Re: Differential GPS

> > What I don't understand is how they are claiming accuracy within
inches
> when GPS is within a meter.

Differential GPS.

A point in the close vicinity, at Lat X Long Y Height Z gets told via
its GPS that it's actually at X +deltaX, Y +deltaY Z +deltaZ. You
communicate with this point, and then know that at that time, in that
local area (say a few hundred km), you should fudge your figures by
about -deltaX, -deltaY and -deltaZ. The figures change over time, and
are more accurate the closer you are to this point. If you have a few
points, and using interpolation, you can get quite accurate. A good rule
of thumb is that differential GPS gets you a factor of 10 improvement (
though 100 has been claimed, and might be true).