Wow. Thank you.turtle wrote:Jim, you're thinking of Selective Availability. This "futzing" was the deliberate degradation of the satellite signals in a manner (the addition of a pseudorandom signal) that only the military could characterize (via knowledge of the seed of the pseudorandom sequence) and reverse.
Selective Availability is no longer active. And will likely not be active again for the reason you note. The "averaging" you refer to is actually the technique known as Differential GPS (DGPS).
As far as I know the only factors currently limiting the accuracy of civilian GPS receivers are the inherent limitations of the GPS system. But there are some good solutions to addressing these limitations. Those civilian users willing to pay for units employing these techniques (geologists, surveyors, precision farmers) can enjoy centimeter-level accuracies.
Zip, unless you're both really motivated and really well funded, these units are generally too heavy and expensive for outdoor use. But there are some indications that this may soon change.
So, based on the above links, it looks like the "normal" accuracy of a GPS is +/- ten feet -- assuming you can get a signal from at least four satellites simultaneously. If you had less than four satellites, there would presumably be some degradation of the accuracy.
HJ