GPS usage

[Steve at]

    It's interesting that some GPS users are finding problems
with not enough "open sky" to get a reading.  I've been using a
Magellan 5000 for about a year and a half, and have taken
hundreds of readings in all sorts of habitats, including rain
forests and steep canyon walls which block a good portion of the
sky (I also regularly check the location of my back porch, right
up against the house, to make sure it isn't moving around too
much).  I've only had three cases where I couldn't get a reading,
two when the machine told me not enough satellites were available
as soon as I asked for a position, and once when it told me I was
about 10 degrees off from where I knew I was and at an elevation
of 106,000 meters (obviously a bad reading).  In the last case I
reset the machine (re-initialized it to approximately the current
position), asked for another position, and got a good reading.

     The Magellan works by selecting 4 or 5 satellites it
determines should be available at the present time and location
(location based on the last position determined, or as set by the
user), and which will give a high "geometric quality" (allow the
most accurate calculation of position based on the angles among
the satellites used).  Often the GPS machine will not be able to
find one or two of these originally selected satellites and will,
after a minute or two of trying, drop a satellite and try
another.  I assume that this allows for a position to be
determined even in "closed sky" situations by being able to find
at least 3 satellites.  Our older GPS (Magellan 1000Plus) uses
only a single channel to search for satellites and can take a
long time (5 minutes not uncommon) to get a fix.  The new
machines use multiple channels (5 in the Magellan 5000) and works
much faster (usually about 1.5 minutes, but as fast as 45 seconds
and as long as 3 minutes).

     As for the ability of finding a specific location, the
Magellan won't get you much closer than about 20-30 meters at
best.  There doesn't seem to be any way of getting more acurate
with the current system.  The dual-transmitter system just
released by Magellan (and presumabley others) works like this
(taken from a recent Magellan upgrade notice): "Selective
Availability alters the GPS satellite signals to reduce acuracy.
A precisely surveyed DGPS [differentially corrected GPS] ground
station determines the amount of error in each satellite signal.
A 'differential correction' is then transmitted to a Magellan DBR
[Differential Beacon Receiver] which relays the information to a
Magellan differential-ready receiver to calculate a position fix
accurate to within 10 meters."  According to Magellan, DGPS
beacons are run by the Coast Guard and are currently available in
New England, Mid Atlantic, the Pacific Northwest, and portions of
the Gulf Coast and Great Lakes, with plans for the entire US to
be added over the next few years.  Canada is also currently
covered for Nova Scotia and British Columbia.  I'm not holding my
breath waiting for DGPS to arrive in Australia, and if you're
headed for Morocco, well ....

    So I would assess the current GPS systems as follows.  If you
want approximate lat/long data accurate to a few seconds for any
place on the planet, GPS works great.  If you want to find a
specific tree or bush any place on the planet, you are better off
using an old-fashioned map and compass along with some practice
and common sense.  Put another way, GPS will get you into the
yard of your house, but you are on your own when it comes to
finding the front door.