GLONASS (Russian ГЛОНАСС; ГЛОбальная НАвигационная Спутниковая Система; Global'naya Navigatsionnaya Sputnikovaya Sistema. GLObal NAvigation Satellite System) is a radio satellite navigation system, the Russian counterpart to the United States' GPS system. It is operated for the Russian government by the Russian Space Forces.

At peak efficiency the system offered a standard (C/a) positioning and timing service giving horizontal positioning accuracy within 55 meters, vertical positioning within 70 meters, velocity vector measuring within 15 cm/s and timing within 1 µs. All based on measurements from four satellite signals simultaneously. A more accurate signal (P) was available for Russian military use.

Like GPS, the complete nominal GLONASS constellation consists of 24 satellites, 21 operating and three on-orbit 'spares' placed in three orbital planes. Each plane contains eight satellites identified by "slot" number, which defines the corresponding orbital plane and the location within the plane: 1-8, 9-16, 17-24. The three orbital planes are separated by 120, and the satellites equally spaced within the same orbital plane, 45 apart. The GLONASS orbits are roughly circular, with an inclination of about 64.8 and a semi-major axis of 25,440 km. The planes themselves have 15 argument of latitude displacement.

GLONASS constellation orbits the Earth at an altitude of 19,100 km (slightly lower than that of the GPS satellites). Each satellite completes an orbit in approximately 11 hours, 15 minutes. The spacing of the satellites in orbits is arranged so that a minimum of 5 satellites are in view at any given time.

GLONASS satellite transmits two types of signal: standard precision (SP) and high precision (HP). SP signal L1 have a frequency division multiple access in L-band: L1= 1602MHz + n0.5625MHz, where "n" is frequency channel number (n=0,1.2...).

All satellites have been launched from Tyuratam, Kazakhstan. The first three test satellites were placed in orbit in October 1982 with the first operational satellites entering service in December 1983. The system was intended to be operation in 1991, it was announced to be operational on September 24, 1993 but the constellation was not completed until December 1995.

A characteristic of the GLONASS constellation is, that the satellite orbits repeat after 8 days. As each orbit plane contains 8 satellites, there is a non-identical repeat (i.e., another satellite will occupy the same place in the sky) after one sidereal day. This differs from the GPS identical repeat period of one sidereal day.

Due to the economic situation in Russia there were only eight satellites in operation in April 2002 rendering it almost useless as a navigation aid.

Since the economic situation in Russia has improved, 11 satellites were in operation by March 2004. Additionally, an advanced GLONASS satellite, the GLONASS-M, with an operational lifetime of 7 years, has been developed. A 3-satellite block of this new version was launched on 26 December 2004. Following a joint venture deal with the Indian Government, it is proposed to have the system fully operational again by 2007.

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