Author: Parveen Kaswan ( Author is an Aerospace Engineer and holds a Masters Degree in Engineering Designs from Indian Institute of Science, Bangalore )
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.
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 and Time (PNT) services to users on a variety of platforms with 24×7 service availability under all weather conditions.
IRNSS will offer two services Standard Positioning Service (SPS) and Restricted Service (RS) with encryption, according to Space Applications Centre, ISRO. The fully deployed IRNSS system consist of 3 satellites in GEO orbit and 4 satellites in GSO orbit, approximately 36,000 km altitude above earth surface. Each satellite is configured around I1K bus which means it is continuously monitored and maintained by ground segment.
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.
The Indian Regional Navigation Satellite System-1A (IRNSS -1A), first navigational satellite, is set to be launched from Sriharikota spaceport at on 1st July, 2013. The IRNSS, will be helpful in both civilian and military sectors. As in recent times we have seen the usefulness of navigation services in disaster management and also in rescue operations. It will provide terrestrial, aerial and marine navigation services and will help in fleet management. In recent years India has developed many sophisticated missiles, and the two most important part of any missile is its control and navigation. To enhance the effectiveness of a weapon delivery system we are using advanced inertial navigation systems augmented with GPS technologies. As it is known that GPS is a sole property of US army and it can be manipulated according to their will. Developing our own satellite navigation system provide India an unmatched flexibility in times of military operations. Also the signals provided to global players are not of military grade, they are degraded signals which provides of 36 metre only. Now when India is developing advanced and accurate cruise missiles like ‘Nirbhay’ accuracy is very important factor for their precise hit.
India also planned to be a part of European Space Agency’s Galileo programme but there also military rights of the system were denied. China is already building its BeiDou system and has committed to provide military grade signals to Pakistan. In these circumstances IRNSS is a much sought military asset.
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.