"Phil" <phild...@aol.com> wrote in message
Water may attenuate GPS signals, such as when foliage gets wet,
or if you get the case of your GPS wet in the area of the antenna
(e.g. drops of water and/or a film of water).
You can use Google Groups to search the archives of this newsgroup
for various discussions about the effects of "weather" on GPS signals.
the Degree Confluence Project
I have a 76S also, and generally have not had the poor reception experience
in the woods that you describe. Is it possible that your unit is not in a
suitable/optimal orientation; i.e., your body is blocking signals, or the
unit is not positioned vertically? Hope this helps!
"Mike" <mi...@nospam.com> wrote in message
definitely! Moisture in canopy effects both attenuation and multipath.
o Parkinson & Spilker, "Global Positioning System: Theory and Applications",
Vol. I, AIAA (1996), Ch. 14 Multipath Effects, Ch. 15 Foliage Attenuation
for Land Mobile Users
wet foliage is almost impenetrable for the 1.5 GHz GPS signals.
It's better without the rain, but leaves are always a problem.
I usually don't get any signal when I walk in a forest under
closed canopy. I sometimes get one when there are gaps in the
canopy, and it helps to stand still for a while.
No mail, please.
(email 2004 not 2001)
Mike <mi...@nospam.com> wrote in message news:<hnQP9.356803$pN3.37151@sccrnsc03>...
This is not normal behavior for the GPSMap 76S. I tested one (actually a
GPSMap76) almost every day one summer, under all kinds of canopy conditions
(with lots of leaves on trees). Generally it did a good job, only losing
lock a few times, in the most trying conditions. There were a few things I
always did to help matters out:
1) Always allow the unit to lock on for at least 10-15 minutes under an open
sky before entering the woods.
2) Leave the unit on as you enter the woods. Don't expect it to get a cold
fix under heavy canopy conditions.
3) Hold the unit away from your body and as high as possible. If it does
lose lock, just move a few meters and it should regain it.
During the winter months, rain will make matters somewhat worse, but not as
badly as in summer, when the leaves are still on the trees. This is not
true, of course, for evergreen forests, which are problematic sometimes,
even if it isn't raining.
Ron Wilson <rmwi...@comcast.net>
GPS Nuts http://www.gpsnuts.com
> Thanks for the tips. I did all this except for the 10-15 minutes under
> open sky. I hope I don't have a lemon. I think the trees were just too
> tall and thick. I hear the pacific northwest isn't prime GPS territory
> due to the trees.
I doubt that you have a "lemon". Go back out and give it another try. The
10-15 minutes may be overkill, but the important factor is to have the unit
acquire lock under an OPEN sky. This allows the receiver to acquire
ephemeris data from all potentially available sats. It must see each sat
for at least an uninterrupted 30 second period in order to acquire this
data. It is important to have the ephemeris data for all visible sats,
because without it, the receiver can't use that sat for a position
determination. In the woods, you need all the help you can get, so the more
sats you have data for, the better the chance of finding enough to determine
a position solution. As time goes by, some sats will go over the horizon
and others will appear. Also ephemeris data becomes old after a couple of
hours. Because of this, it is helpful to try to occasionally get an open
view of the sky, even after you have initialized the unit, as one of the
other posters suggested.
I was reminded of this just recently, while testing out a new Trimble GeoXT
(a $5000 unit) that was supposed to deliver sub-meter accuracy. When I
first turned it on one day, it was under partial canopy conditions and I
didn't leave it out for very long. At one of the NGS benchmarks that was
under dense canopy, the unit had trouble maintaining a fix and gave a 15+
meter error. I thought about this for awhile and returned a couple of days
later, making sure to let the unit initialize out in the open. This time I
got an error of just under 1 meter. I have since returned several times and
acquired similar results.
Another suggestion if you are using yours under canopy a lot is to use an
amplified external antenna. I have a Mighty Mouse II that works with both
my Magellan and Garmin units, and it substantially improves the performance
under canopy. Although the signal with an external antenna under canopy is
still not as good as with an external antenna in a meadow, it is good enough
to be functional.
Especially with my Magellan unit it is important to establish a "lock"
before entering the canopy or it will never happen. I find that leaving the
GPS on the hood of my truck while I gather my gear (10 minutes?) is long
enough. It must have a relatively clear view and be stationary. Don't walk
around with it during this time. The Garmin 12xl is better, but the same
process helps here too.
"Ron Wilson" <rmwi...@comcast.net> wrote in message
I've temporarily posted the possible satellite positions for about
midday at Portland at: (This link will only be good through Feb. 2003)
In general there are few satellites at high elevation angles. I would
not be surprised that the forest was dense enough to block the lower
satellites enough to cause problems getting a position fix. Besides
the other tips given in this thread for using a GPSR under canopy, I
would also recommend paying attention to the satellite constellation.
You will get the same constellation four minutes earlier the next day
and/or use some satellite prediction programs. If you go at the right
time, you probably won't have any trouble keeping a position fix
(using normal mode, not power save, and getting the initial fix in a
clear sky area).
(email change 2001 to 2004)
Mike <mi...@nospam.com> wrote in message news:<187Q9.507150$P31.162974@rwcrnsc53>...