Editing Transit (satellite) (section)

== History ==
[[File:Transit-1A.jpg|thumb|right|Transit 1A]]
[[File:Transit 1B (cropped).jpg|thumb|Transit 1B]]
[[File:330-PSA-286-60 (USN 710826) (21464583995).jpg|thumb|Transit 3A]]
[[File:Transit 5-A.jpg|thumb|Transit 5A]]
The Transit satellite system, sponsored by the Navy and developed jointly by [[DARPA]] and the Johns Hopkins [[Applied Physics Laboratory]], under the leadership of Dr. Richard Kershner at Johns Hopkins, was the first satellite-based geopositioning system.<ref>{{cite book | title=Transit to Tomorrow. Fifty Years of Space Research at The Johns Hopkins University Applied Physics Laboratory | author=Helen E. Worth and Mame Warren | year=2009 | url=https://space.jhuapl.edu/sites/space.jhuapl.edu/files/2021-06/Transit-to-Tomorrow-eBook.pdf | access-date=2024-01-21}}</ref><ref>{{cite web | url=http://www.darpa.mil/WorkArea/DownloadAsset.aspx?id=2565 | archive-url=https://web.archive.org/web/20110629003311/http://www.darpa.mil/WorkArea/DownloadAsset.aspx?id=2565 | url-status=dead | archive-date=2011-06-29 | title=The Story of GPS | author=Catherine Alexandrow | date=Apr 2008 }}</ref><ref name="gap">{{cite book | url=http://www.darpa.mil/about/history/first_50_years.aspx | archive-url=https://web.archive.org/web/20110506103713/http://www.darpa.mil/About/History/First_50_Years.aspx | url-status=dead | archive-date=2011-05-06 | title=DARPA: 50 Years of Bridging the Gap | date=Apr 2008 }}</ref>  Just days after the [[Soviet Union|Soviet]] launch of [[Sputnik 1]], the first man-made Earth-orbiting satellite on October 4, 1957, two physicists at APL, William Guier and George Weiffenbach, found themselves in discussion about the radio signals that would likely be emanating from the satellite.  They were able to determine Sputnik's orbit by analyzing the [[Doppler shift]] of its radio signals during a single [[pass (spaceflight)|pass]].<ref name="guier">{{cite web |url=https://www.jhuapl.edu/Content/techdigest/pdf/V19-N01/19-01-Guier.pdf |title=Genesis of Satellite Navigation |author=Guier & Weiffenbach |year=1998}}</ref> Discussing the way forward for their research, their director Frank McClure, the chairman of APL's Research Center, suggested in March 1958 that if the satellite's position were known and predictable, the Doppler shift could be used to locate a receiver on Earth, and proposed a satellite system to implement this principle.<ref>{{cite book | title=The Legacy of Transit: Guest Editor's Introduction by Vincent L. Pisacane, Johns Hopkins APL Technical Digest, Vol 19, Number 1, 1998.| url=http://www.jhuapl.edu/techdigest/td/td1901/pisacane.pdf| archive-url=https://web.archive.org/web/20150920234035/http://www.jhuapl.edu/techdigest/td/td1901/pisacane.pdf| archive-date=2015-09-20}}</ref>

Development of the Transit system began in 1958, and a prototype satellite, '''Transit 1A''', was launched in September 1959.<ref>{{cite web |url=http://sd-www.jhuapl.edu/Transit/ |title=Navy Navigation Satellite System |publisher=APL}}</ref> That satellite failed to reach orbit.<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=TRAN1 |title=Transit 1A – NSSDC ID: TRAN1 |publisher=NASA Space Science Data Coordinated Archive}}</ref> A second satellite, '''Transit 1B''', was successfully launched April 13, 1960, by a [[Thor-Ablestar]] rocket.<ref>{{cite web |url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=1960-003B |title=Transit 1B – NSSDC ID: 1960-003B |publisher=NASA Space Science Data Coordinated Archive}}</ref> The first successful tests of the system were made in 1960, and the system entered Naval service in 1964. A fully operational constellation of 36 satellites was in place in 1968.<ref>{{Cite journal |last=Wyatt |first=Theodore |date=1981 |title=THE GESTATION OF TRANSIT AS PERCEIVED BY ONE PARTICIPANT |url=https://secwww.jhuapl.edu/techdigest/Content/techdigest/pdf/V02-N01/02-01-Wyatt.pdf |journal=Johns Hopkins APL Technical Digest |volume=2 |issue=1 |pages=33}}</ref>

The Chance Vought/LTV [[Scout (rocket family)|Scout]] rocket was selected as the dedicated launch vehicle for the program because it delivered a payload into orbit for the lowest cost per pound. However, the Scout decision imposed two design constraints. First, the weight of the earlier satellites was about {{convert|300|lb}} each, but the Scout launch capacity to the Transit orbit was about {{convert|120|lb}}, which was later increased significantly. A satellite mass reduction had to be achieved, despite a demand for more power than APL had previously designed into a satellite. The second problem concerned the increased vibration that affected the payload during launching because the Scout used solid rocket motors. Thus, electronic equipment that was smaller than before and rugged enough to withstand the increased vibration of launch had to be produced.
Meeting the new demands was more difficult than expected, but it was accomplished. The first prototype operational satellite ('''Transit 5A-1''') was launched into a polar orbit by a Scout rocket on 18 December 1962.
The satellite verified a new technique for deploying the solar panels and for separating from the rocket, but otherwise it was not successful because of trouble with the power system. '''Transit 5A-2''', launched on 5 April 1963, failed to achieve orbit. '''Transit 5A-3''', with a redesigned power supply, was launched on 15 June 1963. A malfunction of the memory occurred during powered flight that kept it from accepting and storing the navigation message, and the oscillator stability was degraded during launch. Thus, 5A-3 could not be used for navigation. However, this satellite was the first to achieve [[gravity-gradient stabilization]], and its other subsystems performed well.<ref>{{cite web |url=http://techdigest.jhuapl.edu/td/td1901/danchik.pdf |title=An Overview of Transit Development, by Robert J. Danchik. Johns Hopkins APL Technical Digest, Volume 19, Number 1 (1998), pp. 18–26 |access-date=2013-10-02 |archive-date=2017-08-21 |archive-url=https://web.archive.org/web/20170821140444/http://techdigest.jhuapl.edu/td/td1901/danchik.pdf |url-status=dead }}</ref>

Surveyors used Transit to locate remote [[Benchmark (surveying)|benchmarks]] by averaging dozens of Transit fixes, producing sub-meter accuracy.<ref>{{cite journal |title= Using GPS with ARC/INFO|first=Arthur F.|last=Lang |journal=Proceedings of the Eleventh Annual ESRI User Conference |date=1991|volume=1 |page=523 |url=https://books.google.com/books?id=PChz-kj55KIC&q=transit+receiver+sub+meter+averaging+benchmarks&pg=PA523 |publisher=ESRI |access-date=April 23, 2021}}</ref> In fact, the elevation of [[Mount Everest]] was corrected in the late 1980s by using a Transit receiver to re-survey a nearby benchmark.<ref>{{cite news |last1=Shaw |first1=Terry |title=Measuring a Mountain |url=https://www.washingtonpost.com/archive/1998/01/14/measuring-a-mountain/34fe6c45-f382-4776-bfdc-236a1bfc3bae/ |access-date=April 23, 2021 |newspaper=The Washington Post |date=January 14, 1998}}</ref>

Thousands of warships, freighters and private watercraft used Transit from 1967 until 1991. In the 1970s, the [[Soviet Union]] started launching their own satellite navigation system [[Parus (satellite)|''Parus'']] (military) / [[Tsikada]] (civilian), which is still in use today besides the next generation [[GLONASS]].<ref>[http://www.astronautix.com/craft/tsikada.htm Encyclopedia Astronautica: Tsikada] {{webarchive|url=https://web.archive.org/web/20130522015121/http://www.astronautix.com/craft/tsikada.htm |date=2013-05-22 }}</ref> Some Soviet warships were equipped with [[Motorola]] NavSat receivers.<ref>{{cite journal |last1=McDowell |first1=Jonathan |title=1998 Launch Log Special Edition |journal=Jonathan's Space Report |date=January 1, 1999 |issue=384 |url=https://planet4589.org/space/jsr/back/news.384.txt |access-date=April 23, 2021}}</ref>

The Transit system was made obsolete by the [[Global Positioning System]] (GPS), and ceased navigation service in 1996. Improvements in electronics allowed GPS receivers to effectively take several fixes at once, greatly reducing the complexity of deducing a position. GPS uses many more satellites than were used with Transit, allowing the system to be used continuously, while Transit provided a fix only every hour or more.

After 1996, the satellites were kept in use for the Navy Ionospheric Monitoring System (NIMS).<ref>{{cite web |url=http://www.jhuapl.edu/techdigest/td/td1901/tucker.pdf |title=Computerized Ionospheric Tomography, by Arnold J. Tucker. Johns Hopkins APL Technical Digest, Volume 19, Number 1 (1998), pp. 66–71 |access-date=2015-03-30 |archive-date=2015-09-20 |archive-url=https://web.archive.org/web/20150920211346/http://www.jhuapl.edu/techdigest/TD/td1901/tucker.pdf |url-status=dead }}</ref>