| |
 |
|
|
Science Forum Index » Geology - Satellite Navigation Forum » Hoisting up the anetenna under wet canopy
Page 1 of 1
|
| Author |
Message |
| Sam Wormley |
 Posted: Fri Mar 02, 2007 9:20 pm |
|
|
|
Guest
|
Hoisting up the antenna under wet canopy
In response to private email...
You are doing the right thing--Increasing the sensitivity
will buy you little. Moisture in canopy effects both attenuation
and multipath. By raising the GPS antenna under wet canopy, you
reduce both attenuation and multipath.
References:
o http://www.fs.fed.us/database/gps/gpsusfs.htm
o http://www.fs.fed.us/database/gps/mtdc/gps2000/Nav_3-2001.htm
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
First paragraph of Chapter 15 of the reference above, "Foliage Attenuation
for Land Mobile Users", by J.J. Spilker Jr.:
"Land mobile users are expected to be one of the largest categories of
GPS users, and it is important to examine specific GPS propagation
issues for this environment. It has already been shown that the
optimum geometric dilution of precision (GDOP) is provided when several
of the GPS satellites are at low elevation angle near the horizon.
However, the land mobile user environment differs from that of aircraft
in flight or ships at sea in that the users driving along a road or
freeway [or hiking in the woods] is often subject to shadowing,
diffraction, ans scattering of the satellite signal by trees, utility
poles, buildings, or hills. These effects are accentuated by the need
for operation a low elevation angles for at least some of the GPS
satellites. In addition, the requirement for receiver simplicity, and
the need to track several satellites widely spaced in angle
simultaneously, generally dictates the use of an omni- directional or
hemispherical antenna. Thus, while receiving the direct line-of-sight
ray from the satellite, the user has little means to discriminate
against multipath signals scattered from ground reflections, tree limbs
and foliage, or other scattering elements. In addition, the direct ray
may itself be attenuated by tree foliage".
No matter what quality of L1 C/A receiver, an HDOP (B) that is twice
as great as HDOP (A) will result in increasing a probability error
circle (ellipse) by about a factor of two. If a certain specified
level of accuracy is required for a particular purpose, careful
attention must be paid to the potential obstacles to achieving that
level of accuracy.
-Sam Wormley
http://edu-observatory.org/gps/gps_accuracy.html |
|
|
| Back to top |
|
| |
|
Page 1 of 1
All times are GMT - 5 Hours
The time now is Fri Aug 29, 2008 10:44 pm
|
|