Beginning April 2013, the
Smithsonian National Air and Space Museum will examine the cultural and technological history of precise timekeeping and navigation at sea, in the air, and in space, and the impact of satellite navigation on our everyday lives. The exhibit,
TIME and NAVIGATION, will explore 'how revolutions in timekeeping over three centuries have influenced how we find our way.'
On display, NTS-2 is the first satellite completely designed and built by the U.S. Naval Research Laboratory (NRL) under GPS Joint Program funding—a working model was launched June 23, 1977, aboard an Atlas E/F rocket from Vandenberg Air Force Base, Calif.
The first of a four-satellite constellation, NTS-2 was configured to demonstrate instantaneous navigation positioning. The effect of relativity on the onboard cesium atomic clocks were measured and corrected so that a GPS receiver on Earth could observe that the rate of GPS time was the same as Coordinated Universal Time (UTC). The clock frequency stability specification of two parts per 1013 was met.
NTS-2 was the first demonstration satellite in the NAVSTAR GPS constellation managed by the NAVSTAR GPS Joint Program Office at the
Space and Missile Systems Center, Los Angeles Air Force Base, Calif.
Exploiting space-based systems of geodesy, navigation, and timing, NRL research physicist, Roger Easton, laid the foundation for modern day global positioning systems—
GPS.
Proving that a system using a passive ranging technique, combined with highly accurate [atomic] clocks, Easton developed the basis for a new and revolutionary navigation system with three-dimensional coverage (longitude, latitude, and altitude) around the globe.
Sponsored in 1964 by the
Naval Air Systems Command, Easton tested his concepts of time-navigation, dubbed TIMATION, executing the development and launch of the TIMATION satellite in 1967.
With the deployment of three additional experimental satellites, TIMATION II in 1969; the first satellite to fly two rubidium standards, Navigation Technology Satellite (NTS-I) in 1974; and the first satellite to fly two cesium atomic frequency standards in a 12-hour GPS orbit, NTS-2, in 1977, Easton had unequivocally proven the practicality and unprecedented accuracy of satellite-based atomic clocks.
Using time measurements from NTS-2, Einstein's theory of relativity was demonstrated, resulting in the need for a relativistic offset correction that remains in use by every satellite in the GPS constellation.
Easton, as a scientist and engineer at NRL, developed his concept for a time-based navigational system with passive ranging, circular orbits, and space-borne high precision clocks synchronized to a master clock. His concept of passive ranging demonstrated the ability to instantaneously measure the range to an orbiting satellite in an interval of significantly less than one second. With passive ranging, and its rapid measurement capability, the dimensionality of the fix could be determined by the number of satellites in view, two satellites – 2D, four satellites – 4D.
It was well understood that as many satellites needed to be in view as there were unknowns, said Pete Wilhelm, director, NRL Naval Center for Space Technology. In the more stressing and general case of four unknowns—latitude, longitude, altitude and time—four satellites would need to be in view globally at all times.
To minimize the number of satellites in the required constellation, NTS-1 and NTS-2 were placed in higher orbits, each equipped with atomic clocks.
Proving his concept worked, Easton submitted patent application, Navigation System Using Satellites and Passive Ranging Techniques, received Oct. 8, 1970 by the U.S. Patent and Trademark Office. Although the patent illustrations were, as Easton stated, 'dimensionally distorted and extremely simplified for the purpose of disclosure,' the main purpose of the patent was to illustrate the concept of passive ranging from satellites. Once that was understood the rest of the details would become quite evident. On January 29, 1974 it was awarded U.S. Patent 3,789,409.
While initially designed for use by the military, GPS has been adapted for civilian navigation needs ranging from commercial aviation to portable handheld and wristwatch-type devices. Today, GPS is a constellation of 32 Earth-orbiting satellites providing precise navigation and timing data to military and civilian end-users around the globe.