Global Positioning System

GLOBAL POSITIONING SYSTEM Q. What is a GPS? Ans. GPS or Global Positioning System is a space based radio navigation system developed by the US Department of Defense, now available for civilian users worldwide. For anyone with a GPS receiver, the system provides location and time. Q. What are the components of the GPS? Ans. The GPS is based on the Global Navigation Satellite System (GNSS), a constellation of 24-32 medium earth orbit satellites that transmit microwave signals to the GPS receiver. From these signals the receiver calculates the latitude, longitude, altitude and time for the specific user. Its official name is NAVSTAR GPS. The GPS can be divided into three parts: satellites orbiting the earth; control and monitoring stations on earth; and the receivers. Q. What is the basic principle of GPS? Ans. Suppose you are somewhere in India but don’t know exactly where. A passerby tells you that you are 115 km from Chittorgarh. You now know that you are somewhere on a circle around Chittorgarh with a radius of 115 km. Another person tells you that you are 185 km from Mount Abu. Now you have two circles and mathematically two circles have two intersection points, so you know that you are either in Udaipur or Rajsamand. Now a third person tells you that you are 262 km from Ahemdabad. Mathematically, three circles can intersect at only one point and hence you can deduce that you are in Udaipur. GPS uses a similar principle, except that instead of using two dimensional circles three dimensional spheres are used to locate positions. Q. How does it work? Ans. The original GPS design consisted of 27 satellites, of which 24 were operational and three for backup purpose. Each of these was making two complete rotations of the earth everyday. The satellites were so arranged that from any place on earth at least four were visible. A GPS receiver gets continuous feeds from the visible satellites. The satellites transmit messages which include the time and precise orbital information of the satellite. A receiver on earth getting feeds from the satellites can use ‘trilateration’ to calculate the point of intersection of three spheres each centred at three satellites- say A, B, C. This point of intersection is his position. Q. Why a minimum of 4 satellites? Ans. Three satellites would be enough if only three variables- latitude, longitude and altitude- were involved. But a fourth variable is introduced since the satellites are moving at high speed and minor time lag between the received signals of two satellites can cause large positional error. Hence a fourth variable, which is the time of sending of the signal. is introduced and the receiver has to solve four equations to derive four variables and hence the need for a minimum of four

satellites. But the original design has been changed for more precision and the current GPS constellation consists of 31 actively broadcasting satellites. Q. What are its applications? Ans. Primarily developed for determining position, GPS has become a mainstay of transportation systems, providing navigation for aviation, ground, and maritime operations. Apart from this banking, mobile phone operations, and even the control of power grids, depends on GPS for accurate timing provided by the system. Q. How reliable is the GPS? Ans. GPS signals at terrestrial receivers tend to be relatively weak and hence the signals get distorted by other source of electromagnetic radiation like solar flares, the earth’s magnetic field, or man made electromagnetic interfaces. GPS also includes a feature called selective availability (SA) that can tell about the random errors in reception. But this service is currently deactivated for civilian use. To make it more accurate, the Differential Global Positioning System (DGPS) is used for civil navigation purposes. The DGPS also uses a network of fixed, ground-based reference stations. Differences between the positions indicated by the satellite systems and the known fixed positions are used to correct errors. Q. What are the other global navigation satellite systems? Ans. At present, GPS is the only fully operational GNSS. The Russian GLONASS system was also operational for a short period but fell into disrepair with the collapse of the Russian economy. GLONASS is being restored with the help of ISRO and will be fully operational with all 24 satellites by 2010. Apart from this, there are several ongoing projects. China is developing Beidou while EU with the help of India, China and Israel is developing Galileo. India is also developing Indian Regional Navigational Satellite System (IRNSS) based on seven satellites and it will provide positions throughout India and approximately 2,000 km around it.