Friday, 13 May 2016
NAVIC, India’s Very Own GPS-lite
With the launch of the final satellite completing the satnav system, the IRNSS has been renamed NAVIC – Navigation with Indian Constellation India is currently in the process of launching its own indigenous regional satnav system, called the Indian Regional Navigation Satellite System (IRNSS). It will consist of seven satellites working in tandem to provide highly accurate coverage over the Indian subcontinent, up to 1,500 km from Indian borders, a region making up the primary service area. A less accurate extended service area would cover the region enclosed by latitude 30º south to 50º north, longitude 30º east to 130º east – that’s all of South East Asia except parts of Japan, northwest Australia, the Middle East, China and Mongolia, East Africa, and Madagascar. The region IRNSS will cover, enclosed. Source: Sandhya Ramesh The region IRNSS will cover, enclosed. Source: Sandhya Ramesh Satnav – satellite navigation – systems are used the world over to accurately track and know the location and positioning of vehicles, persons, crafts, shipments, electronic devices, and pretty much anything with an appropriate receiver and transmitter on it. The most widely used satnav system is the American Global Positioning System. The GPS that’s used by our phones and Google Maps and the Garmins in our cars are operated with the help of the American-owned and American-military-controlled GPS system. Russia too developed its own global satnav system called GLONASS (GLObal NAvigational Satellite System) at the same time as the US. However, due to financial constraints, it took a long time to be completed and made truly available. As of today, GLONASS and GPS are compatible with each other and both nations make use of each other’s satellites for their own systems. Satellites belonging to navigation systems constantly orbit the earth at different locations (designated by degrees), providing real-time global imagery of the Earth’s surface. So when you step out of your house, there are most likely at least four different satellites looking down at you from different positions in the sky. All satellites belonging to one system operate together in what’s called a constellation. The receivers on our mobile devices receive data transmitted by the satellites and relay it to us in the form of readable maps. Global satnav constellations use more than 20 satellites. GPS uses 31 active satellites and GLONASS uses 24. These two are the only functioning satnav systems today with global coverage. The EU plans to launch its own constellation of 30 satellites called Galileo, and China will launch the BeiDou Navigation System (BDS, not to be confused with the existing regional BeiDou-1) with 35 satellites. Both of them are planned to function independently of GPS and GLONASS. Regional satnav systems that provide coverage only to a certain part of the world require far fewer satellites. IRNSS is a regional system and so its constellation will consist of seven satellites. Three of these will be geostationary over the Indian Ocean, i.e., they will appear to be stationary in the sky over the region, and four will be geosynchronous – appearing at the same point in the sky at the same time every day. This configuration ensures each satellite is being tracked by at least one of fourteen ground stations at any given point of time, with a high chance of most of them being visible from any point in India. IRNSS satellites are numbered from 1A to 1G. Five of these have been launched already; ISRO launched the fifth, IRNSS-1E, on January 20 aboard the heaviest variant of its PSLV launch-vehicle, the PSLV-XL. As with all other satnav systems, IRNSS will provide two services: a free one for civilians and a strongly encrypted one for the military. The civilian service is called the Standard Positioning Service (SPS) and will have an accuracy of up to 20 metres while the encrypted Restricted Service (RS), for authorised users only, could possibly have an accuracy of up to 10 cm — standard for most military systems. However, just as mobile phones come with a GPS receiver, users will require a special bit of hardware to receive and use IRNSS data. These receivers are currently being manufactured in the country and will be able to download signals from the GPS and GLONASS systems as well. IRNSS is independent of India’s existing regional satnav system, GAGAN (GPS Aided GEO Augmented Navigation), built by ISRO in conjunction with the Airports Authority of India. Today, GAGAN is primarily used by the commercial airline industry and by scientists studying the ionosphere. As the name suggests, GAGAN takes the help of the American GPS constellation for its functioning. IRNSS, on the other hand, aims to be autonomous and independent while still maintaining compatibility with GPS and GLONASS. Since the average lifespan of the IRNSS satellites is 9.5 years for geostationary and 11 for geosynchronous, ISRO plans to build four more satellites as backup to be augmented to the existing constellation. The last two primary satellites, IRNSS-1F and IRNSS-1G, are expected to go into orbit in February and March of this year, after which India’s very own satnav system will be available for civilian use. Sandhya Ramesh is a science writer focusing on astronomy and earth science.
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