_US_LR.pdf|title=datasheet Trimble Resolution SMT GG|access-date=2014-01-22|archive-date=2013-06-22|archive-url=https://web.archive.org/web/20130622223112/http://www.trimble.com/timing/pdf/022542-039A_Resolution_SMT_GG_DS_0412_US_LR.pdf|url-status=live}}</ref><ref>{{cite web|url=https://www.u-blox.com/sites/default/files/NEO-LEA-M8T-FW3_DataSheet_%28UBX-15025193%29.pdf|title=datasheet u-blox NEO/LEA-M8T|access-date=2017-04-11|archive-date=2017-04-12|archive-url=https://web.archive.org/web/20170412061545/https://www.u-blox.com/sites/default/files/NEO-LEA-M8T-FW3_DataSheet_%28UBX-15025193%29.pdf|url-status=live}}</ref> The recent revival and enhancement of [[LORAN]], a land-based radio navigation system, will provide another multiple source time distribution system.

===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>

===Astronomy timekeeping===
Although any [[satellite navigation]] 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 is often not as precise as the internal clock. Most inexpensive navigation receivers have one [[CPU]] that is multitasking. The highest-priority task for the CPU is maintaining satellite lock—not updating the display. Multicore CPUs for navigation systems can only be found on high end products.

For serious precision timekeeping, a more specialized GPS device is needed. Some amateur astronomers, most notably those who time [[grazing lunar occultation]] events when the moon blocks the light from stars and planets, require the highest precision available for persons working outside large research institutions. The Web site of the International Occultation Timing Association<ref>{{Cite web |url=http://www.lunar-occultations.com/iota/ |title=International Occultation Timing Association |access-date=2006-07-19 |archive-date=2006-07-20 |archive-url=https://web.archive.org/web/20060720062504/http://www.lunar-occultations.com/iota/ |url-status=live }}</ref> has detailed technical information about precision timekeeping for the amateur astronomer.