Transit (satellite)
Transit was the first satellite navigation system in the world and the predecessor of the U.S. GPS.
Function
Its transmission frequencies were at 150 and 400 MHz. Transit was in 1958 by the U.S. Navy initially under the designation Navy Navigation Satellite System ( NNSS ) developed, used and civil military from 1964 and 1967. The satellite system is transit since 31 December 1996 out of service.
It was the first operational satellite navigation system at all, and was originally developed for the guidance of ballistic missiles on submarines and aircraft carriers of the United States Navy.
The accuracy was based on the reception technique between 500 and 15 m. In the final phase of the operation of the constellation of six satellites, in function was three and three as a backup system. They moved in a polar satellites orbit at an altitude of about 1200 km.
Principle of measurement
The satellites transmit their ephemerides and a time signal. Movement relative to the receiver causes a frequency shift of the carrier frequency due to the Doppler effect. High accepts the change with approach to, and indeed faster the closer the satellite passes through the receiver. Quantitatively, the carrier frequency of 400 MHz changes by a maximum of / - 9.8 kHz. From the transmitted time, the exact position of the satellite and the Doppler shift, the position can own the world, even at the poles, determine. The signal is also influenced by:
- The latitude of the receiver. The Earth's rotation at the equator is greatest.
- The proper motion of the receiver.
- The ionosphere.
According to the principle of the transit systems Argos and DORIS work.
Satellite system
Four ground stations operated by the Naval Astronautics Group ( NAG) in Point Mugu (California), led the satellites and correction data relayed every twelve hours. The Transit satellites orbited the Earth on polar orbits at an altitude of about 1200 km, but due to slight braking through the high atmosphere decreased slightly and in some cases has been corrected. The 5-6 orbital planes were evenly spaced pivoted (at 5 levels every 36 °, see picture).
Transit 4A, launched in 1961, was the first nuclear -powered energy spacecraft, the first time a radioisotope generator ( RTG) has been used in space.
Transit 5BN -3 not reached after the launch on April 21, 1964 its orbit. When entering the satellite burned up, the plutonium in the RTG (type SNAP -9A ) contained was released into the atmosphere.