Global Positioning Systems
Since prehistoric times, people have been trying to discover a reliable way to tell where they are, to help guide them to where we are going, and to get them back home again. The latest application for determining our position is the Global Positioning System.The Global Positioning System (GPS) is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations. Using satellites and radio signals to pin-point a user's position, GPS provides users with accurate information about their position and velocity, as well as the time, anywhere in the world and in all weather conditions.
The GPS is comprised of three segments: the space segment, the control segment and the user segment. The 24 satellites of the space segment are arranged into six circular orbits (4 satellites per orbit) at an altitude of 20100 kilometres , inclined at 60 degrees to each other and at 55 degrees to the Earth's equator (to ensure coverage of polar regions).
Each satellite orbit's the Earth will always be in radio contact with at least 4 satellites . Powered by solar cells, the satellites continuously adjust themselves to point their solar panels towards the sun and their antennas towards the earth. Each satellite contains four atomic clocks.
The control segment consists of ground stations (five of them located around the world) that make sure the satellites are working properly. It uses measurement collected by the monitor stations to predict the behaviour of each satellite's orbit and clock. The prediction data is transmitted (via radio waves) to the satellites for transmission to the users. The control segment also ensures that the GPS satellite orbits and clocks remain within acceptable limits.
The user segment of the GPS is basically the receiver which the user can hold in...