Global Positioning Systems

By: John Ronan
Did you know that there is a constellation of 24 satellites currently in orbit around the earth twice a day at an altitude of about 12,000 miles?
For twenty-four hours a day, these satellites broadcast high-frequency radio signals containing position, time and date; and enabling anyone with a Global Positions System (GPS) receiver to determine their location anywhere on earth. The GPS signals are available to an unlimited number of users at the same time.
With the same general shape and size a GPS receiver looks a little bit like a mobile phone. However, with this device and using the information transmitted from the GPS satellites you can pinpoint your location from anywhere in the world as long as you can ‘see’ the sky. Your coordinates are then displayed on a map on the GPS receiver’s LCD Display. The GPS unit has several MegaBytes of built in memory, including a database of cities worldwide, political boundaries, roads, parks, waterways and railways.
Every point on the Earth can be identified by a specific address. By using two sets of numbers referred to as coordinates, which represent the exact spot where a horizontal line (latitude) crosses a vertical line (longitude), you can represent any location precisely. GPS receivers report and record your current position – or the position of any place you’ve been or would like to be – with latitude and longitude coordinates. GPS receivers also produce other critical navigation information, including heading, bearing, distance-to-go, time-to-go, anytime, anywhere, in any weather.
The basis of GPS technology is precise time and position information. Using atomic clocks (accurate to within one second every 70,000 years) and location date, each satellite continuously broadcasts the time and its own position. A GPS receiver uses signals from three or more satellites at once to determine the user’s position on earth.
By measuring the time interval between the transmission and the reception of a satellite signal, the GPS receiver calculates the distance between the user and
each satellite. Using the distance measurements of at least three satellites in an algorithm computation, the GPS receiver arrives at an accurate position fix. Information must be received from three satellites in order to obtain two-dimensional (latitude and longitude) fixes, and four satellites are required for three-dimensional (latitude, longitude and altitude) positioning. The position information in a GPS receiver may be displayed as longitude/latitude, Military Grid or other system coordinates.
In the 1970s the U.S.
Department of Defence began development of the GPS satellite navigation system to provide continuous, worldwide positioning and navigation data to the U.S. military forces around the globe. However, GPS has even broader civilian applications. Position and navigation information is vital for many professional and personal activities, including boating, surveying, aviation, vehicle tracking, navigation, and more.
To meet these different needs, two levels of GPS services were implemented, one for civilian access and the second encrypted for exclusive military use.
Initially the civilian GPS, signals were subjected to Selective Availability (SA) interference by the United States Government, which meant there were random errors in the data transmitted by the satellites to reduce the civilian GPS signal accuracy to 100 meters for security purposes.
However, on May 1st, 2000, the U.S. government removed SA from GPS signals, which resulted in ten times greater accuracy for public users of GPS, i.e. position fixes that are usually within 10 meters and there are methods by which one can increase the accuracy of position readings so that they are accurate to within three meters or better.