What is GPS?
The Global Positioning System (GPS) was developed by the Department of Defense and consists of a group of 24 satellites which are monitored by five ground stations. It essentially allows you to pinpoint your location anywhere on the surface of the Earth, even in cloudy weather, with the use of a GPS receiver. The GPS receiver is a navigational device that uses these satellites as reference points to calculate your position on the ground. It does this by triangulating your position between at least 3 satellites. The GPS receiver uses the time it takes the coded radio signal to get from the satellite to the receiver to calculate the distance it is from that satellite. So, by accurately measuring the distance from the ground to these satellites, it can triangulate your position.
GPS consists of 3 basic parts:
The control part is the central part of GPS. This is where the satellites are monitored and adjustments (atmospheric corrections, timing corrections, etc.) are made. There are 5 stations located worldide and each satellite passes over a monitoring station twice a day.
The space part is the NAVigation Satellite Timing And Ranging (NAVSTAR) group of satellites that bradcast the GPS signals. There are 24 satellites orbiting at about 20,200km above the Earth. They each make one revolution approximately every 12 hours.
Consists of a user and a GPS receiver. The possible applications of GPS are limitless.
GPS is based on satellite ranging. This technique of measurement is based on timing how long it takes a radio signal to travel from the satellite to the GPS receiver and then using this time to calculate the distance. There are basically two factors involved. The speed of the radio signal (approximately 300,000km/s) and the time it takes for the signal to reach Earth. The timing is the critical part and requires the use of a very accurate clock, and to facilitate this the satellites do have atomic clocks on board. "Because if the timing is off just a thousandth of a second, at the speed of light, that translates to almost 200 miles of error" (Trimble).
Trilateration refers to the geometric approach to measuring the distance from at least three satellites to determine a position on the surface of the Earth. Basically, the radio waves transmitted from the satellites radiate out in a spherical pattern at the speed of light. Since trigonometry requires three perfect measurements to determine a position in three-dimensional space, the position where these signals intersect with the surface of the Earth and bisect each other (position P on diagram) determines your position on the surface. In three-dimensional space this actually narrows it down to two positions, however one of those positions will generally be rejected mathematically by the receiver as...