How GPS works?

The NAVSTAR-GPS (NAVigation System with Timing And Ranging Global Positioning System) commonly known as GPS, is a U.S. space based radio navigation system that provides reliable positioning, navigation and timing services to users on a continuous worldwide basis, freely available to all. GPS was originally intended for military applications, but in the 1980’s the government made the system available for civilian use. GPS works in all weather conditions, anywhere in the world, 24 hours a day. GPS was created by the U.S. Department of Defence (DOD) and was originally run with 24 satellites and there are currently 28 operational satellites orbiting the earth at a height of 20,180 km on 6 different orbital planes. Their orbits are inclined at an angle of 55° to the equator, ensuring that at least 4 satellites are in radio communication with any point on the planet. Each satellite orbits the earth in approximately 12 hrs and has 4 atomic clocks on board.

Before getting into the details of how the GPS works we should first know what signal transit time is and how it is measured. At some point or the other on a stormy night, you might have wondered how far away you are from a flash of lightning. This distance can be measured quite easily:

Distance = time the lightning flash was first perceived (start time) until the thunder is heard (stop time) multiplied by the speed of sound. The difference between the start and stop time is called transit time.

Hence distance = transit time * speed of sound

The GPS system exactly works under the same principle. In order to calculate one’s position, all that needs to be measured is the signal transit time between the point of observation and four different satellites whose positions are known.

Each satellite transmits its exact position and its precise on board time to earth at a frequency of 1575.42 Mhz. These signals are transmitted at the speed of light (3,00,000 km\s) and therefore require approx. 67.3 milli seconds to reach a position on the earth’s surface. The signals require a further 3.33 micro seconds for every extra km of travel.

Measuring the signal transit time and knowing the distance from a satellite isn’t enough to calculate one’s own position in 3-D space. To achieve this, 4 independent transit time measurements are required. It is for this reason that signal communication with 4 satellites is needed to calculate one’s exact position. All this time we have been assuming that the signal transit time that we measured was precise and highly accurate, but an error of 1 micro second can produce a positional error of 300m. As the clocks on all the satellites are synchronised, the transit time in all the measurements is inaccurate by the same amount. From the basic knowledge of mathematics we know that to solve a problem with n no. of unknowns, we need n no. of equations. To precisely determine the position of a person we need 4 equations i.e. one each for longitude, latitude, height and time error hence 4 satellites are required.

The most important applications of GPS are to determine one’s exact location and the precise time anywhere on the earth. The traditional fields of application for GPS are surveying, shipping and aviation. However with the increasing demand and popularity of the GPS, now it is being used in archaeology, geology, cartography, forestry and agricultural sciences, planning and managing of plantations, fleet management, navigation systems, trekking and sports activities and most importantly military services for which it was mainly developed for.