How it works?

2. GPS transceiver triangulates from the satellites. Using distance measurements from three or more satelites, it calculates the user’s position on earth.

3. Distance is measured by timing how long it takes for a signal sent from the satelite to arrive at the receiver. Both the satellite and the receiver simultaneously generate the same pseudo-random code. The time delay before both codes will synchronize multiplied by the speed of light gives the distance.

4. Satellites have atomic clocks that can make precise time measurements, while available GPS receivers don’t. To correct this a fourth satellite distance measurement is made, providing perfect timing or atomic-accuracy clock measurement.

5. The GPS satellites are constantly monitored by the Department of Defense (DoD). Thus, they are able to measure minor orbit variations. The error information are sent to the satellite and are transmitted along with the timing signals.

6. The US Government practices a policy called “Selective Availability” to make sure that no hostile or terrorist forces can use the GPS to make accurate weapons. They introduce noise into the satellite data to prevent too accurage position calculations. Military receivers use a decryption key to remove these SA errors.

7. Differential GPS has been developed to correct the various inaccuracies in the GPS system. The earths’s ionosphere and atmosphere may cause delays in the GPS signal that may result to positioning errors. Using mathematics and modeling, these errors are factored mathematically to provide accuracy that will support the GPS’ commercial and consumer applications.