24 GPS satellites have been launched in space and are at a distance of 20,200 km revolving around the Earth. Whether its day or night, sun or rain, heavy snow or light drizzle, these satellites continue to provide signals that can be received by any GPS receiver to locate its position on the Earth. Officially this system was named NAVSTAR GPS (Navigation Signal Timing and Ranging Global Positioning System). The project was started in 1978 when they launched their first satellite for testing. In 1994, the project of launching 24 satellites for global positioning completed and these 24 satellites revolve in six planes with each plane containing four satellites.

There are three main segments of GPS – space, control and user. Space, as you may have already guessed, consists of the 24 operational satellites in six orbital planes. Any GPS receiver receives three signals from four satellites in one of these six planes. The control unit consists of monitor stations, ground antennas and master control station. Monitor stations are located at Hawaii, Kwajalein, Ascension Island, Diego Garcia, Colorado Springs. The user segment is composed of receiver devices that receive signals and provide positioning and velocity information to the user.

GPS has its major application in the field of Global Information Systems (GIS). It greatly facilitates the process of map-making and other similar applications. Any GPS receiver will pinpoint its own location by receiving time signals from three satellites and determining the longitude and latitude. Most GPS receivers these days are designed with a visual map display screen. These kinds of receivers are found in almost all new car models and they tell you exactly where you are by showing your position on a map. These receivers sometimes also tell you the speed at which you are moving and the direction, as in North, South, East or West.

Mountain climbers and hikers now make use of a GPS handheld device instead of compass. Popular GPS-based services are MapQuest and TomTom. Stolen mobile phones that come with integrated GPS can be easily tracked down. Mobile phone GPS systems have enabled other applications to be developed in this field, such as location-based games for entertainment purposes. These kinds of games are like network games, except that they are played by physically moving here and there and tracking down your rivals.

GPS has many applications in the fields of geology, geophysics, seismic data processing etc. There are two levels of services provided by GPS – the Standard Positioning Service (SPS) and the Precise Positioning Service (PPS). SPS is available to all users across the globe and it is provided on the GPS L1 frequency. This frequency contains a C/A (Coarse Acquisition) code. PPS on the other hand is meant for accurate military positioning. This service is authorized by the US precision code. C/A has a 1.023 MHz chip rate, whereas Precision code has a rate of 10.23 MHz.

A few years ago GPS receivers were huge in size and could not be carried around easily. They were mostly designed to be used for special and emergency purposes only. Today these receivers have been reduced to the size of a cell phone or any other handheld device. They are meant to be carried around in a pocket. These systems are personalized to be used as a personal tracking system, vehicle tracking system or engine board series.