Subject: Re: post-processing vs. FM correction
Author:  Jeff Kennedy  at NBS-Internet-Gateway
Date:    12/1/99 8:26 PM


Ecologgers,

Just a small correction to Leonard Pearlstein's posting to the list
regarding GPS correction signal availability. Although the US Coast Guard
is the primary broadcaster of these signals, they are available inland in
most of the US and southern Canada for several reasons. First, the Great
Lakes/St. Lawrence Seaway is covered by the USCG, as is the
Mississippi/Missouri corridor. Second, cooperating state and provincial
agencies (e.g., state depts. of transportation) are also setting up inlan=
d
DGPS beacon antennas, with a stated national goal of continental coverage
by the end of the year 2000. For more information, see:

    http://www.csi-dgps.com/dgps/index.htm    and for maps, see:

    http://www.csi-dgps.com/dgps/b-maps.htm

Note that all of the lower 48 states is currently covered except for the
Rocky Mts/Great Basin/West Texas corridor. That should change in the next
year.

There's another point to be aware of. There are two types of antennas tha=
t
you can attach to your portable DGPS beacon receiver: (1) an E-Field
antenna that comes with the receiver and must be grounded to get a strong
signal and (2) an H-Field antenna that doesn't require a ground. The form=
er
looks like a stubby whip antenna, while the latter looks like a square
flattened muffin. In a pinch, you can ground the E-Field antenna to the
negative terminal of your battery, but the signal strength will be at lea=
st
10dB less than when using the H-Field antenna in the same spot. This is a
huge difference in the field, especially if you work in mountainous terra=
in
and/or under tree canopies. Normally, E-Field antennas are used in marine
setting, where the antenna can be grounded to a submerged hull-mounted
ground plate. The H-Field signal also seems to be less susceptible to
attenuation due to topography and overhead foliage.

Having tried both antennas in the rugged terrain of California's Big Sur
Coast, I can say from personal experience that the H-Field antenna is
definitely worth the extra ~$250 it costs. Using a Garmin 12XL (~$200), a
West Marine DGPS Beacon Receiver (~$399 with an E-Field antenna), an
H-Field antenna (~$250), and an external GPS antenna made by Lowe
Electronics (~$65), see: http://www.lowe.co.uk/gpspage.html  and with
position averaging for 1-2 minutes, I have been able to get fixes in
1000-ft deep canyons in Redwood Forests with an accuracy (FOM or "Figure =
of
Merit" in Garmin-speak) of 1.6-3 meters =B11 std dev. of the time. This i=
s
equivalent to 3.2-6 meter accuracy =B12 std dev. of the time, which is th=
e
criterion used by Trimble and other high-end receivers. That small radius=
,
together with plot photos of my transect marker locations allows me to
relocate my plots. And this is in an area where the beacon receiver is 10=
2
miles away (the recommended limit is 90 miles max). I was often unable to
get differential correction behind ridges using the E-Field antenna, but
the H-Field antenna gave me a fix every time. Pretty impressive. Hope thi=
s
helps.

Jeff Kennedy
Ph.D. Candidate
Ecosystems & Landscape Ecology
UC Davis

>Alaa:
> ... The beacon is a correction signal from
>the US Coast Guard. It works very well in the experience of the people I
>have known as long as you are within the range of the signal -- since th=
ey
>are intended for coastal navigation, you must be near the coast. ...

>-Leonard


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Jeff Kennedy                         Home Ofc:       510-658-4083
Center for Spatial Technologies      (For mail: call or email for

  & Remote Sensing (CSTARS)           home address)
Land, Air & Water Resources Dept.
Veihmeyer Hall                       CSTARS Lab:     530-752-5092
University of California              http://cstars.ucdavis.edu/
One Shields Avenue                   Campus fax:     530-752-3350
Davis, California 95616              E-mail: jakennedy@ucdavis.edu
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