5.4 - GPS ERRORS

Intentional Errors! As hard as it may be to believe, the same government that spent $12 billion to develop the most accurate navigation system in the world is intentionally degrading its accuracy. The policy is called "Selective Availability" or "SA" and the idea behind it is to make sure that no hostile force or terrorist group can use GPS to make accurate weapons. Basically the U.S. Department of Defense introduces some "noise" into the satellite's clock data which, in turn, adds noise (or inaccuracy) into position calculations. The DoD may also be sending slightly erroneous orbital data to the satellites which they transmit back to receivers on the ground as part of a status message. Together these factors make SA the biggest single source of inaccuracy in the system. Military receivers use a decryption key to remove the SA errors and so they're much more accurate.