🚀 go-pugleaf

Regarding the availability of GPS units, they most definitely are
available for land, sea, and air applications.  As mentioned by an
earlier poster, the Department of Defense "fuzzes" the signal slightly
(this is called "Selective Availability", SA for short) to prevent bad
guys from using the system for pinpoint accuracy.

The GPS system can be used by aircraft, but has not been certified by
the FCC as the sole positioning system for an aircraft.  Because not
all the satellites are up yet, there can still be times when you can't
get a good fix because too few satellites are currently in view.

Note that there _are_ units that are small enough for backpacking,
etc.  Furthermore, there are units that allow you to defeat the
Selective Availability "feature".  Basically, you have two units, one
with you (the mobile station) and one fixed (the base station).  Both
simultaneously record the same signals from the same satellites (so
they must be within about 500 km of each other).  Since the base
station doesn't move, it "knows" where it is and generates corrections
for the SA-degraded signal.  The corrections can be applied to the data
from the mobile unit, fixing the SA uncertainty.  The cheapest way to
do this is to post-process the data later on a PC, but there are units
that will broadcast the corrections in realtime ($, as you might
imagine).

I used one of these post-processing correction systems this summer off
the Florida coast (tracking lobsters, of all things) and it worked
beautifully.  The uncorrected data wander all over the place (within a
few hundred meters).  After correction, the data were _far_ tighter.  I
was most impressed.  The system we used was the Pathfinder from Trimble
Navigation (800-TRIMBLE, 408-481-8000).  They'll happily sell you one
or tell you who rents them (we rented).  Very nice system.

-Dean