You’ll no longer have to depend on GPS to find your way, as Navigation through Indian Constellation (NAVIC) is here. A user’s guide
Arup Dasgupta | May 31, 2016
The Indian Regional Navigation Satellite, IRNSS-1G, went into geosynchronous transfer orbit on April 28 and by May 3 it has taken up its position in the geostationary orbit at 129.5 degree east. With this the IRNSS constellation of seven satellites is in position. The constellation consists of three geostationary earth orbit (GEO) spacecraft and four spacecraft in geosynchronous orbit (GSO) inclined at 29 degree to the equator. IRNSS will provide two types of services: standard positioning services (SPS), provided to all users, and restricted services (RS), provided to authorised users.
Three satellites IRNSS-1C, 1F and 1G at 5 deg inclination are called GEO satellites. IRNSS-1F is placed at 32 deg E, 1C at 83 deg E and 1G at 129.5 deg E. The four GSO satellites, IRNSS-1A, B, D and E, are placed in inclined orbit with longitude crossover of equatorial plane at 55 deg E and 111.75 deg E. GSO satellites are placed in two planes which are 180 deg apart. These seven satellites will cover a service area of 1,500 km around India with an SPS accuracy of 20 metres or better.
The IRNSS satellites carry two types of payloads: navigation payload and CDMA payload. The navigation payload operating in L5-band and S-band will transmit navigation service signals to the users. It also has a highly accurate rubidium atomic clock. The ranging (CDMA) payload consists of a C-band transponder which facilitates accurate determination of the range of the satellite. It also carries corner cube retro reflectors (CCRR) for laser ranging.
How do navigation satellites work?
They work on the principle of trilateration. Position of an object is determined by its latitude, longitude and height above mean sea level. If at the time of measurement the instantaneous position of three satellites are known and the distance of the point of measurement from each of these three satellites is known then the latitude, longitude and height of the point can be determined using simple distance formula.
How does a GPS receiver determine its distance from each satellite? It does so by comparing a code generated by a satellite with the same code generated internally in the receiver. The time difference between the two codes multiplied by the speed of light gives the distance. That requires a very stable signal source on the satellites which is provided by the rubidium clock which is used to generate the code and the carrier signal for the code. Each satellite has a unique code; hence the receiver can identify each satellite in its view. The carrier also contains the precise orbital parameters of the satellite which is updated regularly. The rubidium clocks on the satellites are also synchronised regularly. The IRNSS satellite downlink signals are in L band and S band. The L band signal cannot be received by standard GPS receivers as the L band frequency of IRNSS is different from the standard GPS satellites. Therefore, both the L and S band will require new receivers.
The precise position of each satellite is determined through CDMA Ranging using the C-band ranging transponder and the Corner Cube Reflectors which reflect laser ranging signal transmitted from the Laser Ranging ground stations. There are 14 IRNSS Range and Integrity Monitoring Stations (IRIMS) located in different parts of India and more are planned in countries outside India. The IRIMS are very precisely located. They receive signals from the IRNSS satellites and establish the integrity of the constellation. Data from the IRIMS, CDMA and Laser ranging stations are fed to the IRNSS Navigation Centre at Byalalu where the entire system is controlled and all orbital and timing updates are generated and fed back to the satellites.
Why are IRNSS satellites not in perfect geostationary orbits?
Since IRNSS is a national system it makes sense to have them in geostationary orbits so that they are always visible. However this would limit the service area to only a part of India. By adopting GEO and GSO orbits the service area is extended from 40 degree East to 140 degree East and from 40 degree North to 40 degree South. More specifically this arrangement can give about 20 m accuracy for about 1,500 km around India and better on the Indian mainland. The choice of seven satellites ensures that any receiver in the service area will always see at least four satellites at any time. Even though three satellites are ideally enough to determine a three-dimensional position, a fourth satellite is required to determine the bias between the satellite clocks and the receiver clock.
The need for Indian GPS
Why did India opt for such a system when other systems are already available like GPS, GLONASS and Galileo? All these systems are controlled by other nations and their continuity of service for global users is not guaranteed. Location is not just for finding the closest pizza parlour when you are hungry! It is of utmost importance for locating and tracking mobile assets like trucks, earth-moving machinery and containers to name a few. Other mission critical areas are disaster management and most importantly the C4ISR programme of the armed forces.
ISRO and the Airports Authority of India (AAI) joined hands to develop GPS Aided Geo Augmented Navigation (GAGAN), a wide area augmentation service for the Indian region using GPS signals and real-time corrections of the GPS signals to assure a positional error of less than one metre for aircraft navigation. Similar systems are in operation in the US as WAAS, Europe as EGNOS and Japan as MSAS. This experience and the experience of developing advanced satellites for remote sensing and communications gave ISRO the confidence to plan a regional navigation satellite system which was christened IRNSS and is now Navigation through Indian Constellation (NAVIC).
When will we get to use NAVIC?
According to Accord Software and Systems, a private company which is working with ISRO and AAI on the GAGAN project and now on the IRNSS user hardware, the easiest way is to use an add on IRNSS dongle to any smartphone having USB On-The-Go port and a software in the phone to view the dongle output.
Smartphones with NAVIC support in-built is some way off and will depend on how fast the system picks up in the commercial market. This is because an additional receiver and antenna is needed in the smartphone to pick up IRNSS signals which are in the L5 and S bands. Today’s smartphones in India use receiver chips that mainly support GPS. Some also support GLONASS and Beidou. All these work around the L1 band which has resulted in a quicker development of multi-constellation receivers for GPS, GLONASS and Beidou.
Who all can use NAVIC?
For commercial users there are several other products which can be used for both navigation and asset tracking. Hardware and software for using IRNSS for other technical applications like precise timing and mapping and geodetic data capture are also available. Interestingly, GAGAN will not use NAVIC. This is because AAI has to cater to international users whose aircraft are already equipped for using WAAS, EGNOS and MSAS. GAGAN has to maintain compatibility with these systems. The biggest immediate users for NAVIC will be the armed forces who will also have access to the very accurate Restricted Service signals on the L5 and S bands. Other users are likely to be government agencies like the railways, survey of India and ONGC. It is hoped that with the pickup of usage in the public and defence sectors the receiver costs will reduce to a level where industry will begin to look at market possibilities in the commercial and consumer sectors where volumes will result in further reduction of prices.
IRNSS, now NAVIC, is a bold venture which is the hallmark of Made in India. The scientists and technologists have done their bit. It is for the industry to take off from here and make NAVIC a commercial success not only in India but in neighbouring countries as well.
Prof Dasgupta is a fellow of the Indian Society of Geomatics (ISG) and the IETE, India. He worked in the Space Applications Centre, ISRO from 1970 to 2005.
(The column appears in the June 1-15, 2016 issue of Governance Now)
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