Re: 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).

- by using the fixed value of the DGPS broadcast point, accuracies
improved
substantially - the value dropped off the moment the dither factor was
lifted - still usefull under very bad weather conditions but otherwise
expensive and bulky woth no current value.... (a standard GPS needs
additional reciever hardware to pick up dgps broadcasts).

Brendan