So far as I know there is no deliberate tweaking of the GPS timing to produce an inaccuracy anymore. Current inaccuracies are inherent limitations of the system. One of the last things Bill Clinton did before leaving office was sign an executive order eliminating Selective Availability (the deliberate errors). Of course there is a caveat that in times of national emergency, the DOD reserves the right to re-introduce it. But by now, that would probably so damage the economy that they wouldn't dare. Even farmers use GPS to guide their tractors! (Can't fight a war if your soldiers are hungry!)

I think you might be confusing WAAS (Wide Area Augmentation System) with differential GPS. Assuming you are out in the open so you can see lots of satellites without reflections from buildings or power lines and there is no deliberate jamming (try riding next to a military base), the biggest error in GPS comes from the ionosphere. The free electrons in the ionosphere produce an index of refraction for the radio waves that GPS uses. In other words the waves travel through the ionosphere slower than the speed of light. Since the ionosphere is variable (depends on time of day, solar activity, latitude, etc.) this is an unpredictable delay in GPS timing. The WAAS uses a network of ground stations (I believe about a dozen over the US) to measure the time delays due to the ionosphere. These are then uplinked to geostationary satellites which broadcast the ionospheric delays (on a coarse grid). WAAS enabled receivers can use these broadcasts to make a correction for the ionosphere. The correction can only be perfect at the locations of the ground stations that measured the delays at the time the delays were measured.

Differential GPS uses receivers at a fixed location (an airport say) to measure the errors from each satellite as seen at that location. These are then broadcast locally and a differential GPS receiver can correct its GPS signals to produce much higher accuracy positions and elevations near the airport. Landing planes totally under the control of differential GPS was demonstrated some years ago.

The air force is currently launching the next generation of GPS satellites which have two civilian frequencies. (The current generation has only one frequency for civilians.) With two frequencies, the GPS receiver can measure the ionosphere delay to each satellite without the need for WAAS and accuracies of several feet (rather than tens of feet) should be possible.

OK, lecture mode OFF!

- Ed