Editing Global Positioning System (section)

=== Space segment ===
{{See also|GPS satellite blocks|List of GPS satellites}}

[[File:160921-F-0000U-001.jpg|thumb|GPS II underwent a four-month series of qualification tests in the AEDC Mark I Space Chamber to determine whether the satellite could withstand extreme heat and cold in space, 1985.]]
[[File:GPS24goldenSML.gif|thumb|upright=1.35|A visual example of a 24-satellite GPS constellation in motion with the Earth rotating. Notice how the number of ''satellites in view'' from a given point on the Earth's surface changes with time. The point in this example is in Golden, Colorado, USA ({{coord|39.7469|N|105.2108|W}}).]]

The space segment (SS) is composed of 24 to 32 satellites, or Space Vehicles (SV), in [[medium Earth orbit]], and also includes the payload adapters to the boosters required to launch them into orbit. The GPS design originally called for 24&nbsp;SVs, eight each in three approximately circular [[orbital plane (astronomy)|orbits]],<ref>{{cite journal |first=P. |last=Daly |date=December 1993 |title=Navstar GPS and GLONASS: global satellite navigation systems |journal=Electronics & Communication Engineering Journal |volume=5 |issue=6 |pages=349–357 |doi=10.1049/ecej:19930069|doi-broken-date=December 7, 2024 }}</ref> but this was modified to six orbital planes with four satellites each.<ref>{{cite web|last=Dana|first=Peter H.|format=GIF|url=http://www.colorado.edu/geography/gcraft/notes/gps/gif/oplanes.gif|title=GPS Orbital Planes|date=August 8, 1996|access-date=February 27, 2006|archive-url=https://web.archive.org/web/20180126111533/https://www.colorado.edu/geography/gcraft/notes/gps/gif/oplanes.gif|archive-date=January 26, 2018}}</ref> The six orbit planes have approximately 55° [[inclination]] (tilt relative to the Earth's [[equator]]) and are separated by 60° [[right ascension]] of the [[orbital node|ascending node]] (angle along the equator from a reference point to the orbit's intersection).<ref name="GPS overview from JPO">[https://www.losangeles.spaceforce.mil/?id=5325 GPS Overview from the NAVSTAR Joint Program Office] . Retrieved December 15, 2006.</ref> The [[orbital period]] is one-half of a [[sidereal day]], ''i.e.'', 11 hours and 58 minutes, so that [[Satellite revisit period|the satellites pass over the same locations]]<ref>[http://metaresearch.org/cosmology/gps-relativity.asp What the Global Positioning System Tells Us about Relativity] {{webarchive |url=https://web.archive.org/web/20070104191143/http://metaresearch.org/cosmology/gps-relativity.asp |date=January 4, 2007 }}. Retrieved January 2, 2007.</ref> or almost the same locations<ref name="The GPS Satellite Constellation">{{cite web|url=http://www.gmat.unsw.edu.au/snap/gps/gps_survey/chap2/222sats.htm |title=The GPS Satellite Constellation |website=gmat.unsw.edu.au |archive-url=https://web.archive.org/web/20111022020714/http://www.gmat.unsw.edu.au/snap/gps/gps_survey/chap2/222sats.htm |archive-date=October 22, 2011 |access-date=October 27, 2011}}</ref> every day. The orbits are arranged so that at least six satellites are always within [[Line-of-sight propagation|line of sight]] from everywhere on the Earth's surface (see animation at right).<ref>{{cite web|url=http://www.navcen.uscg.gov/?pageName=gpsFaq|title=USCG Navcen: GPS Frequently Asked Questions|access-date=January 31, 2007|archive-url=https://web.archive.org/web/20110430020428/http://www.navcen.uscg.gov/?pageName=gpsFaq|archive-date=April 30, 2011|url-status=live}}</ref> The result of this objective is that the four satellites are not evenly spaced (90°) apart within each orbit. In general terms, the angular difference between satellites in each orbit is 30°, 105°, 120°, and 105° apart, which sum to 360°.<ref name=avionicswest>{{cite web|last=Thomassen|first=Keith|title=How GPS Works|url=http://avionicswest.com/Articles/howGPSworks.html|publisher=avionicswest.com|access-date=April 22, 2014|archive-url=https://web.archive.org/web/20160330083710/http://avionicswest.com/Articles/howGPSworks.html |archive-date=March 30, 2016}}</ref>

Orbiting at an altitude of approximately {{convert|20200|km|mi|abbr=on}}; orbital radius of approximately {{convert|26600|km|mi|abbr=on}},<ref>{{cite book|title=Global Positioning: Technologies and Performance |first1=Nel |last1=Samama |publisher=John Wiley & Sons |year=2008 |isbn=978-0-470-24190-5 |page=[{{google books|plainurl=y|id=EyFrcnSRFFgC|page=65 |title=Extract of page 65}} 65] |url={{google books|plainurl=y|id=EyFrcnSRFFgC}}}},</ref> each SV makes two complete orbits each [[sidereal day]], repeating the same [[ground track]] each day.<ref>{{cite journal|title=Finding the repeat times of the GPS constellation|author1=Agnew, D.C.  |author2=Larson, K.M.|author-link2=Kristine M. Larson|journal=GPS Solutions|volume=11|pages=71–76|year=2007|doi=10.1007/s10291-006-0038-4|issue=1|s2cid=59397640 }} [http://spot.colorado.edu/~kristine/gpsrep.pdf This article from author's web site] {{webarchive |url=https://web.archive.org/web/20080216041650/http://spot.colorado.edu/~kristine/gpsrep.pdf |date=February 16, 2008 }}, with minor correction.</ref> This was very helpful during development because even with only four satellites, correct alignment means all four are visible from one spot for a few hours each day. For military operations, the ground track repeat can be used to ensure good coverage in combat zones.

{{As of|2019|2}},<ref>{{cite web |url=https://www.gps.gov/systems/gps/space |title=Space Segment |publisher=GPS.gov |access-date=July 27, 2019 |archive-url=https://web.archive.org/web/20190718190908/https://www.gps.gov/systems/gps/space/ |archive-date=July 18, 2019 |url-status=live }}</ref> there are 31 satellites in the GPS [[satellite constellation|constellation]], 27 of which are in use at a given time with the rest allocated as stand-bys. A 32nd was launched in 2018, but as of July 2019 is still in evaluation. More decommissioned satellites are in orbit and available as spares. The additional satellites improve the precision of GPS receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a nonuniform arrangement. Such an arrangement was shown to improve accuracy but also improves reliability and availability of the system, relative to a uniform system, when multiple satellites fail.<ref>{{cite journal|last=Massatt|first=Paul|author2=Wayne Brady|url=http://www.aero.org/publications/crosslink/summer2002/index.html|title=Optimizing performance through constellation management|journal=Crosslink|date=Summer 2002|pages=17–21|archive-url=https://web.archive.org/web/20120125065043/http://www.aero.org/publications/crosslink/pdfs/CrosslinkV3N2.pdf|archive-date=January 25, 2012 }}</ref> With the expanded constellation, nine satellites are usually visible at any time from any point on the Earth with a clear horizon, ensuring considerable redundancy over the minimum four satellites needed for a position.