GPS: An Introduction

Global Positioning System (GPS) is satellite-based navigation system which records and displays location of an object on earth (in terms of coordinates, e.g. latitude and longitude). It also shows height of a place above mean sea level. Like many other remote sensing and GIS devices, application of GPS devices was also first started for military and defense purposes. Slowly these devices made entry into public domain for various civilian uses. Now GPS has become so popular that it is being integrated in new generation mobile phones. 

A GPS device receives signals from satellites and these signal codes are processed & converted into values showing position, time and velocity (what we see on screen of GPS device). The instrument to which we generally call ‘GPS’ is basically a GPS receiver and it is a small part of a large system. A complete Global Positioning System consists of three segments- space segment, control segment and user segment. 

A Handheld GPS

Space Segment 

Space segment consists of dedicated satellites for Global Positioning System and are referred as space vehicles (SVs). There are twenty four satellites in a nominal GPS constellation. Out of these twenty four satellites three are spare satellites which start operating if some of the functional twenty one satellites have some operational problem. These satellites remain in six orbital planes (four in each) and are positioned such that five to eight of these SVs visible from any place on the earth at a given point of time. 

Control Segment 

The control segment consists of tracking stations located around all over world. There are master control station and monitoring stations in control segment. 

User Segment User segment of GPS is what we uses i.e. GPS receiver. It also includes the GPS user community. A GPS receiver consists of a screen for displaying information, buttons to operate it and antenna to receive signals from satellites. 

The antenna may be in-built in receiver instrument or it may be externally attached to it. Signals from minimum four satellites are required for a GPS receiver to compute location (x, y & z) and time (t). GPS receivers are essential part of navigation system of air crafts and ships. Now-a-days these are also being integrated in surface-based navigation (trains, buses, cars & other vehicles).

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