IRNSS and Other Satellite Navigation Systems: Key Differences

Under the satellite navigation system, several satellites are placed around Earth in different orbits to provide geo-spatial positioning (location on or above the Earth in three dimensions). The satellite navigation system can be used to locate latitude, longitude, altitude, velocity and the time information. The satellites in the navigation system transmit a signal, which contains the orbital data and timing information to the receiver. To maintain the synchronization of all satellites in the constellation, the satellites use an atomic clock to measure the accurate time. The receiver receives the data from several satellites at the same time and it uses the triangulation method to calculate its position. The accuracy of the position may vary from a couple of metres to few centimetres. The more satellites the receiver can track, the more accurate the position. To calculate the four parameters of latitude, longitude, altitude and the time, the receiver needs to see at least four satellites. The result with fewer satellites will be incorrect.

Triangulation

Triangulation is a process through which the location of a point can be determined by measuring the angles to it from two known points. But the calculation becomes more complex as the satellites move very fast and the Earth is a curved surface. Because of the curvature there will be some error in the location of the receiver. To rectify the error, local augmentation systems are used. The receiver can use the regional data that can better describe the local geographical features to get the more accurate position.

In the initial days of using GPS, an error code was transmitted along with satellite signals. Because of this error, there was inaccuracy in locating the position of an object. Later terrestrial beacons were used to augment the system and account the error. But to receive the augmented signals, separate receivers were required and this had made the GPS system as commercially unviable. The error code was deactivated in 2000 and the GPS use has become widespread. But, there are chances of activation of error code or complete turn off of the signal by the US for its strategic advantage in case of its fight with any other country. This has encouraged the other countries to develop their own satellite navigation systems.

Major Satellite Navigation Systems

The satellite navigation systems around the world are of two types: global and regional. Global satellite navigation systems are used to provide coverage all over the world whereas regional systems provide coverage to one area. The regional systems generally augment the global systems but they can also be used as stand-alone systems. Today, the operational global satellite navigation systems in the world are GPS and GLONASS. The other global navigation satellite systems that are in development phase are Compass and Galileo. The regional navigation systems include IRNSS, Beidou, and QZSS.

Global Positioning System (GPS)

The GPS system of US consists of 3 segments – the satellite constellation, ground control network and use equipment. The baseline constellation of GPS consists of 24 satellites positioned  in 6 earth-cantered orbital planes with four operation satellites and a spare satellite slot in each orbital plane. The system can support a constellation of up to 30 satellites in orbit. The GPS satellites operate in circular 20,200 km 12-hour orbits at an inclination of 55 degrees. They are not in geo-stationary orbit.  The GPS system GPS can be accessed anywhere on or near the Earth where there is an unobstructed line-of-sight to four or more GPS satellites. The GPS system serves the purposes to military, civil and commercial users across the world and is freely available to anyone with a GPS receiver.

Global Satellite Navigation System (GLONASS)

GLONASS was developed by the USSR. It is composed of 24 satellites operating in three orbital planes, with eight evenly spaced satellites on each. It also provides global coverage.

Galileo

The Galileo global satellite navigation system is being under development by the European Union and European Space Agency intended primarily for civilian use. The 30-satellite system is expected to be completed in 2019.

Compass or Beidou-2

It is a global satellite navigation being under development by China. It is the second generation of China’s Beidou regional satellite navigation system. It consists of 35 satellites- five in geostationary orbit and 30 in medium Earth orbit (MEO), and is expected to complete by 2020.

Quasi-Zenith Satellite System (QZSS)

QZSS is a proposed 3 satellite regional time transfer system and Satellite Based Augmentation System for the Global Positioning System that would be receivable within Japan and Australia.

Beidou

The Beidou of China consists of 3 satellites and it offers limited coverage and applications.

Indian Regional Navigational Satellite System (IRNSS)

IRNSS is India’s indigenous satellite navigation system. The IRNSS constellation will have seven satellites in total and  two more satellites are expected to be kept ready as reserve on ground. Presently four satellites (IRNSS-1A, 1B, 1C, and 1D) have been launched. The minimum requirement is of four satellites to make it operational is successful. The launch of remaining three satellites (IRNSS 1E, 1F and 1G) by March 2016 will make it fully operational. IRNSS will offer two kinds of services:

  • Standard Positioning Service (SPS), which will be available to all the users, and
  • Restricted Service (RS), which is an encrypted service provided to specific users.

The total system would have a lifetime of minimum of 10 years.

How IRNSS is different from other satellite navigation systems?

There are several differences between India’s IRNSS and other similar systems. Firstly, IRNSS is a regional navigation system unlike the global GPS, GLONASS, Galileo and the Compass. Secondly, IRNSS is expected to provide better signal even for the civilian usages. The reason of this is that the navigation payload {IRNSS 1-B has two payloads viz. navigation and ranging} operates in L5 and S frequency bands whereas other global systems operate in L1 and L2 frequencies which are not able to correct the error that are caused due to the changes in the ionosphere characteristics. The ranging payload consists of C-band transponder for determining the range of the satellite. IRNSS is designed to provide accurate position within India and up to 1,500 Km outside of the country’s boundaries. The system would provide accuracy of better than 20 metres in the primary service area. Thirdly, IRNSS follows different configuration than other global navigational systems. Generally the navigation satellites, like the US GPS, are positioned in medium Earth orbit (MEO). In case of IRNSS, the four already launched satellites are in geosynchronous orbits and the remaining three satellites will be in the geostationary orbit. This is a departure from the current global practices and has its own challenges. However, positioning of IRNSS satellites in geo-synchronous orbit at a height of 35,786 kilometres has strategic importance when considering anti-satellite missiles. It makes IRNSS out of range of solid-fuelled intercontinental missiles and makes it a more challenging task for liquid-fuelled launch vehicles to reach this strategic height. At that height it can be easily traced by Indian government in order to take necessary actions as per the situation.

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