Early the next year, Frank Mc Clure, the deputy director of the APL, asked Guier and Weiffenbach to investigate the inverse problem — pinpointing the user's location, given that of the satellite.
(At the time, the Navy was developing the submarine-launched Polaris missile, which required them to know the submarine's location.) This led them and APL to develop the TRANSIT system. Navy developed the Timation satellite, which proved the feasibility of placing accurate clocks in space, a technology required for GPS.
Satellite orbital position errors, induced by variations in the gravity field and radar refraction among others, had to be resolved.
A team led by Harold L Jury of Pan Am Aerospace Division in Florida from 1970–1973, used real-time data assimilation and recursive estimation to do so, reducing systematic and residual errors to a manageable level to permit accurate navigation.
It used a constellation of five satellites and could provide a navigational fix approximately once per hour. In the 1970s, the ground-based OMEGA navigation system, based on phase comparison of signal transmission from pairs of stations, became the first worldwide radio navigation system.
Limitations of these systems drove the need for a more universal navigation solution with greater accuracy.
The United States government created the system, maintains it, and makes it freely accessible to anyone with a GPS receiver. Announcements from Vice President Al Gore and the White House in 1998 initiated these changes. During the 1990s, GPS quality was degraded by the United States government in a program called "Selective Availability"; this was discontinued in May 2000 by a law signed by President Bill Clinton.
The Russian Global Navigation Satellite System (GLONASS) was developed contemporaneously with GPS, but suffered from incomplete coverage of the globe until the mid-2000s.China's Bei Dou Navigation Satellite System is due to achieve global reach in 2020.There are also the European Union Galileo positioning system, and India's NAVIC. Getting of The Aerospace Corporation, and Bradford Parkinson of the Applied Physics Laboratory are credited with inventing it.The nuclear triad consisted of the United States Navy's submarine-launched ballistic missiles (SLBMs) along with United States Air Force (USAF) strategic bombers and intercontinental ballistic missiles (ICBMs).Considered vital to the nuclear deterrence posture, accurate determination of the SLBM launch position was a force multiplier.