From: Brian (bmcewen_at_comcast.net)
Date: Mon Aug 11 2003 - 02:20:07 PDT
>Today, there is a new correctional system in development called WAAS (EGNOS=
>in Europe, and a few other things in Japan, and Asia). The use of WAAS al=
>lows you to get even further refined readings while operating, on the order=
>of 3-5 meters. As such, most handheld GPS units are taking advantage of t=
I expect Matt knows this, but for everyone else: WAAS, since in
development, can't be trusted to be present- they turn it off and on.
Somewhere on the FAA web pages you can find a notice of this. Plus since
they are equatorial satellites providing the WAAS correction, ground
reception once you are enar the latitude of the US/Canadian border the
reception gets spotty (this isn't a problem for its intended use since the
system is designed for airplanes a few miles up).
Real time sub-meter precision and accuracy exists costs about $12k or $14k,
depending on the model (see Trimble.com, someting like 10cm and sub-minute
acquisition times supposedly) plus $800 per year for a satellite-based
commercial correction service (which I'm sure the military workers get "for
free" (free at least to the guy in the field with the right backpack). You
can apply a correctional factor yourself in the if you have lots of time on
your hands (you have to wait until you and the other station you are basing
your correction upon have chatted with the same group of satellites, which
can take a while) and are within 30? miles of a dedicated (ground) base
station- either one provided by gov't or one you placed yourself. Since
you usually can't expect much density of gov't provided units outside the
US (depending on your country; we work in Mexico (see Newtons Around the
World :) where usually there is often only one base station per State), and
45 min wait times can happen with the other method, plus if there isn't a
free station to use for the correction, you have to buy 2 units yourself at
$5k each (one for planting, one for walking around with), getting reliable
sub-5m accuracy reliably is for pretty dedicated projects.
I borrowed a Garmin eTrek recently, I think the cheapest thing you could
get last year, it reported with 5m (15ft) accuracy most of the time, found
its satellites pretty fast, and could do a HECK of a lot of stuff- perfect
for hiking or fishing on big lakes or whatever (map the route you took,
find your shortest path home, route you to a given waypoint, stuff like
that). It was way cool for the cost, I was impressed. It can't do WAAS
but WAAS might not work well where I live/hike/fish, in Michigan, since
here we go south to get to Canada :) but units that have WAAS are only
about $30 more. And just a touch more $ than that gives you prebuilt
scrollable maps, etc.
For comparison, an older Trimble handheld we used for teaching Arcview, (4
or 5 years old I guess), takes a lot longer to find its satellites, and
when plotted in the lab, could be 30-50m off from day to day, which would
be enough to put you on the wrong street if you made a map and were driving
by it.
It would be really fun to use a PC Card GPS in the Newt, but it's likely
older so just be aware that you might have issues sometimes. You might
have a little more fun with a newer handheld ($100 or $150 will get you
going) hooked to your messagepad, most handhelds have a serial port and
NMEA is understood by GPS Map Lite. Somewhere there is a little util for
parsing TSIP, Trimble's binary "standard" as well, search the archives for
TSIP). Like Eric asked, off the shelf can/will give you meter accuracy,
starting at about 5m :) or so, sometimes 3m if you get lucky with the WAAS
reception and availability, at least at this time.
Brian
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