--- Dave Herzstein <[EMAIL PROTECTED]> wrote:
<snip>
> A GPS is not necessarily accurate to within 10
> meters. Heavy tree cover, obscured horizon (such as
> a deep canyon) or multipath distortion (reflections
> from buildings) can skew the position report wildly.
Generally speaking, you must have line-of-sight access
to at least four satellites in the constellation to
get the claimed maximum accuracy, which can be
difficult in some terrestrial applications. Airborne
and over water, this isn't a huge problem, so they are
hard to beat there.
> However, its better than nothing and its usually
> very good.
It can get VERY good. If you have a differential
receiver nearby and have a mobile receiver that can
use its signals, you can get sub-centimeter accuracy.
The way this works is that a stationary receiver in a
surveyed location receives a signal and determines how
far off its computed location is from its actual
location. It then transmits a second signal to mobile
receivers on an extra pair of channels. When this is
received by a mobile receiver, the positional error
can be subtracted out.
This is how the FAA certified GPS for sole-source
navigation and Category 3 precision approaches in
aircraft.
What's really nice is that technology, fallout from
the Cold War, is available to Joe Consumer at a price
that doesn't bar applications like photography. Kinda
like cheap sonar for fish finders and surplus
nightvision scopes... swords to plowshares.
MadMat
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