Global Positioning System

Now days GPS is a very common feature in a smart phone, even in the entry level mobile phone. But only few people knows how it works exactly. GPS is a satellite based service e available for position And velocity determination of any vehi me, boats or ships, aircrafts or any moving body even though animals. This service is can also be useful for civil construction companies.

In this total 21nos. of satellites are needed for complete coverage of the earth surface. When full coverage  becomes avaible, a constellation of 21 satellite placed in nearly 20,000km high circular orbit will provide highly precise, continuous data.

Principle of GPS fix

GPS satellites are not geostationary. These are orbiting the earth all the time. Satellites continuously transmit their own precise orbital data (called 'ephemeris'). The GPS identifies computers & their locations by receiving this data. Using this data, GPS receiver measures very accurate distances to the satellite. Here the modulation technique used is 'spread spectrum modulation'. Since satellite locations and their distances are known already, GPS can now fix its own position by triangulation method.

Principle of Operation

In August 1974, after many years of development and refinement, a project by the name GPS/Navstar was announced by the US Air-force. Under this program, a 24 satellites have been placed in a six-plane, 12-hour orbit. Each satellite transmits a number of signals that can be used for position and time measurement.

 Two L-band carrier frequencies L1 and L2 are modulated by accurately timed pseudo-random binary sequences. Thease sequences includes information used to determine accurately the time these signals were transmitted, satellite clock correction and where the satellite was located at that time.

The GPS receiver, using advanced mathematical techniques(triangulation techniques), measure the distance of at least four satellites from the user point. The receiver measures the time of arrival of each signal. This determines the range of each satellite based on the speed of radio-wave. Knowing the range and the number of satellites and satellites positions determines its own end user's position. By this, user's longitude, latitude and altitude are determined.



 A typical GPS navigator receiver operates on what is known as L1 carrier frequency and tracks the C/A code. The receiver has multiple channels(usually 4) and provides continuous tracking. But it is not accurate enough. 

With the use of improved techniques known as differential GPS, higher accuracy are obtained. In DGPS, a reference station computes the errors in positional solution at some known location and sends these errors to mobile users to enable applying correction factors. With these, accuracy of 5 to 10 meters are obtained.