GLOBAL POSITIONING SYSTEM

Before executing any project the station points used in surveying are to be identified: surveyors used permanent objects as reference points and made reference sketches of station points for this purpose. Navigators used stars and sun as references. Sometimes when the project is taken up so called parmanent object (as building corner) may not exist when the execution of project work is taken up. Weather conditions may obstruct the observations for navigations. Today by using an instrument called Global Positioning System (GPS) this problem is over- come. This was developed by United States defence department and which was called as Navigational System with Time and Ranging Global Positioning System (NAVSTAR) or which is simply known as GPS now.

By US air force there are 24 geostationary satellites positioned around the earth. To locate any point on the earth these satellites are used as reference points. These satellites are at an altitude of 20200 km above the earth. The 24 satellites are positioned such a way that from any point on the earth a minimum of 4 satellites are visible.

A user require only GPS receiver. From satellites the receiver measures the travel time of the signals and calculate position (latitude and longitude) and with reference to a selected datum the elevation altitude of the station. The advantages of using GPS are following:

1. Not only in day but also in night it can be used.

2. Intervisibility of the 2 stations on the earth is not a requirement.

3. Time required in establishing the position of a point is much less.

4. Requirement of man power is less.

5. Accuracy is very high. Most expensive GPS provide accuracies within 10 mm.