About nine years back, the Indian Space Research Organisation’s scientists and engineers began to look at the possibility of establishing a navigation satellite system for the country, rather like America’s Global Positioning System (GPS).
Like the GPS, the Indian satellites would continually transmit data that allowed suitably equipped receivers to establish their location with considerable precision. The GPS requires a constellation of 24 orbiting satellites, supported by a global network of ground stations, to cover every part of the world. That kind of global system is expensive.
ISRO had a more limited goal — creating a system wholly in India's control for providing navigation signals over this country and surrounding areas. The cost of such a system was a major consideration.
“We looked at many thousands of configurations,” said one person who was involved in those early studies. The configuration that was finally chosen for the Indian Regional Navigation Satellite System (IRNSS) required just seven satellites.
All seven IRNSS satellites will be at a height of about 36,000 km, taking a whole day to circle the Earth. Three of the satellites will be placed over the equator, in what is known as the geostationary orbit, where they match the Earth's rotation and therefore appear from the ground to remain at a fixed position in the sky. The remaining four satellites will be in pairs in two inclined geosynchronous orbits. From the ground, these satellites will appear to travel in figures of ‘8’ during the course of a day.
The project to establish the IRNSS at a cost of Rs. 1,420 crores was approved by the Union Government in June 2006. The primary service area for the system covers India and up to 1,500 km beyond its borders.
If necessary, the coverage area around India could be enhanced by adding four satellites, the ISRO Chairman, K. Radhakrishnan, told The Hindu.
The first of the IRNSS satellites is scheduled go into space aboard the Polar Satellite Launch Vehicle on July 1.
Navigation satellites periodically transmit their precise position in orbit along with the exact time when each transmission takes place. Since these microwave signals travel at the speed of light, the delay between a signal’s transmission and its reception allows a receiving device to compute its distance from the satellite involved. With data from four satellites, the receiver can then work out its own position.
Navigation satellites therefore need to carry extremely accurate clocks. Each IRNSS satellite is equipped with three rubidium atomic clocks, which keep precise time down to a few tenths of a trillionth of a second in an hour. Although currently these sophisticated clocks are imported, research efforts to make them indigenously are in progress.
But for the atomic clocks to function properly, they have to be kept at within one degree Celsius of their optimum operating temperature. They must also be protected from excessive vibration and electromagnetic interference. The IRNSS satellites had therefore to be designed to ensure such an environment.
Using the time provided by the clocks and taking into account the satellite’s own position in orbit, an onboard navigation payload generates the signal that will be broadcast.
IRNSS satellites transmit signals in two microwave frequency bands known as L5 and S. The system will provide two types of services, the ‘Standard Positioning Service’ that will be accessible to anyone and an encrypted ‘Restricted Service’ that will be available only to the military and other government-authorised users.
The system “is expected to provide a position accuracy better than 20 metres in the primary service area,” according to an ISRO brochure. However, the space agency’s technical personnel believe its actual performance is likely to be better than that and match single-frequency U.S. GPS receivers’ position accuracy of about 15 metres.
Those who wish to use the IRNSS will need receivers equipped to pick up and utilise the data transmitted by the Indian satellite system. A standard GPS receiver will not do.
ISRO's Space Applications Centre at Ahmedabad, along with industry, were in the process of developing suitable receivers, said the space agency’s chairman.