Article Posted on : - Sep 17, 2009

The Global Navigation Satellite System (GNSS) is being progressively used throughout the world to overcome many of the limitations of today’s air navigation infrastructure. With its accurate, seamless, all-weather three-dimensional coverage, GNSS offers a satellite navigation service that satisfies many of the Area Navigation requirements.

The GNSS allows small electronic receivers to determine their location within a few metres using time signals transmitted along line-of-sight from satellites. Receivers on the ground with a fixed position can also be used to calculate the precise time as a reference for scientific experiments.

At present, the United States' Global Positioning System (GPS) is the only fully operational GNSS. The Russian GLONASS is a GNSS in the process of being restored to full operation. The European Union's Galileo positioning system is a GNSS in initial deployment phase, scheduled to be operational in 2013.

The NAVSTAR Global Positioning System (GPS) is a U.S. space-based radio navigation system that provides reliable positioning, navigation, and timing services to civilian users on a continuous worldwide basis -- freely available to all. Galileo is Europe's initiative for a state-of-the-art global navigation satellite system, providing a highly accurate, guaranteed global positioning service under civilian control.

GLONASS

GLONASS (Global'naya Navigatsionnaya Sputnikovaya Sistema or 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.

GLONASS is based on a constellation of active satellites which continuously transmit coded signals in two frequency bands, which can be received by users anywhere on the Earth's surface to identify their position and velocity in real time based on ranging measurements.
The Purpose of GLONASS is to provide real-time position and velocity determination, initially for use by the Soviet military for navigation and ballistic missile targeting. It was the Soviet Union's second generation satellite navigation system, improving on the Tsiklon system which required one to two hours of signal processing to calculate a location with high accuracy.

By contrast, once a GLONASS receiver is tracking the satellite signals, a position fix is available instantly. It is stated that at peak efficiency the system's standard positioning and timing service provide horizontal positioning accuracy within 57–70 meters, vertical positioning within 70 meters, velocity vector measuring within 15 cm/s, and time transfer within 1 µs (all within 99.7% probability).

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. They are spaced in orbit so a user on the ground can see at least five satellites at any time.

GLONASS satellites transmit precision (SP) and high precision (HP) signals at a frequency around 1.6 GHz. The operational system contains 21 satellites in 3 orbital planes, with 3 on-orbit spares. Glonass provides 100 meters accuracy with its C/A (deliberately degraded) signals and 10-20 meter accuracy with its P (military) signals.

The first GLONASS satellites were launched into orbit in 1982. Two Etalon geodetic satellites were also flown in the 19,100 km GLONASS orbit to fully characterise the gravitational field at the planned altitude and inclination. It was officially declared operational on September 24, 1993 by a decree of the President of the Russian Federation.

In India, Russian Space Agency is having a joint venture with the Indian Space Research Organisation and both have decided to collaborate to restore the system to constant coverage of Russian and Indian territory and therefore be fully operational with all 24 satellites by 2010.

The Glonass system, military sources say, would give India an advantage over most countries in a future when network-centric warfare would be a normal function. In addition it can be used to identify position and velocity in real time based on ranging arrangements. Access to the GPS-type system has important advantages in managing traffic, roadways and ports. It is also an important tool for police and security agencies to track stolen vehicles or those being driven by criminals. It has implications for national disaster warning and will be useful in commercial transactions dealing with sale and exchange of geographical and economic data.

The satellites currently in use are of two modifications - GLONASS and its updated version GLONASS-M. The latter satellites have a longer service life of seven years and are equipped with updated antenna feeder systems and an additional navigation frequency for civilian users.

The future modification, GLONASS-K, is an entirely new model and is a small-sized spacecraft that are considerably lighter than their previous models, which makes them less costly to put into orbit.

The first three GLONASS -M's are expected to be launch from the Baikonur Cosmodrome on or about September 25, 2009 with a second set of three GLONASS satellite to be launched in December 2009. Another launch with three more GLONASS-M's is expected in September 2010 to be followed in December 2010 with two additional GLONASS-M's and the test GLONASS-K series spacecraft. That would hopefully bring the system up to a full 24 active satellites constellation.

Through September 2008 there were 17 operational GLONASS satellites with 22 expected to be operational before the year is over with six more to be launched into the operational satellite system. It is planned to have 18 satellites covering the Russian Federation territory with a total of 24 satellites to cover the globe by 2012. The Russian Federal Space Agency hopes to have 30 satellites in place by some time in 2011.

And also in India, the GPS Aided Geo Augmented Navigation or GPS and Geo Augmented Navigation system (GAGAN) is a planned implementation of a regional Satellite Based Augmentation System (SBAS) by the Indian government. It is a system to improve the accuracy of a GNSS receiver by providing reference signals.

Till now, 4.7 billion ribles or ($200 million ) was allocated in 2006 with 9.9 billion rubles ($418.25 million) in 2007 allocated. Prime Minister Vladimir Putin on September 15, 2008 approved the addition of 67 billion rubles ($2.6 billion) in a work meeting with the government ministers. He also stated that he would approve an additional 45 billion rubles ($1.8 billion) for the Federal Space Program. On December 26, 2008 three more GLONASS-M satellites were orbited making it a 20 operational satellite system.

- Courtesy:-
        Inside GNSS website
        Global Security
        Gps.gov
        Galileo Information Centre website