Editing Global Positioning System (section)

== Structure ==
{{more citations needed section|date=March 2015}}
The current GPS consists of three major segments. These are the space segment, a control segment, and a user segment.<ref name="breeze-19790916-97"/> The [[U.S. Space Force]] develops, maintains, and operates the space and control segments. GPS satellites [[broadcast signal]]s from space, and each GPS receiver uses these signals to calculate its three-dimensional location (latitude, longitude, and altitude) and the current time.<ref name="gps.gov">{{cite web|url=http://www.gps.gov/systems/gps |title=Global Positioning System |publisher=Gps.gov |access-date=June 26, 2010 |archive-url=https://web.archive.org/web/20100730173245/http://www.gps.gov/systems/gps |archive-date=July 30, 2010 }}</ref>

=== 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.

=== Control segment ===
[[File:GPS monitor station.jpg|right|thumb|upright=0.8|Ground monitor station used from 1984 to 2007, on display at the [[Air Force Space and Missile Museum]]]]

The control segment (CS) is composed of:
# a master control station (MCS),
# an alternative master control station,
# four dedicated ground antennas, and
# six dedicated monitor stations.

The MCS can also access [[Satellite Control Network]] (SCN) ground antennas (for additional command and control capability) and NGA ([[National Geospatial-Intelligence Agency]]) monitor stations. The flight paths of the satellites are tracked by dedicated U.S. Space Force monitoring stations in Hawaii, [[Kwajalein Atoll]], [[Ascension Island]], [[Diego Garcia]], [[Colorado Springs, Colorado]] and [[Cape Canaveral]], Florida, along with shared NGA monitor stations operated in England, Argentina, Ecuador, Bahrain, Australia and Washington, DC.<ref>United States Coast Guard. [https://archive.today/20120712041201/http://igs.bkg.bund.de/root_ftp/IGS/mail/igsmail/year2005/5209 General GPS News 9–9–05].</ref> The tracking information is sent to the MCS at [[Schriever Space Force Base]] {{convert|25|km|mi|abbr=on}} ESE of Colorado Springs, which is operated by the [[2nd Space Operations Squadron]] (2&nbsp;SOPS) of the U.S. Space Force. Then 2&nbsp;SOPS contacts each GPS satellite regularly with a navigational update using dedicated or shared (AFSCN) ground antennas (GPS dedicated ground antennas are located at [[Kwajalein]], [[Ascension Island]], [[Diego Garcia]], and [[Cape Canaveral]]). These updates synchronize the atomic clocks on board the satellites to within a few [[nanosecond]]s of each other, and adjust the [[ephemeris]] of each satellite's internal orbital model. The updates are created by a [[Kalman filter]] that uses inputs from the ground monitoring stations, [[space weather]] information, and various other inputs.<ref>[[USNO]] [http://tycho.usno.navy.mil/gpsinfo.html NAVSTAR Global Positioning System] {{Webarchive|url=https://web.archive.org/web/20060208110241/http://tycho.usno.navy.mil/gpsinfo.html |date=February 8, 2006 }}. Retrieved May 14, 2006.</ref>

When a satellite's orbit is being adjusted, the satellite is marked ''unhealthy'', so receivers do not use it. After the maneuver, engineers track the new orbit from the ground, upload the new ephemeris, and mark the satellite healthy again. The operation control segment (OCS) currently serves as the control segment of record. It provides the operational capability that supports GPS users and keeps the GPS operational and performing within specification.

OCS replaced the 1970s-era mainframe computer at Schriever Air Force Base in September 2007. After installation, the system helped enable upgrades and provide a foundation for a new security architecture that supported U.S. armed forces.

{{anchor|OCX}}OCS will continue to be the ground control system of record until the new segment, Next Generation GPS Operation Control System<ref name="losangelesmil" /> (OCX), is fully developed and functional. The U.S. Department of Defense has claimed that the new capabilities provided by OCX will be the cornerstone for enhancing GPS's mission capabilities, enabling U.S. Space Force to enhance GPS operational services to U.S. combat forces, civil partners and domestic and international users.<ref>{{Cite web |title=DoD Decision Breathes New Life into Critical OCX Satellite Program |url=https://www.defense.gov/News/News-Stories/Article/Article/974228/dod-decision-breathes-new-life-into-critical-ocx-satellite-program/https://www.defense.gov/News/News-Stories/Article/Article/974228/dod-decision-breathes-new-life-into-critical-ocx-satellite-program/ |access-date=2023-11-26 |website=U.S. Department of Defense |language=en-US}}{{dead link|date=April 2025|bot=medic}}{{cbignore|bot=medic}}</ref><ref>{{Cite web |title=GPS.gov: Next Generation Operational Control System (OCX) |url=https://www.gps.gov/systems/gps/control/OCX/ |access-date=2023-11-26 |website=www.gps.gov}}</ref> The GPS OCX program also will reduce cost, schedule and technical risk. It is designed to provide 50%<ref>{{cite web|url=http://www.defenseindustrydaily.com/The-USAs-GPS-III-Satellites-04900/|title=The USA's GPS-III Satellites|date=October 13, 2011|publisher=Defense Industry Daily|access-date=October 27, 2011|archive-url=https://web.archive.org/web/20111018184806/http://www.defenseindustrydaily.com/The-USAs-GPS-III-Satellites-04900/|archive-date=October 18, 2011|url-status=live}}</ref> sustainment cost savings through efficient software architecture and Performance-Based Logistics. In addition, GPS OCX is expected to cost millions of dollars less than the cost to upgrade OCS while providing four times the capability.

The GPS OCX program represents a critical part of GPS modernization and provides information assurance improvements over the current GPS OCS program.
* OCX will have the ability to control and manage GPS legacy satellites as well as the next generation of GPS III satellites, while enabling the full array of military signals.
* Built on a flexible architecture that can rapidly adapt to changing needs of GPS users allowing immediate access to GPS data and constellation status through secure, accurate and reliable information.
* Provides the warfighter with more secure, actionable and predictive information to enhance situational awareness.
* Enables new modernized signals (L1C, L2C, and L5) and has M-code capability, which the legacy system is unable to do.
* Provides significant information assurance improvements over the current program including detecting and preventing cyber attacks, while isolating, containing and operating during such attacks.
* Supports higher volume near real-time command and control capabilities and abilities.

On September 14, 2011,<ref>{{cite web|url=http://www.comspacewatch.com/news/viewpr.html?pid=34625|title=GPS Completes Next Generation Operational Control System PDR|date=September 14, 2011|publisher=Air Force Space Command News Service|archive-url=https://web.archive.org/web/20111002043642/http://www.comspacewatch.com/news/viewpr.html?pid=34625|archive-date=October 2, 2011}}</ref> the U.S. Air Force announced the completion of GPS OCX Preliminary Design Review and confirmed that the OCX program is ready for the next phase of development. The GPS OCX program missed major milestones and pushed its launch into 2021, 5 years past the original deadline. According to the Government Accounting Office in 2019, the 2021 deadline looked shaky.<ref>{{cite web|url=https://www.gao.gov/assets/700/699234.pdf|title=GLOBAL POSITIONING SYSTEM: Updated Schedule Assessment Could Help Decision Makers Address Likely Delays Related to New Ground Control System|date=May 2019|publisher=US Government Accounting Office|access-date=August 24, 2019|archive-date=September 10, 2019|archive-url=https://web.archive.org/web/20190910233141/https://www.gao.gov/assets/700/699234.pdf|url-status=live}}</ref>

The project remained delayed in 2023, and was (as of June 2023) 73% over its original estimated budget.<ref>{{Cite news |date=June 21, 2023 |title=Raytheon's $7 Billion GPS Stations Are Running 73% Over Estimates |language=en |work=Bloomberg.com |url=https://www.bloomberg.com/news/articles/2023-06-21/raytheon-s-7-billion-ocx-gps-ground-stations-draw-the-ire-of-house-panel |access-date=2023-11-26}}</ref><ref>{{Cite web |last=Albon |first=Courtney |date=June 9, 2023 |title=Space Force sees further delays to 'troubled' GPS ground segment |url=https://www.c4isrnet.com/battlefield-tech/space/2023/06/09/space-force-sees-further-delays-to-troubled-gps-ground-segment/ |access-date=2023-11-26 |website=C4ISRNet |language=en}}</ref> In late 2023, Frank Calvelli, the assistant secretary of the Air Force for space acquisitions and integration, stated that the project was estimated to go live some time during the summer of 2024.<ref>{{Cite web |last=Hitchens |first=Theresa |date=November 7, 2023 |title=Next-gen GPS ground system expected to come online this summer: Calvelli |url=https://breakingdefense.com/2023/11/next-gen-gps-ground-system-expected-to-come-online-this-summer-calvelli/ |access-date=2025-03-15 |website=Breaking Defense |language=en-US }}</ref>

=== User segment ===
{{further|GPS navigation device}}

[[File:GPS Receivers.jpg|thumb|right|upright=0.8|GPS receivers come in a variety of formats, from devices integrated into cars, phones, and watches, to dedicated devices such as those shown above.]]
[[File:Leica WM 101 at the National Science Museum at Maynooth.JPG|thumb|right|The first portable GPS survey unit, a Leica WM 101, displayed at the [[Irish National Science Museum]] at [[Maynooth]]]]

The user segment (US) is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial and scientific users of the Standard Positioning Service. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highly stable clock (often a [[crystal oscillator]]). They may also include a display for providing location and speed information to the user.

GPS receivers may include an input for differential corrections, using the [[RTCM]] SC-104 format. This is typically in the form of an [[RS-232]] port at 4,800&nbsp;bit/s speed. Data is actually sent at a much lower rate, which limits the accuracy of the signal sent using RTCM.{{Citation needed|date=August 2011}} Receivers with internal DGPS receivers can outperform those using external RTCM data.{{Citation needed|date=August 2011}} {{As of |2006}}, even low-cost units commonly include [[Wide Area Augmentation System]] (WAAS) receivers.

[[File:SiRF Star III основанный на GPS приёмнике с интегрированной антенной.jpg|thumb|right|upright=0.8|A typical GPS receiver with integrated antenna]]

Many GPS receivers can relay position data to a PC or other device using the [[NMEA 0183]] protocol. Although this protocol is officially defined by the National Marine Electronics Association (NMEA),<ref>{{cite web|url=http://www.nmea.org/content/nmea_standards/nmea_standards.asp|title=Publications and Standards from the National Marine Electronics Association (NMEA)|publisher=National Marine Electronics Association|access-date=June 27, 2008|archive-url=https://web.archive.org/web/20090804071335/http://www.nmea.org/content/nmea_standards/nmea_standards.asp|archive-date=August 4, 2009}}</ref> references to this protocol have been compiled from public records, allowing open source tools like [[gpsd]] to read the protocol without violating intellectual property laws.{{Clarify|What does it mean to "compile references to a protocol"?|date=February 2013}} Other proprietary protocols exist as well, such as the [[SiRF]] and [[MediaTek|MTK]] protocols. Receivers can interface with other devices using methods including a serial connection, [[Universal Serial Bus|USB]], or [[Bluetooth]].