From: "David W. Inouye"
Subject: GPS receivers
I think you will find that any of the current generation of GPS receivers
that can track up to 12 satellites simultaneously is a significant
improvement over previous models that tracked only up to 6. A colleague
who works in rainforest in Brazil tells me that the newer models do much
better in the forest than the older models. The least expensive models
(under $300) don't have built-in maps, but should work fine for field work
where you just want to record latitude and longitude. Garmin and Magellan
are probably the two best-known manufacturers, and their products are
available from sporting good stores or mail order companies (e.g., REI,
Cabela's). The least expensive source for Garmins that I have found is a
company called Ball Variometers in Boulder, Colorado, which manufactures
navigation equipment for hot air balloon, hang-glider and paraglider
pilots. I have enjoyed using a Garmin III+ connected to my laptop with
DeLorme's Street Atlas and Topo USA (topographic map) programs.
If you need better resolution than +/- about 40-60 feet (as long as the
military continues the 'selective availability' feature) you'll have to
jump up to about $12,000 I think, for a system that can store your data for
later correction with data from a base station that can filter out the
fudge factor that's still being broadcast. And if you don't have access to
such a base station you'll need several thousand dollars more for your own.
The Forestry Suppliers catalog has information on such systems.
David Inouye, di5@umail.umd.edu
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I have been delaying any response to this question hoping someone more
knowledgeable will kick in after the weekend is over. However, Dave may be
providing faulty information: From what I can tell from the specs, the
Garmin GPS 48 does have 12 parallel channel receivers but I don't know if
these are sequencing or not. A non-sequencing 12 channel parallel receiver
should provide better reception under canopy because once it locks onto a
satellite signal it does not need to drop it (sequence) to look for the next
one. If the unit is sequencing, it will not do a very good job under canopy
because of the difficulty in sequentially relocating enough satellites (at
least 3) to get a position fix. My little consumer grade Eagle Explorer is
good in this respect; better than the older Magellan and Garmin consumer
models. Also, you don't need $12,000 to set youself up with a good
professional grade GPS receiver and data recorder that will allow you to make
postprocessed differential corrections using the proper computer software and
base station data. I think prices are now running about $3 to $5 thousand
for GPS unit and software packages provided by Trimble, Magellan, Topcon and
others. Dave may have been referring to a surveyor's package that also
includes a receiver for real-time differential correction, but this requires
you to be within range of a base station beacon, something that's not too
likely in a tropical forest.
Warren
------------------------------
I wish to thank everyone who sent me information regarding the use of
GPS under forest canopies. In response to some messages asking that
I summarize and repost the information, here goes:
All GPS signals, being high frequency, are attenuated by interference
from tree canopies (mainly due to leaves) and are corrupted by
multiple reception of the same signal as it bounces off of tree
trunks (called multipathing).
In order to get good signals, the general rules are to
1) use a receiver that can track 12 satellites *simultaneously,* as
opposed to sequentially. This maximizes the probability of getting a
simultaneous reading of the 3 or 4 satellites necessary for a fix.
2) work during the times that the satellites are in optimal position overhead
3) use an external antenna, preferably on a pole placed as high as possible.
In addition, one can
1) use a US government GPS through the USGS or Forest Service, which
has access to a better quality signal (avoids what's known as
Selective Availability, a deliberate degrading of GPS signals by the
US military)
2) use a GPS receiver that gets both US and Russian signals
(expensive) (essentially increasing the number of satellites
available?)
3) use a Trimble professional grade receiver (also expensive)
4) use what's known as Differential GPS, which corrects in-the-field
errors with readings taken from a separate, fixed-location base
station (which can be a second handheld unit)
5) Use a backpack unit, particularly one called the Ashtech reliance
sub-meter unit, which one respondent successfully field-tested and is
using in riparian forest, with an antenna that can be lifted over the
canopy. (again, in the several thousands of dollars)
6) Take a reading from an area where the signal can be received and
then map other points using compass, clinometer and measuring tapes.
Some of the models suggested (in addition to those above) are
Garmin III, Garmin 12, Garmin 12XL, Eagle Explorer (inexpensive end),
Magellan 315
Of course, there are hundreds of models out there and it seems,
dozens of manufacturers.
David Inouye suggested Ball Variometers in Boulder, CO as an
inexpensive supplier of Garmins.
I also did some searching on the web and found an extraordinarily
useful set of pages, starting at
(http://www.bridge.de/~tom/garmin.htman.htmlphid.htmlga.aspPostingNumber=3D=sc/wh), which has tons of information
on Garmins, including posts from a GPS aficionado who successfully
used a Garmin 12XL with an external antenna under secondary forest
canopy in Vancouver. (I don't know if the success was due to the
model, the secondary forest canopy, or Vancouver, or all of the
above.) Many GPS users post to this set of web pages, the kind that
know as much about the machines as the engineers do, and there is
also a link to a GPS newsgroup, which I did not take advantage of.
One important piece of information from these web pages is that the
same model of Garmin can have its 'firmware' and hardware upgraded,
sometimes several times in a single year. Upgrades can significantly
improve performance over earlier versions of the same model. Also,
the latest versions are immune to Y2K and other date problems, as are
most GPS's manufactured in the previous five years. If you own a
GPS, you should check on upgrades as a cheap way to wring better
performance out of your machine.
In the end, based on your recommendations, I have settled on a Garmin
12XL (199 British pounds) plus a Lowe external antenna (36 pounds,
available from www.lowe.co.uk). (You will notice that prices are
higher in the UK than in the US: 1.65 dollars to the pound.) This
model fits my idiosyncratic requirements of low price, extreme
portability, rainproofness, max error of 100m, low power consumption,
storage of waypoints, and the fact that I am already used to climbing
trees to get a fix. Hopefully, I will be able to take advantage of
nearby forest gaps and/or climb less often.
Thanks again to all.
Doug Yu
_______________________________________________________________________
Douglas W. Yu
NERC Centre for Population Biology
Imperial College at Silwood Park
Ascot, Berks SL5 7PY, UK
Tel: UK 1344-294-544, (F) 873-173
(Efax) US 707-222-7112 (I receive these faxes by email)