From: ronkean@juno.com
Date: Sun Dec 19 1999 - 23:26:19 MST
On Sun, 19 Dec 1999 13:06:52 -0800 Spike Jones <spike66@ibm.net> writes:
> Eugene Leitl wrote:
>
> > 1) GPS signal is effectively shielded by a few mm of tissue atop
> of
> > the antenna...
>
> Is someone here up to speed enough on GPS to confirm this? I
> was told by someone that a GPS antenna will work indoors, if
> the roof is not metallic. That sounded pretty far-fetched to me,
> but the radio works indoors. I havent found a website that mentions
> it. spike
>
I have several hand-held GPS receivers and I have experimented
extensively with them. GPS is not intended to work indoors, and its
performance is terrible, nearly always unusable, when the receiver is
surrounded by tall buildings. In a forest, performance may be
significantly degraded, depending on how densely wooded. GPS uses a
carrier frequency of 1542 MHz (I think that's the right number), which is
chosen because that frequency propagates well thru rain, snow, clouds
and fog. As a result, GPS works well under open sky in virtually all
kinds of weather, including thunderstorms.
If I wrap my hand tightly around the antenna, the signal strength drops
off dramatically, so if there were usable signals from say, 6 satellites,
only one or two signals, much weakened, may remain. A minimum of 3
satellite signals is required for a 2D position, four for a 3D position.
In a steel shed, there is no signal. In an automobile, even with the
receiver and antenna sitting on the dashboard, it is unusable. But with
an external antenna on top of the roof of the car, it works fine under
mostly open sky.
In an attic, under a pitched roof made of plywood with one layer of
roofing paper and one layer of asphalt/fiberglass shingles, performance
is greatly degraded, but it is usually possible to maintain a lock on 3
or four satellites. GPS receivers seem to work fine in a vinyl tent with
steel struts, but the owner's manual for one receiver advises that even a
single layer of canvas can affect performance somewhat. GPS almost
always works very well on the open sea so long as the open sky is
unobstructed. There are 24 active satellites at any given time, plus
some ready spares in orbit which can be turned on when needed, so there
are usually about 8 or more satellites above the horizon at any given
time. Some receivers are designed to receive signals from up to 12
satellites at once, but the less expensive hand-held units are usually
able to process signals from no more than 8 satellites at once.
Obstructions aside, signal strength is a function of a satellite's angle
of elevation above the horizon. Satellites below 10 or 20 degrees
generally do not yield signals strong enough to be useful.
GPS alone seems not very suitable for tracking people who do not wish to
be tracked, partly because GPS works well only under open sky with no
obstruction other than a thin layer of plastic surrounding the antenna,
and partly because GPS alone only provides location data to the receiver,
but not to a third party remote from the receiver. It would seem that
transponders with embedded ID codes would be more effective. The
transponders could be excited from cell phone base station transmitters,
with the base station receivers being used to receive the signals from
the transponders. Since cell phone base stations have multiple element
antenna arrays in a horizontal plane, they can be used as precise
azimuthal direction finders, with suitable software and calibration. If
the signal from the transponder can be received by three base stations,
the position of the transponder can be determined within a few feet, in
principle.
Ron Kean
.
.
.
.
___________________________________________________________________
Why pay more to get Web access?
Try Juno for FREE -- then it's just $9.95/month if you act NOW!
Get your free software today: http://dl.www.juno.com/dynoget/tagj.
This archive was generated by hypermail 2.1.5 : Fri Nov 01 2002 - 15:06:08 MST