NCST played a pivotal historical role in position and time determination from its earliest development of systems for satellite tracking to the beginnings of the Global Positioning System (GPS).
...adapted from an NRL Review 1985 feature article, "Space Navigation and Time," by J.A. Murray and R.L. Beard; and from a Journal of the Institute of Navigation 1995 article, "A History of Satellite Navigation," by Bradford Parkinson, Ronald Beard, and others.
L-R in photo: CDR William (Bill) G. Huston, USN, first Navy Deputy to the GPS Joint Program Office; Roger L. Easton, TIMATION Program Manager, NRL Space Metrology Branch; Peter G. Wilhelm, Director for NRL Spacecraft Technology Center.
The strong interest of NRL in position and time determination considerably predates the launching of the first satellites in the late 1950's. The Laboratory had been active in developing clocks and timing systems for the U.S. Naval Observatory and had been working on new radio-navigation and time-dissemination systems using all parts of the radio spectrum.
However, the lower radio frequencies that could be transmitted over long distances were degraded by the ionosphere, and higher frequencies were limited to line of sight. Satellites overcame these limitations and offered new opportunities for improving both time and position accuracy. Satellites could provide worldwide coverage; therefore, new concepts and technology for navigation and time dissemination were needed.
The MINITRACK system, which was developed in the late 1950s for the NRL Vanguard Satellite Program, used the signals emitted by Sputnik and later satellites to determine their positions and orbits. This evolved into tracking nonradiating or noncooperative satellites by signals reflected off of them.
The MINITRACK system became known as the Naval Space Surveillance System (NAVSPASUR) and was commissioned as an operational command in 1961.
TRANSIT, the Navy Navigation Satellite System, was developed in the early 1960s by the Johns Hopkins Applied Research Lab. Using the Doppler principle, a radio signal from a satellite in a known orbit could be used to determine the unknown position of a radio receiver on the ground. TRANSIT demonstrated that worldwide accurate navigation was possible from space and its operational constellation of satellites in circular polar orbits are still in use today.
NRL scientist Rogerl L. Easton formulated a concept in April 1964 for transmitting ranging signals along with the primary CW signal such that the distance to the target satellite could also be measured, making early orbit determination. This idea led to the concept of a new generation satellite-based navigation system, called TIMATION, which would use ranging from satellites.
NRL began the TIMATION project to provide both accurate position and precise time to passive terrestrial observers. (A "passive" user is not required to emit a signal.) Precise spaceborne clocks would be regularly updated by a master clock on the ground. These would be linked to the user's receiving equipment by ranging signals broadcast by the satellites.
NRL scientists recognized that the TIMATION concept not only could accelerate and simplify the positioning process but could make it more accurate. However, passive ranging requires an accurate and stable oscillator in the satellite. The TIMATION Project, under the direction of Roger L. Easton, concentrated on developing an improved quartz frequency standard for satellites and determining the most effective satellite constellation for providing worldwide coverage.
NRL also identified orbits that would maintain a consistant ground track so that an almanac of satellite positions could be published and the satellite would not be required to transmit its position. Simplified receiving equipment could then be used, and the techniques of celestial navigation could be employed for simple and accurate navigation and time determination.
TIMATION I was a small, power-limited satellite.
(Click on the image for an enlarged view)
It was launched May 31, 1967 on a Thor-Agena vehicle into a 500 nautical mile polar orbit.
It was tracked from several experimental ground stations built and operated by NRL, so that the orbit could be accurately known. The sidetone range signals were transmitted at approximately 400 MHz, which was close to the frequency used by TRANSIT.
Since the satellite was small and power limited, it was turned on while in range of NRL's Blossom Point Satellite Tracking Station. Experiments were performed, and then it was turned off as it set. Small boats, aircraft, and trucks were navigated by passive ranging from the satellite. In the original experiments, only one satellite was needed, since instantaneous positioning was not required.
TIMATION 2 was a larger satellite designed for continuous operation. It was launched in 1969 into a 500 nmi polar orbit and incorporated sidetone range signals transmitted at 150 and 400 MHz. Higher frequency range tones (Up to 1 MHz) increased the range measurement resolution to 10 M. The experiments with this satellite pointed the way to the concept for satisfying the JCS requirements.
The TIMATION Project and an Air Force Project called 621B, which had developed a sophisticated ranging signal concept, gave birth to the NAVSTAR Global Positioning System (GPS) Program. While the Air Force became the executive service, the Navy, and in particular NRL, participated heavily in the system concept and technologies.
The NAVSTAR GPS development plans in 1973 called for NRL to continue the technology efforts begun in the TIMATION Project. NRL's work was considered vital to the time-based system concept, and the Deputy Secretary of Defense called for Navy participation with emphasis on the clock developments. A cesium standard development proposed by NRL was selected the as the most likely satellite clock candidate for meeting the ultimate requirements.
NRL's third TIMATION satellite was renamed Navigation Technology Satellite 1 at its launch in July 1974, one year after the start of the GPS Program. It was the first satellite to fly atomic clocks, and examined two experimental rubidium frequency standards operating in space.NTS-2 was launched in June 1977 and orbited the first cesium clock in space. NTS-2 was used to evaluate the stations, software, and systems for the first developmental NAVSTAR satellites that were launched in 1978.