From: ronkean@juno.com
Date: Tue Dec 21 1999 - 10:34:22 MST
On Tue, 21 Dec 1999 00:59:41 -0800 "Michael M. Butler"
<butler@comp-lib.org> writes:
>
> Eyeballing things, I'd say my dash positioning gives a view of about
> 2/3 sky. '85 Mazda GLC, Left corner of dash, with plenty of driver-side
> window in view. If you have a modern car with a slant dash and airbags,
> your dash geometry may not permit as much sky to be seen. Score one
more for
> preregulation technology.
>
> You specifically said
>
> > > > In an automobile, even with the
> > > >receiver and antenna sitting on the dashboard, it is unusable.
>
> It was the word "unusable" that I was picking on. I use it. It
> works. I do not deny that an external antenna would be better, but I've
been
> being a cheapskate. :)
>
> MMB
>
You are right that I should not have made a blanket statement that a
hand-held GPS receiver is unusable inside a car, and your experience
shows otherwise. I should have said that based on the testing I did of
GPS receivers inside cars, the indications were that it was unusable, or
at least not very satisfactory. I was mostly using a 1970 VW Beetle,
which has a nearly vertical windshield and a nearly non-existent
dashboard 'shelf', and could not find any place for the receiver inside
the car where it would work satisfactorily with the car stationary. In
another test, using a 1986 Nova with a very slanted windshield, and a
Magellan receiver with a built-in wedge-shaped antenna, with the antenna
placed near the front of the windshield, the receiver was able to
marginally track position during a 20 mile drive, but it did not seem
reliable. If the direction in which the car is pointing changes
frequently, it may make up somewhat for a large portion of the sky being
blocked by the roof of the car, as the receivers appear to integrate the
data over a period of a minute or so.
For an external antenna test, I used a Garmin GPS50 with a detachable
antenna, with the GPS antenna taped to a wooden stick attached to the
AM/FM radio whip antenna. 5 feet of 50 ohm RG-58/U coax ran to the GPS
receiver inside the car. That worked, but the signals were substantially
attenuated. RG-58/U cable is not really spec'ed for 1.5 GHz, and the
attenuation at that frequency is probably about 1 db per foot. To work
well, an 'active' (amplified) external antenna is needed, but I was too
cheap to buy one. I have since learned that Lowe Electronics in the U.K.
sells active GPS antennas via the web, but of course most hand-held GPS
receivers do not support an external antenna.
Ron Kean
.
.
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