Editing Global Positioning System (section)

=== Predecessors ===

When the [[Soviet Union]] launched its first artificial satellite ([[Sputnik 1]]) in 1957, two American physicists, William Guier and George Weiffenbach, at [[Johns Hopkins University]]'s [[Applied Physics Laboratory]] (APL) monitored its radio transmissions.<ref name="guier-weiffenbach">{{cite journal|last1=Guier|first1=William H.|last2=Weiffenbach|first2=George C.|title=Genesis of Satellite Navigation |journal=Johns Hopkins APL Technical Digest|volume=19|issue=1|pages=178–181|year=1997|url=http://www.jhuapl.edu/techdigest/td/td1901/guier.pdf|access-date=April 9, 2012|archive-url=https://web.archive.org/web/20120512002742/http://www.jhuapl.edu/techdigest/td/td1901/guier.pdf|archive-date=May 12, 2012}}</ref> Within hours they realized that, because of the [[Doppler effect]], they could pinpoint where the satellite was along its orbit. The Director of the APL gave them access to their [[UNIVAC I]] computer to perform the heavy calculations required.

[[File:Managers for the Timation program.jpg|thumb|The [[Naval Research Laboratory]]’s managers for the Timation program and, later, the GPS program: [[Roger L. Easton]] (left) and [[Al Bartholemew]].]]

Early the next year, Frank McClure, the deputy director of the APL, asked Guier and Weiffenbach to investigate the inverse problem: pinpointing the user's location, given the satellite's. (At the time, the Navy was developing the submarine-launched [[UGM-27 Polaris|Polaris]] missile, which required them to know the submarine's location.) This led them and APL to develop the [[Transit (satellite)|TRANSIT]] system.<ref>{{citation |author=Johnson |first=Steven |title=Where good ideas come from, the natural history of innovation |year=2010 |place=New York |publisher=Riverhead Books}}</ref> In 1959, ARPA (renamed [[DARPA]] in 1972) also played a role in TRANSIT.<ref>{{cite book |last1=Worth |first1=Helen E. |url=http://space50.jhuapl.edu/pdfs/book.pdf |title=Transit to Tomorrow. Fifty Years of Space Research at The Johns Hopkins University Applied Physics Laboratory |last2=Warren |first2=Mame |year=2009 |access-date=March 3, 2013 |archive-url=https://web.archive.org/web/20201226045330/http://space50.jhuapl.edu/pdfs/book.pdf |archive-date=December 26, 2020 |url-status=live}}</ref><ref name="Alexandrow">{{cite web |author=Alexandrow |first=Catherine |date=April 2008 |title=The Story of GPS |url=http://www.darpa.mil/WorkArea/DownloadAsset.aspx?id=2565 |archive-url=https://web.archive.org/web/20130224065525/http://www.darpa.mil/WorkArea/DownloadAsset.aspx?id=2565 |archive-date=February 24, 2013}}</ref><ref name=gap>{{cite book
|url=http://www.darpa.mil/about/history/first_50_years.aspx|title=DARPA: 50 Years of Bridging the Gap|date=April 2008|archive-url=https://web.archive.org/web/20110506103713/http://www.darpa.mil/About/History/First_50_Years.aspx|archive-date=May 6, 2011}}</ref>

TRANSIT was first successfully tested in 1960.<ref>{{cite web|last=Howell|first=Elizabeth|title=Navstar: GPS Satellite Network|url=http://www.space.com/19794-navstar.html|publisher=SPACE.com|access-date=February 14, 2013|archive-url=https://web.archive.org/web/20130217140737/http://www.space.com/19794-navstar.html|archive-date=February 17, 2013|url-status=live}}</ref> It used a [[satellite constellation|constellation]] of five satellites and could provide a navigational fix approximately once per hour. In 1967, the U.S. Navy developed the [[Timation]] satellite, which proved the feasibility of placing accurate clocks in space, a technology required for GPS.<ref>{{Cite web |title=NRL Launched First Time-Based Navigation Satellite in 1967 |url=https://www.nrl.navy.mil/Media/News/Article/3411925/nrl-launched-first-time-based-navigation-satellite-in-1967/ |access-date=2025-01-05 |website=U.S. Naval Research Laboratory |language=en-US}}</ref>

In the 1970s, the ground-based [[Omega (navigation system)|OMEGA]] navigation system, based on phase comparison of signal transmission from pairs of stations,<ref>{{cite web |author=Proc |first=Jerry |title=Omega |url=http://www.jproc.ca/hyperbolic/omega.html |url-status=live |archive-url=https://web.archive.org/web/20100105155410/http://www.jproc.ca/hyperbolic/omega.html |archive-date=January 5, 2010 |access-date=December 8, 2009 |publisher=Jproc.ca}}</ref> became the first worldwide radio navigation system. Limitations of these systems drove the need for a more universal navigation solution with greater accuracy.

Although there were wide needs for accurate navigation in military and civilian sectors, almost none of those was seen as justification for the billions of dollars it would cost in research, development, deployment, and operation of a constellation of navigation satellites. During the [[Cold War]] [[arms race]], the nuclear threat to the existence of the United States was the one need that did justify this cost in the view of the United States Congress. This deterrent effect is why GPS was funded.{{citation needed|date=September 2024}} It is also the reason for the ultra-secrecy at that time. The [[nuclear triad]] consisted of the United States Navy's [[submarine-launched ballistic missile]]s (SLBMs) along with [[United States Air Force]] (USAF) [[strategic bomber]]s and [[intercontinental ballistic missile]]s (ICBMs). Considered vital to the [[nuclear strategy|nuclear deterrence]] posture, accurate determination of the SLBM launch position was a [[force multiplication|force multiplier]].

Precise navigation would enable United States [[ballistic missile submarine]]s to get an accurate fix of their positions before they launched their SLBMs.<ref>{{cite web |url=http://www.trimble.com/gps/whygps.shtml#0|archive-url=https://web.archive.org/web/20071018151253/http://www.trimble.com/gps/whygps.shtml#0|archive-date=October 18, 2007|title=Why Did the Department of Defense Develop GPS?|publisher=Trimble Navigation Ltd|access-date=January 13, 2010}}</ref> The USAF, with two-thirds of the nuclear triad, also had requirements for a more accurate and reliable navigation system. The U.S. Navy and U.S. Air Force were developing their own technologies in parallel to solve what was essentially the same problem. To increase the survivability of ICBMs, there was a proposal to use mobile launch platforms (comparable to the Soviet [[RT-23 Molodets|SS-24]] and [[RT-2PM Topol|SS-25]]) and so the need to fix the launch position had similarity to the SLBM situation.

In 1960, the Air Force proposed a radio-navigation system called MOSAIC (MObile System for Accurate ICBM Control) that was essentially a 3-D LORAN System. A follow-on study, Project 57, was performed in 1963 and it was "in this study that the GPS concept was born". That same year, the concept was pursued as Project 621B, which had "many of the attributes that you now see in GPS"<ref>{{cite web |url=http://www.aero.org/publications/crosslink/summer2002/01.html|title=Charting a Course Toward Global Navigation|publisher=The Aerospace Corporation|access-date=October 14, 2013|archive-url=https://web.archive.org/web/20021101215923/http://www.aero.org/publications/crosslink/summer2002/01.html|archive-date=November 1, 2002<!--, 01:01:18-->}}</ref> and promised increased accuracy for U.S. Air Force bombers as well as ICBMs.

[[File:Navigation Technology Satellite – II.jpg|thumb|Navigation Technology Satellite – II (Timation IV): NTS-II, the first satellite completely designed and built by NRL under GPS Joint Program funding. Launched June 23, 1977.]]

Updates from the Navy TRANSIT system were too slow for the high speeds of Air Force operation. The [[United States Naval Research Laboratory|Naval Research Laboratory]] (NRL) continued making advances with their [[Timation]] (Time Navigation) satellites, first launched in 1967, second launched in 1969, with the third in 1974 carrying the first [[atomic clock]] into orbit and the fourth launched in 1977.<ref>{{cite web|url=http://support.radioshack.com/support_tutorials/gps/gps_tmline.htm|title=A Guide to the Global Positioning System (GPS) – GPS Timeline|publisher=Radio Shack|access-date=January 14, 2010|archive-url=https://web.archive.org/web/20100213100725/http://support.radioshack.com/support_tutorials/gps/gps_tmline.htm|archive-date=February 13, 2010}}</ref>

Another important predecessor to GPS came from a different branch of the United States military. In 1964, the [[United States Army]] orbited its first Sequential Collation of Range ([[SECOR]]) satellite used for geodetic surveying.<ref>{{cite web|title=Geodetic Explorer – A Press Kit|date=October 29, 1965|access-date=October 20, 2015|publisher=NASA |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19660002550_1966002550.pdf|archive-url=https://web.archive.org/web/20140211071631/http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19660002550_1966002550.pdf|archive-date=February 11, 2014|url-status=live}}</ref> The SECOR system included three ground-based transmitters at known locations that would send signals to the satellite transponder in orbit. A fourth ground-based station, at an undetermined position, could then use those signals to fix its location precisely. The last SECOR satellite was launched in 1969.<ref>{{cite encyclopedia|url=http://www.astronautix.com/craft/secor.htm|title=SECOR Chronology|encyclopedia=Mark Wade's Encyclopedia Astronautica|access-date=January 19, 2010|archive-url=https://web.archive.org/web/20100116213013/http://astronautix.com/craft/secor.htm|archive-date=January 16, 2010}}</ref>