What is GPS & Glonass

What is GPS & Glonass

The Global Positioning System, usually called GPS, is the only fully-functional satellite navigation system. A constellation of more than two dozen GPS satellites broadcasts precise timing signals by radio to GPS receivers, allowing them to accurately determine their location (longitude, latitude, and altitude) in any weather, day or night, anywhere on Earth.

United States Department of Defense developed the system, officially named NAVSTAR GPS (Navigation Signal Timing and Ranging GPS), and the satellite constellation is managed by the 50th Space Wing at Schriever Air Force Base. Although the cost of maintaining the system is approximately US$400 million per year, including the replacement of aging satellites, GPS is available for free use in civilian applications as a public good.

GPS has become a vital global utility, indispensable for modern navigation on land, sea, and air around the world, as well as an important tool for map-making, and land surveying. GPS also provides an extremely precise time reference, required for telecommunications and some scientific research, including the study of earthquakes.

In late 2005, the first in a series of next-generation GPS satellites was added to the constellation, offering several new capabilities, including a second civilian GPS signal called L2C for enhanced accuracy and reliability. In the coming years, additional next-generation satellites will increase coverage of L2C and add a third and fourth civilian signal to the system, as well as advanced military capabilities.

The Wide-Area Augmentation System (WAAS), available since August 2000, increases the accuracy of GPS signals to within 2 meters (6 ft) for compatible receivers. GPS accuracy can be improved further, to about 1 cm (half an inch) over short distances, using techniques such as Differential GPS (DGPS).

GPS is used by people around the world as a navigation aid in cars, airplanes, and ships. The system can also be used by computer controlled harvesters, mine trucks and other vehicles. Hand-held GPS receivers can be used by mountain climbers and hikers. Glider pilots use the logged signal to verify their arrival at turnpoints in competitions. Low cost GPS receivers are often combined in a bundle with a PDA, car computer, or vehicle tracking system.

More costly and precise receivers are used by land surveyors to locate boundaries, structures, and survey markers, and for road construction.

GLONASS (Russian & Global'naya Navigatsionnaya Sputnikovaya Sistema. GLObal NAvigation Satellite System) is a radio satellite navigation system, the Russian counterpart to the United States' GPS system and the European Union's embryonic Galileo positioning system. It is operated for the Russian government by the Russian Space Forces.

At peak efficiency the system offered a standard (coarse-acquisition or C/A) positioning and timing service giving horizontal positioning accuracy within 57-70 meters, vertical positioning within 70 meters, velocity vector measuring within 15 cms and timing within 1 µs, all based on measurements from four satellite signals simultaneously. A more accurate signal (precision or P(Y)) 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= 1602 MHz + n0.5625 MHz, where n is frequency channel number (n=0,1,2...).

All satellites have been launched from Baikonur Cosmodrome. 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 operational 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, 17 satellites were in operation by December 2005. 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. A further improved GLONASS-K satellite, with a reduced weight and an increased operational lifetime of 10-12 years, is due to enter service in 2008. Reducing its weight by 50% will allow to significantly lower launch costs. Following a joint venture deal with the Indian Government, which will launch two GLONASS-M satellites on its GSLV rockets, it is proposed to have the system fully operational again by 2008 with 18 satellites, providing full coverage of Russia territory, and by 2010 with all 24 satellites. During the December 2005 summit between Indian Prime Minister Manmohan Singh and Russian President Vladimir Putin, it was agreed that India would share the development costs of the GLONASS-K series and launch them from India.