Global Positioning System

The Global Positioning System

Introduction

1.   There had been a requirement for a world wide based navigational aid   with an accuracy of a few meters for quite some time. With the introduction of global postioning system, this requirement has been very closely met.   The development of nav star GPS is a real time postion fixing system and is a more accurate navigational aid than any other existing system with   world coverage.   As per the plan an absolute accuracy of 100m will be available to general users of all the   countries and an accuracy of 16 m to the US & NATO military and other authorised users.

2.   Besides position fixing in three dimension (longitude, latitude and altitude above sea level) GPS provides the user with velocity information and with Universal time coordinated. The construction and working principle of GPS will be covered under the following heads:-

(a) GPS Satellite orbits

(b) General description

(c) GPS position fixing
(i)   GPS clocks
(ii)   Determining the position of a satellite
(iii) Equations for ranging
(iv)   Determination of the user's position
(v)   Determination of the user's velocity
(d)   Satellite frequencies

(e)   Accuracy of GPS

(f)   Other existing and future satellite systems

GPS Satellite Orbits

3.   There are 18 satellites in this system which circle the earth in orbit with a period of 12 sidereal hours.   The satellite travels at a velocity of 3.9 km/sec   at an altitude of 20200km.   The advantage of such a great altitude is that the orbits will be less affected by irregularities caused by the unequal distribution of masses in the earth.

    GPS measures the velocity of the user by means of the doppler effect.   For this application the greater altitude is a disadvantage because the doppler curves becomes flatter, thus affecting the accuracy of the measurements. As near the centre of the orbit, there is almost no doppler effect.

4.   The 18 satellite constellation is configured in six...