From:    Jeff Kennedy 
Subject: URL for Lowe GPS Antenna; Terminology Clarification

>Dear Jeff,
>     Where can I get the Lowe antenna?
>Thank you,  Tom
>tom@vims.edu

Tom,

Go to the URL shown below and click on the "high performance magnetic GPS
antenna" button.

http://www.lowe.co.uk/gpspage.html

You buy directly from Lowe and it costs $65, including airmail shipping
from England. It's a great deal. I also like the thin diameter coax cable
they use.

Confusion with the terms "magnetic" and "ground":
________________________________________________

To Ecolog readers at large:

    You'll note that the Lowe web site refers to a "magnetic GPS
antenna," which can be the source of some confusion. In this case, the te=
rm
"magnetic" refers to an actual physical magnet embedded in the bottom of
the antenna case that allows it to be attached to a suitably magnetic
metallic surface (aka a "ground plane"), such as a car roof.

    However, when you research differential correction antennas (which
are _separate_ antennas from the GPS antenna, although the two can be
physically bundled in the same plastic housing), you'll find that the
antennas come in two "flavors": an E-field antenna that picks up the
electric field component of an electromagnetic (EM) wave and an H-field
antenna that picks up the magnetic component of an EM wave. The H-field
(aka "magnetic") differential beacon receiver antenna has no physical
magnet. It's usually screwed onto a mast, although to complicate things
further, it can be screwed onto a magnetic base for attachment to a car o=
r
truck. The GPS antenna receives signals from the GPS satellites. The
E-field or H-field antennas receive signals from a ground-based
differential beacon antenna operated by the US Coast Guard or by
cooperating state agencies.

    As a further source of confusion, the E-field antenna requires a
physical, electrical ground to get decent signal to noise ratios. (While
you can "ground" an E-field antenna to the negative terminal of your
battery, it doesn't work nearly as well as a real electrical ground, as I
discovered the hard way.) The H-field antenna does not require an
electrical ground to operate, and as a consequence, it's the type most
commonly used in terrestrial field work. (Wouldn't you know it, H-field
antennas cost about $100 more than their E-field cousins.).

    Douglas Yu's confusion over the use of the term "ground" with the
Lowe GPS antenna stems from the name for the metal plate than needs to be
placed under the GPS antenna: it's called a "ground plane," I presume
because it becomes the reference plane for measured GPS elevations. These
elevations must be corrected for antenna height to get the actual ground
surface elevation. This use of the term "ground" has nothing to do with a=
n
electrical ground. Note, for example, that a car roof is electrically
insulated from the earth by its rubber tires.

    The error on GPS elevation readings is typically twice that for x-y
coordinate readings. Note that since position averaging with differential
correction gives a 95% error radius of 3.4-6.4 m with the Garmin 12XL, my
elevation errors become =B16.8-12.8 m. Thus, an antenna height above the
ground of 3-4 meters is significant, unless corrected for.

    Now, just when you think you've got it all straight, here's another
wrinkle: From the above discussion, you might get the impression that the
ground plane disk/surface must be a magnetic metal. But an engineer once
told me that the plate simply needs to be a good electrical conductor -- =
an
aluminum disk would work. But since magnetic attachment is so convenient,
the ground plane is commonly iron-based.

    No wonder it's so easy to be confused by the terminology! Hope this
helps.

Jeff Kennedy

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Jeff Kennedy                      Home Ofc:     510-658-7645
Center for Spatial Technologies   CSTARS Lab:   530-752-5092
  & Remote Sensing (CSTARS)      http://cstars.ucdavis.edu/
Land, Air & Water Resources Dept.   jakennedy@ucdavis.edu
University of California at Davis  E-MAIL ME FOR POSTAL ADDR
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