ISRO Navigation Centre (INC), the critical ground segment of the Indian Regional Navigation Satellite System (IRNSS), an independent navigation satellite system being developed by the country has inaugurated recently. The navigation centre will function as the main ground station for the satellite system, which will be equipped with high precision atomic clocks and transmit navigation signals to multiple users round the clock.
The navigation centre will also be responsible for time reference, generation of navigation messages and monitoring and control of ground facilities. The key to global positioning system (GPS)-based navigation support is the time reference to which all ground-based systems and satellite clocks are synchronized. The time reference is generated by the high precision timing facility at the navigation centre, which is equipped with high stability, high precision atomic clocks. The first satellite will be tested in its geostationary orbit for about six months before commissioning for operational use. It will have a ten-year lifespan. The remaining six satellites (of IRNSS constellation) will be launched by 2015.
A network of 21 ranging stations located across the country will provide data for the orbit determination of the satellites and monitoring of the navigation signal. The data from the ranging and monitoring stations will be relayed to the data processing facility at the centre on real-time basis to generate navigation messages, which are in turn transmitted to the satellites through the spacecraft control facility of the space agency at Hassan (180 km from Bengaluru) and Bhopal in Madhya Pradesh.
The navigational satellites will provide an accurate real time position, navigation time (PNT) services to users in air, sea and land on a variety of platforms under all weather conditions. It will provide two types of services- standard positioning service for civilian use and restricted service, which is encrypted for authorized users, particularly the ministry of defence.
According to ISRO, IRNSS is an independent regional navigation satellite system designed to provide position accuracy better than 10m over India and the region extending about 1500 kms around India. It will provide an accurate real time Position, Navigation
The proposed system would consist of a constellation of seven satellites and a support ground segment. Three of the satellites in the constellation will be placed in geostationary orbit. These GEOs will be located at 34 East 83 East and 132 East longitude. Two of the GSOs will cross the equator at 55 East and two at 111 East. Such an arrangement would mean all seven satellites would have continuous radio visibility with Indian control stations. The satellite payloads would consist of atomic clocks and electronic equipment to generate the navigation signals.
What is GPS
When people talk about “a GPS,” they usually mean a GPS receiver. The Global Positioning System (GPS) is actually a constellation of 27 Earth-orbiting satellites (24 in operation and three extras in case one fails or 3 are acting as backup). GPS is a type of navigational system and broadly it comes under Satellite Navigation System. The U.S. military developed and implemented this satellite network as a military navigation system, but soon opened it up to everybody else.
Each of these 3,000- to 4,000-pound solar-powered satellites circles the globe at about 12,000 miles (19,300 km), making two complete rotations every day. The orbits are arranged so that at any time, anywhere on Earth, there are at least four satellites “visible” in the sky or the same point can be tracked by at least four satellites.
A GPS receiver “knows” the location of the satellites, because that information is included in satellite transmissions. By estimating how far away a satellite is, the receiver also “knows” it is located somewhere on the surface of an imaginary sphere centered at the satellite. It then determines the sizes of several spheres, one for each satellite. The receiver is located where these spheres intersect. The accuracy of a position determined with GPS depends on the type of receiver. Most hand-held GPS units have about 10-20 meter accuracy. Other types of receivers use a method called Differential GPS (DGPS) to obtain much higher accuracy. DGPS requires an additional receiver fixed at a known location nearby. Observations made by the stationary receiver are used to correct positions recorded by the roving units, producing an accuracy greater than 1 meter.
So in short IRNSS system will not work on the GPS receivers, hence special receivers will be required for the vehicles which are going to utilize IRNSS.