> With differential GPS, you can achieve much greater accuracy, but
> you need a fixed data reference. The DoD is putting up extra satellites
> to implement something called WAAS, which will also improve the
> accuracy ... but still. It's never going to be better than a few feet.
WAAS is up and working, has been for quite some time (although there are
still more improvements and satellites to come). We have demonstrated
long-term accuracy using WAAS to less than a foot (1 sigma RMS), although
"advertised" is 1.5 feet. Basically all new receivers, except the very
cheapest ones (which typically aren't new), can use WAAS/EGNOS corrections
today.
Oh, and WAAS is not a DOD project. The FAA runs it, so GPS can provide
sufficient accuracy, redundancy and reliability to be used for IFR flight
and landing.
Even without corrections, GPS can give very good velocity and speed.
Granted most low-cost receivers don't do very well at this, but it is
possible. And the low-cost guys will get better as they figure out that
it's just SMOP (Small Matter Of Programming), and that there is a demand for
it.
There are also other correction services available, that can be received by
the same receiver. One such is StarFire (which my employer operates) ...
offers accuracies down to 4 inches or so.
With a local reference, we offer performance better than 1 inch. But to
paraphrase someone, performance costs money. How much performance do you
want ?
Randall Young
Senior Software Engineer
NavCom Technology, Inc.
A John Deere Company
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