Reception in forest

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Dec 29, 2002, 11:57:49 PM12/29/02
I took my new GPSMAP 76S into the woods the other day and found out how
worthless it is under a canopy of tall trees. Even in an area of
deciduous trees with no leaves, it had great difficulty tracking. It
was raining lightly that day and the trees were wet. Does anybody know
if I can I expect better results when the trees are dry?



Dec 30, 2002, 1:33:32 AM12/30/02
Probably not. But get an amplified external antenna and it will make a BIG
difference. At least it did for me!
"Mike" <> wrote in message


Dec 30, 2002, 2:01:07 AM12/30/02
Make sure you have it on your hat, girls think its soooo sexy. :-)


"Phil" <> wrote in message

Dave Patton

Dec 30, 2002, 4:02:06 AM12/30/02
Mike <> wrote in news:hnQP9.356803$pN3.37151@sccrnsc03:

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.

Dave Patton
Canadian Coordinator
the Degree Confluence Project
49°16'07"N 123°09'07"W

Scott Dickman

Dec 30, 2002, 7:30:12 AM12/30/02

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" <> wrote in message

Sam Wormley

Dec 30, 2002, 7:31:07 AM12/30/02

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

-Sam Wormley

Hans-Georg Michna

Dec 30, 2002, 8:28:51 AM12/30/02
Mike <> wrote:


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.

Dan Anderson

Dec 30, 2002, 10:31:50 AM12/30/02
Would you be willing to post where this occurred (approximate
latitude/longitude, etc) and when (local and UTC)?

(email 2004 not 2001)

Mike <> wrote in message news:<hnQP9.356803$pN3.37151@sccrnsc03>...

Ron Wilson

Dec 30, 2002, 4:43:42 PM12/30/02

"Mike" <> wrote in message


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 <>

GPS Nuts


Dec 30, 2002, 9:18:37 PM12/30/02
This was in a city green space in Portland, OR. I was looking for a
geocache (GC786D at at N45 28.714 W122 41.043. I never
found it. It's on a hill side (the hill blocks anything below 30
degrees to the west) and under fir trees 100 feet or more tall. I had a
lock on several satellites before entering the forest and lost reception
as I descended down the trail. I started to get a lock on a couple of
satellites when I was under some deciduous trees that would have a
closed canopy in the summer. I held the GPS vertical at arms length
slightly above my head. I think the hills and tall trees were just too



Dec 30, 2002, 9:22:15 PM12/30/02
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.


Ron Wilson

Dec 31, 2002, 12:53:10 AM12/31/02

"Mike" <> wrote in message

> Ron Wilson wrote:
> > "Mike" <> wrote in message
> > news:hnQP9.356803$pN3.37151@sccrnsc03...
> >

> 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.
> Mike.


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.

Good luck!

Pieter Litchfield

Dec 31, 2002, 8:19:19 AM12/31/02
I think this is excellent advice - I have a 12xl and a Magellan Tracker.
The Tracker is far worse than the 12xl, but both suffer signal loss under
canopy, as do ALL GPSs to my knowledge.

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" <> wrote in message

Dan Anderson

Dec 31, 2002, 12:10:29 PM12/31/02
Thanks for the additional information.

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 <> wrote in message news:<187Q9.507150$P31.162974@rwcrnsc53>...


Jan 1, 2003, 1:27:01 AM1/1/03
Hey, cool graphs. If I had known you were going to do this, I would
have checked my saved track and told you I was out there between 1:40pm
and 2:00pm on Dec 26. I also bookmarked your web page. It looks like
some good reading.


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