Monday, April 20, 2015

Indian GPS or We call it Desi GPS.


      India is all set to rule skies with its own navigational system. The fourth in the series of seven navigational satellites IRNSS-1D is successfully launched from Sriharikota. With this launch, the country is poised to operationalise the Indian Regional Navigation Satellite System.

       The latest in IRNSS series was IRNSS-1D having a mission life of 10 years. The rocket, the Polar Satellite Launch Vehicle (PSLV-C27), standing around 44 metres tall and weighing around 320 tonnes carried 1,425 kg IRNSS-1D. The rocket blasted off from the second launch pad at the Satish Dhawan Space Centre here, around 80 km from Chennai.  

     The two solar panels of IRNSS-1D consist of Ultra Triple Junction solar cells which generate about 1660 Watts of electrical power. Sun and Star sensors as well as gyroscopes provide orientation reference for the satellite. Special thermal control schemes have been designed and implemented for some of the critical elements such as atomic clocks. The Attitude and Orbit Control System (AOCS) of IRNSS-1D maintains the satellite's orientation with the help of reaction wheels, magnetic torquers and thrusters. Its propulsion system consists of a Liquid Apogee Motor (LAM) and thrusters.

     IRNSS-1D is first launched into a sub Geosynchronous Transfer Orbit (sub GTO) with a 284 km perigee (nearest point to Earth) and 20,650 km apogee (farthest point to Earth) with an inclination of 19.2 deg with respect to the equatorial plane. After injection into this preliminary orbit, the two solar panels of IRNSS-1D are automatically deployed in quick succession and the Master Control Facility (MCF) at Hassan took control of the satellite and performed the initial orbit raising manoeuvres consisting of one manoeuvre at perigee (nearest point to earth) and three at apogee (farthest point to earth). For these manoeuvres, the Liquid Apogee Motor (LAM) of the satellite is used, thereby finally placing it in the circular geostationary orbit at its designated location.

      IRNSS-1D satellite has two payloads: a navigation payload and CDMA ranging payload in addition with a laser retro-reflector which can be used for range calibration to determine precisely the spacecraft’s position in space. The payload generates navigation signals at L5 and S-band. The design of the payload makes the IRNSS system inter-operable and compatible with Global Positioning System (GPS) and Galileo. Reaction wheels and magnetorquers are also fitted to help control the spacecraft’s attitude. 

      The GPS is a space-based satellite navigation system managed by the United States that provides location and time information in all weather conditions, anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites. The system provides critical capabilities to military, civil and commercial users around the world. It is freely accessible to anyone with a GPS receiver. Russia’s GLONASS, China’s Beidou and EU’s Galileo too work on the same scenario.

      The INRSS Architecture will consist of three segments: space, ground and user. 

IRNSS Architecture
IRNSS Architecture

  • The IRNSS Space Segment: consists of a constellation of seven satellites which will focus on the region up to 1,500 km beyond India’s boundaries, between longitude 40° E and 140° E, and latitude ± 40°. Three will be placed in geostationary orbit located at 34° E, 83° E and 131.5° E; the other four in geosynchronous orbit at an inclination angle of 29°, two each with the equator crossing at 55° E and 111° E. The three geostationary satellites will appear fixed in the sky, while the four geosynchronous satellites will appear to move in the figure of ‘8’ when observed from the ground.
  • IRNSS ground segment: consists ground stations for generation and transmission of navigation parameters, satellite control, satellite ranging and monitoring. The critical ground segment of the IRNSS is ISRO Navigation Centre (INC) located at Byalalu, about 40 km from Bangalore. INC is responsible for providing the time reference, generation of navigation messages and monitoring and control of ground facilities including ranging stations of IRNSS. It hosts several key technical facilities for supporting various navigation functions. 
  • The IRNSS User segment: is made of the IRNSS receivers. They will be dual-frequency receivers (L5 and S band frequencies) or single frequency (L5 or S band frequency) with capability to receive ionospheric correction. They will be able to receive and process navigation data from other GNSS constellations and the seven IRNSS satellites will be continuously tracked by the user receiver. The user receiver will have a minimum gain G/T of -27 dB/K.
    IRNSS has a different configuration than other global navigational systems. Normally, navigational satellites, like the American Global Positioning System (GPS), are positioned in medium Earth orbit (MEO). In case of IRNSS, four satellites will be in inclined geosynchronous orbits and the remaining three satellites in the geostationary orbit. This 'desi GPS' will be similar in function to the American Global Positioning System (GPS) but regional in coverage. It will provide two types of services:

  • Standard Positioning Service (SPS for civil usage and will be provided to all the users
  • Restricted Service (RS), an encrypted service provided only to the authorized users, mainly to security and intelligence organizations(military)
IRNSS System
IRNSS System

  The main features of IRNSS include: 

  • It will provide a position accuracy of better than 20 meters in the primary service area.
  • IRNSS consists of a space segment and a ground segment; the space segment comprising seven satellites, with three satellites in geostationary orbit and four satellites in inclined geosynchronous orbit.
  • IRNSS satellites would revolve round the earth at the height of about 36,000 kilometres from the earth's surface. 
  • It will be useful in land, sea and air navigation, disaster management, vehicle tracking and fleet management, integration with mobile phones, provision of precise time, mapping, and navigation aid for hikers and travellers, visual and voice navigation for drivers.
  • It can track people or vehicles and can be of immense use in disaster situations like last year’s flash floods in Uttarahand. 
  • It will be a boon for the railways for tracking wagons.
  • A highly accurate Rubidium atomic clock is part of the navigation payload of the satellite.
  • Highly accurate position, velocity and time information in real time for authorized users on a variety of vehicles.
  • Data with good accuracy for a single frequency user with the help of Ionospheric corrections.
  • All weather operation on a 24 hour basis.

    After America, Russia, Europe, China and Japan, India will be the sixth country in the world to have this system. This is necessary in times of war since most modern precision bombs and missiles depend on accurate positioning. Till now most of us have been relying on the American GPS which is very popular on smart phones but not good enough for military applications as it can't be relied upon for seamless coverage in times of war.

     India’s adversaries, like China and Pakistan, are nuclear weapons countries and they also have a significant inventory of state-of-art missiles. There also exists a possibility that the US could deny others access to GPS during political disagreements. Like in the past, during Kargil war, US suspended GPS for both India as well as Pakistan.  Naturally, India needs to have a reliable and accurate space-based navigational system. Hence our own IRNSS.

   Predecessors IRNSS-1A, 1B and 1C were launched by PSLV-C22, PSLV-C24 and PSLV-C26 in July 2013, April 2014 and October 2014 respectively. All the satellites are functioning satisfactorily from their designated orbital positions. India expects to complete the seven-satellite constellation by end of this year or early next year by launching the remaining three satellites in quick succession.

   IRNSS will provide positional accuracies similar to the GPS: 10 m over the Indian landmass, 20 m over the Indian Ocean. As is the case with GPS and the US military, IRNSS will provide a more accurate restricted service for the Indian armed forces and other special authorised users. It is not so accurate as GPS(accuracy in mm), but in short future ISRO will improve its accuracy in millimetre range. 

   After IRNSS-1D becomes operational in the coming months, ISRO will make India a proud owner of its own indigenous regional, satellite navigation system. Once the regional navigation system is in place, India need not be dependent on other platforms.  India also proposes to offer the IRNSS signals to the states in its neighbourhood, a novel way of using space assets for diplomacy. It appears that for India IRNSS is not about any competition, but instead a way to attain strategic parity.

Please give your views.....


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