Editing Global Positioning System (section)

=== Non-navigation applications ===
{{for|a list of applications|#Applications}}
GPS requires four or more satellites to be visible for accurate navigation.<ref>{{Cite web |last= |first= |last2= |first2= |last3= |first3= |title=The Global Positioning System: Global Positioning Tutorial |url=https://oceanservice.noaa.gov/education/tutorial_geodesy/geo09_gps.html#:~:text=It%20takes%20four%20GPS%20satellites,error%20in%20the%20receiver's%20clock. |access-date=2025-01-05 |website=oceanservice.noaa.gov |language=EN-US}}</ref> The solution of the [[#Navigation equations|navigation equations]] gives the position of the receiver along with the difference between the time kept by the receiver's on-board clock and the true time-of-day, thereby eliminating the need for a more precise and possibly impractical receiver based clock. Applications for GPS such as [[time transfer]], traffic signal timing, and [[IS-95#Physical layer|synchronization of cell phone base stations]], [[#Timekeeping|make use of]] this cheap and highly accurate timing. Some GPS applications use this time for display, or, other than for the basic position calculations, do not use it at all.

<!--This paragraph seems to be in the wrong section, or possibly the section heading needs changing to reflect its current content.-->Although four satellites are required for normal operation, fewer apply in special cases. If one variable is already known, a receiver can determine its position using only three satellites. For example, a ship on the open ocean usually has a known elevation [[tidal range|close to 0m]], and the elevation of an aircraft may be known.{{efn|In fact, the ship is unlikely to be at precisely 0m, because of tides and other factors which create a discrepancy between mean sea level and actual sea level. In the open ocean, high and low tide typically only differ by about 0.6m, but there are locations closer to land where they can differ by over 15m. See [[tidal range]] for more details and references.}} Some GPS receivers may use additional clues or assumptions such as reusing the last known altitude, [[dead reckoning]], [[inertial navigation system|inertial navigation]], or including information from the vehicle computer, to give a (possibly degraded) position when fewer than four satellites are visible.<ref>{{cite web |last1=zur Bonsen |first1=Georg |last2=Ammann |first2=Daniel |last3=Ammann |first3=Michael |last4=Favey |first4=Etienne |last5=Flammant |first5=Pascal |date=April 1, 2005 |title=Continuous Navigation Combining GPS with Sensor-Based Dead Reckoning |url=http://www.gpsworld.com/gpsworld/article/articleDetail.jsp?id=154870&pageID=6 |archive-url=https://web.archive.org/web/20061111202317/http://www.gpsworld.com/gpsworld/article/articleDetail.jsp?id=154870&pageID=6 |archive-date=November 11, 2006 |publisher=GPS World}}</ref><ref name="NAVGPS">{{cite web|title=NAVSTAR GPS User Equipment Introduction|url=http://www.navcen.uscg.gov/pubs/gps/gpsuser/gpsuser.pdf|publisher=United States Government|access-date=August 22, 2008|archive-url=https://web.archive.org/web/20080910184805/http://www.navcen.uscg.gov/pubs/gps/gpsuser/gpsuser.pdf|archive-date=September 10, 2008|url-status=live}} Chapter 7</ref><ref>{{cite web|title=GPS Support Notes|url=http://www.navmanwireless.com/uploads/EK/C8/EKC8zb1ITsNwDqWcqLQxiQ/Support_Notes_GPS_OperatingParameters.pdf|date=January 19, 2007|access-date=November 10, 2008|archive-url=https://web.archive.org/web/20090327051208/http://www.navmanwireless.com/uploads/EK/C8/EKC8zb1ITsNwDqWcqLQxiQ/Support_Notes_GPS_OperatingParameters.pdf|archive-date=March 27, 2009}}</ref>