The GPS Navstar satellite transmits a radio signal unique to each individual satellite. The signal is essentially omnidirectional, although there is a preferential orientation toward the Earth since the satellite’s antennas are located on one side of the vehicle which is, of course, aimed at the Earth. For simplicity’s sake, let’s assume that the signal is truly omnidirectional and that the satellite broadcasts its signal uniformly outward in all directions. If we happen to know that the range (GPS Tracking For Cars ) to a particular satellite is precisely 20,000 kilometers (for example), then the only place in the universe which is that precise distance from the satellite is somewhere on the surface of an imaginary sphere that has a radius of 20,000 kilometers. With only this amount of information there is no way to know where on the sphere we might be located, only that we’re no closer than 20,000 kilometers and no farther than 20,000 kilometers. It could be in any direction. Remember, there is no direction information given in the satellite’s signal.
Selective Availability is far and away the worst source of error in GPS positioning, producing up to 70 meters of positional displacement alone. And it’s deliberate! Selective Availability is the intentional degradation of the GPS signal by either dithering the clocks or the orbital information to produce incorrect satellite positions and, thereby, provide incorrect receiver positions. The purpose is to limit accuracy for non-PPSauthorized users to a 95% probability of 100 meters or less. The amount of error induced into each satellite’s clock and ephemeris data varies from satellite to satellite and is continuously varied in degree over periods of hours. This means that the error can’t be averaged out with data collection periods of less than several hours, effectively eliminating any possibility of acquiring the higher potential accuracy of Portable GPS in real-time.
The diagram at left illustrates what a long-term (several hour) position-plot would look like with Selective Availability turned on. While the actual receiver position is located at zero-zero, the receiver-perceived position drifts over an area as wide as 100 meters (or possibly even more for as much as 5% of the time) from that true point. It’s interesting to note that during the Gulf War a large number of civilian receivers were fielded by the military which were unable “see through” the Selective Availability the way that military-designed receivers can. Consequently, the Department of Defense had to turn SA off for the duration of the war, which is directly opposite the intended purpose of SA! As soon as the war was over, they promptly turned it back on. With the declaration of Full Operational Capability (Wireless GPS Tracker ) in July, 1995, with a full complement of Block II satellites in orbit and operating, Selective Availability has been turned on continuously. However, on March 29, 1996, President Clinton released a Policy Fact Sheet that declared that the S/A would be turned off permanently within the next four years. When exactly can’t be said although most industry analysts believe that it will be considerably sooner rather than later.
More information at http://www.jimilab.com/ .