Global Positioning System
Global Positioning System
The Global Positioning System involves the use of transmission of at least 4 radio wave signals from a “constellation” of 24 earth-orbiting satellites at one time.
A Global Positioning System (GPS) unit consists of a space segment, a control segment, and a user segment. The space segment is a constellation of two-dozen satellites orbiting the earth twice every 24 hours, at approximately 10,900 nautical miles above the earth’s surface. The control segment is a series of monitoring stations located at different sites on earth. These stations update and correct errors in the navigational message of the satellites. The user segment is a receiver that receives radio waves from the satellites in orbit, which keeps track of how far away each satellite is.
In general there are normally 8 or so satellites “visible” to a GPS hand-held receiver at any given moment. Each satellite contains an atomic clock. The satellites send radio wave signals to the GPS receivers so that the receivers can find out how far away each satellite is at a given time. From this, the receiver is able to work out how far it is from the satellite. Since we know how fast radio signals travel — they are electromagnetic waves traveling at the speed of light, about 186,000 miles per second in a vacuum – we can figure out how far they have traveled by figuring out how long it took for them to arrive. The use of these waves are very convenient as they travel extremely fast and can travel through space, and are not damaging to human beings.
With this information from four or more satellites, the receiver is able to calculate its position to quite a high accuracy on the Earth. Information from four satellites is used simultaneously to pinpoint the precise position of the receiver on the earth. Information from the first three satellites narrows down the range of possible locations to two points; one of these is usually illogical and indicates a point not on the earth. A fourth satellite is used to confirm the target location. We also have to be outside in a fairly open area for the GPS receiver to work.
If it does work, the receiving unit uses triangulation to calculate extremely accurate measurements of the user’s position, velocity and time. This is accurate enough to potentially allow an aircraft to make a safe landing on a fog-bound runway, guided only by the GPS, or be used to measure the sluggish drift of the continents. If there is several receivers scattered everywhere, then the accuracy of a GPS can be narrowed down to 5-10 metres.
GPS can also be used to measure distance. At a particular time (let’s say midnight), the satellite begins transmitting a long, digital pattern called a pseudo-random code. The receiver begins running the same digital pattern also exactly at midnight. When the satellite’s signal reaches the receiver, its transmission of the pattern will lag a bit behind the receiver’s playing of the pattern.
The length of the delay is equal to the signal’s travel time. The receiver multiplies this time by the speed of light to determine how far the signal traveled. Assuming the signal traveled in a straight line, this is the distance from receiver to satellite. The signal transferred is the electromagnetic radio wave.
In order to make this measurement, the receiver and satellite both need clocks that can be synchronized down to the nanosecond. To make a satellite positioning system using only synchronized clocks, one would need to have atomic clocks not only on all the satellites, but also in the receiver itself. However atomic clocks cost somewhere between $50 000 and $100 000, far too expensive for ordinary consumer use.
So the Global Positioning System has a clever and effective solution to this problem. Every satellite contains an expensive atomic clock, but the receiver itself uses an ordinary quartz clock, which it constantly resets.
www.howstuffworks.com (Brain M. How a GPS Receiver Works.)