Editing Global Positioning System (section)

=== Timekeeping {{anchor|GPS time|GPS time and date}} ===

==== Leap seconds ====
While most clocks derive their time from [[Coordinated Universal Time]] (UTC), the atomic clocks on the satellites are set to ''GPS time''. The difference is that GPS time is not corrected to match the rotation of the Earth, so it does not contain new [[leap second]]s or other corrections that are periodically added to UTC. GPS time was set to match UTC in 1980, but has since diverged. The lack of corrections means that GPS time remains at a constant offset with [[International Atomic Time]] (TAI) (TAI – GPS = 19&nbsp;seconds). Periodic corrections are performed to the on-board clocks to keep them synchronized with ground clocks.<ref name="NAVGPS" />{{rp|at=Section 1.2.2}}

The GPS navigation message includes the difference between GPS time and UTC. {{As of|2017|1|post=,}} GPS time is 18 seconds ahead of UTC because of the leap second added to UTC on December 31, 2016.<ref>{{cite web|title=Notice Advisory to Navstar Users (NANU) 2016069 |access-date=June 25, 2017 |url=https://www.navcen.uscg.gov/?Do=gpsArchives&path=nanu&year=2016&file=25665&type=messageBody--nanuId--NANUS&name=2016069.txt |archive-url=https://web.archive.org/web/20170525063405/https://www.navcen.uscg.gov/?Do=gpsArchives&path=nanu&year=2016&file=25665&type=messageBody--nanuId--NANUS&name=2016069.txt |archive-date=May 25, 2017 |publisher=GPS Operations Center}}</ref> Receivers subtract this offset from GPS time to calculate UTC and specific time zone values. New GPS units may not show the correct UTC time until after receiving the UTC offset message. The GPS-UTC offset field can accommodate 255 leap seconds (eight bits).

==== Accuracy ====
GPS time is theoretically accurate to about 14 nanoseconds, due to the [[clock drift]] relative to [[International Atomic Time]] that the atomic clocks in GPS transmitters experience.<ref>{{cite book |author=David W. Allan |author2=Neil Ashby |author3=Clifford C. Hodge |url=https://www.hpmemoryproject.org/an/pdf/an_1289.pdf |title=The Science of Timekeeping |publisher=Hewlett Packard |year=1997 |via=HP Memory Project}}</ref> Most receivers lose some accuracy in their interpretation of the signals and are only accurate to about 100 nanoseconds.<ref>{{cite magazine |author=Peter H. Dana |author2=Bruce M Penrod|url=http://www.pdana.com/PHDWWW_files/gpsrole.pdf |title=The Role of GPS in Precise Time and Frequency Dissemination |magazine=GPS World |date=July–August 1990 |access-date=April 27, 2014|archive-url=https://web.archive.org/web/20121215031941/http://www.pdana.com/PHDWWW_files/gpsrole.pdf |archive-date=December 15, 2012|url-status=live |via=P Dana}}</ref><ref>{{cite web|url=http://www.atomic-clock.galleon.eu.com/support/gps-time-accuracy.html|title=GPS time accurate to 100 nanoseconds|publisher=Galleon|access-date=October 12, 2012|archive-url=https://web.archive.org/web/20120514001920/http://www.atomic-clock.galleon.eu.com/support/gps-time-accuracy.html|archive-date=May 14, 2012|url-status=live}}</ref>

==== Relativistic corrections ====
The GPS implements two major corrections to its time signals for relativistic effects: one for relative velocity of satellite and receiver, using the special theory of relativity, and one for the difference in gravitational potential between satellite and receiver, using general relativity. The acceleration of the satellite could also be computed independently as a correction, depending on purpose, but normally the effect is already dealt with in the first two corrections.<ref>{{Citation|last1=Fliegel|first1=Henry F.|last2=DiEsposti|first2=Raymond S.|date=December 1996|publisher=The Aerospace Corporation|location=El Segundo, CA|title=GPS and relativity overview|url=https://apps.dtic.mil/dtic/tr/fulltext/u2/a516975.pdf|access-date=December 7, 2022|archive-date=March 6, 2023|archive-url=https://web.archive.org/web/20230306071351/https://apps.dtic.mil/dtic/tr/fulltext/u2/a516975.pdf|url-status=dead}}</ref><ref>{{Cite journal |last=Ashby |first=Neil |date=2003 |title=Relativity in the Global Positioning System |journal=Living Reviews in Relativity |volume=6 |issue=1 |pages=1 |doi=10.12942/lrr-2003-1 |doi-access=free |issn=1433-8351 |pmc=5253894 |pmid=28163638|bibcode=2003LRR.....6....1A }}</ref>

==== Format ====
{{further|GPS week number rollover}}
As opposed to the year, month, and day format of the [[Gregorian calendar]], the GPS date is expressed as a week number and a seconds-into-week number. The week number is transmitted as a ten-[[bit]] field in the C/A and P(Y) navigation messages, and so it becomes zero again every 1,024&nbsp;weeks (19.6&nbsp;years). GPS week zero started at 00:00:00&nbsp;UTC (00:00:19&nbsp;TAI) on January 6, 1980, and the week number became zero again for the first time at 23:59:47&nbsp;UTC on August 21, 1999 (00:00:19&nbsp;TAI on August 22, 1999). It happened the second time at 23:59:42&nbsp;UTC on April 6, 2019. To determine the current Gregorian date, a GPS receiver must be provided with the approximate date (to within 3,584&nbsp;days) to correctly translate the GPS date signal. To address this concern in the future the modernized GPS civil navigation (CNAV) message will use a 13-bit field that only repeats every 8,192&nbsp;weeks (157&nbsp;years), thus lasting until 2137 (157&nbsp;years after GPS week zero).