Re: It just hit me, duh (GPS for target shooting)
WARNING! More than you ever wanted to know about GPS accuracy....
"Normal" (civilian) satellite navigation receivers compare pseudorandom signals from a minimum of 3 satellites with an internally generated copy of the same signals. Since the signals from the satellites takes time to reach the receiver, the signals do not "line up" properly. (The satellite's copy is delayed in relation to the local copy.) By progressively delaying the local copy more and more, the signals will eventually line up properly. That delay is the time needed for the signals to reach the receiver, and from this the distance from the satellites can be calculated.
The accuracy of the resulting range measurement is generally a function of the ability of the receiver's electronics to accurately compare the signals. In general, receivers are able to align the signals to about 1% of one bit-width. For instance, the coarse-acquisition (C/A) code sent on the GPS system sends a bit every 0.98 microsecond, so a receiver is accurate to 0.01 microsecond, or about 3 meters in terms of distance. The military-only signal sent by the same satellites is clocked ten times as fast, so with similar techniques the receiver will be accurate to about 30 cm. Other effects introduce errors much greater than this, and accuracy based on an uncorrected C/A signal is generally about 15 m.
WAAS-enabled units use a network of ground-based reference stations, in North America and Hawaii, to measure small variations in the GPS satellites' signals in the western hemisphere. Measurements from the reference stations are routed to master stations, which queue the received Deviation Correction (DC) and send the correction messages to geostationary WAAS satellites in a timely manner (about every 5 seconds or better). Those satellites broadcast the correction messages back to Earth, where WAAS-enabled GPS receivers use the corrections while computing their positions to improve accuracy.
The WAAS specification requires it to provide a position accuracy of 7.6 meters or better (for both lateral and vertical measurements), at least 95% of the time. Actual performance measurements of the system at specific locations have shown it typically provides better than 1.0 meter laterally and 1.5 meters vertically throughout most of the contiguous United States and large parts of Canada and Alaska.
Bill
