Transit (satellite navigation system)
First operational satellite navigation system, used by U.S. Navy.
The Transit system—also called NAVSAT or the Navy Navigation Satellite System (NNSS)—was the first operational satellite navigation system. It served primarily the U.S. Navy, giving Polaris ballistic missile submarines precise location data, and was also used by Navy surface ships, hydrographic survey vessels, and geodetic surveyors.
The system grew out of work at the Johns Hopkins Applied Physics Laboratory (APL) shortly after the Soviet Union launched Sputnik 1 in October 1957. Two APL physicists, William Guier and George Weiffenbach, discussed the radio signals the satellite would emit and successfully determined Sputnik’s orbit by analyzing the Doppler shift of its signals during a single pass. Their director, Frank McClure, suggested in March 1958 that if a satellite’s position were known and predictable, the same Doppler shift could locate a receiver on Earth, and he proposed building a satellite system around that principle.
Development began in 1958. The first prototype, Transit 1A, launched in September 1959 but failed to reach orbit. Transit 1B launched successfully on April 13, 1960, aboard a Thor-Ablestar rocket. Initial system tests succeeded in 1960, and the Navy began using it in 1964. A full constellation of 36 satellites was operational by 1968.
The Chance Vought/LTV Scout rocket became the dedicated launch vehicle because it offered the lowest cost per pound to orbit. However, the Scout imposed two constraints: its payload capacity to Transit’s orbit was only about 120 pounds (later increased significantly), while earlier satellites weighed roughly 300 pounds; and its solid rocket motors produced intense vibration. Engineers had to shrink the electronics and make them more rugged. The first prototype operational satellite, Transit 5A-1, launched into polar orbit on December 18, 1962, aboard a Scout. It tested new solar panel deployment and separation techniques but failed due to power system issues. Transit 5A-2 launched on April 5, 1963, but did not reach orbit. Transit 5A-3, with a redesigned power supply, launched on June 15, 1963; a memory malfunction prevented it from storing the navigation message, and oscillator stability degraded during launch, so it could not be used for navigation. It did, however, become the first satellite to achieve gravity-gradient stabilization, and its other systems worked well.
Quick Facts
- Country
- United States
- First launch
- 1959
- Last launch
- 1988
Facts from the source article.
Lore & Background
The Transit satellite system was sponsored by the Navy and developed jointly by the Advanced Research Projects Agency (ARPA) and the Johns Hopkins Applied Physics Laboratory, under the leadership of Dr. Richard Kershner. Just days after the Soviet launch of Sputnik 1 on October 4, 1957, two physicists at APL, William Guier and George Weiffenbach, determined Sputnik's orbit by analyzing the Doppler shift of its radio signals. Their director Frank McClure suggested in March 1958 that if the satellite's position were known, the Doppler shift could be used to locate a receiver on Earth, proposing a satellite system to implement this principle.
Development began in 1958. A prototype, Transit 1A, launched in September 1959, failed to reach orbit. Transit 1B was successfully launched on April 13, 1960, by a Thor-Ablestar rocket. The first successful tests were made in 1960, and the system entered Naval service in 1964. The Chance Vought/LTV Scout rocket was selected as the dedicated launch vehicle, imposing design constraints on satellite weight and vibration resistance. The first prototype operational satellite (Transit 5A-1) was launched into a polar orbit by a Scout rocket on December 18, 1962. Transit 5A-3, launched on June 15, 1963, was the first to achieve gravity-gradient stabilization.
The satellites were placed in low polar orbits at an altitude of about 600 nmi, with an orbital period of about 106 minutes. A constellation of five satellites was required for reasonable global coverage, though at least ten were usually kept in orbit. The system provided single-pass accuracy of roughly 200 m and time synchronization to roughly 50 microseconds. Transit satellites also broadcast encrypted messages as a secondary function.
Reader's Guide
Transit provided continuous navigation satellite service from 1964, initially for Polaris submarines and later for civilian use. Surveyors used Transit to locate remote benchmarks by averaging dozens of fixes, producing sub-meter accuracy; the elevation of Mount Everest was corrected in the late 1980s using a Transit receiver to re-survey a nearby benchmark. 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 systems, including Parus (military) and Tsikada (civilian), which were eventually superseded by GLONASS. Some Soviet warships were equipped with Motorola NavSat receivers.
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 take several fixes at once, greatly reducing the complexity of deducing a position. GPS uses many more satellites, allowing continuous use, 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).
Frequently Asked Questions
When did Transit become operational and how large did its constellation get?
The first successful satellite, Transit 1B, was launched on April 13, 1960, and the system entered regular operational service in 1964. By 1968 the full constellation of 36 satellites was in place, and the network was eventually superseded by the GPS system.
Why is Transit considered a landmark in navigation history?
It demonstrated for the first time that orbiting satellites could provide dependable, precise positioning to fast-moving vessels, laying the conceptual and engineering groundwork that GPS and every modern satellite navigation system still builds upon. Without those early Transit flights, global satellite positioning would almost certainly have arrived much later.
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