The Doppler is dramatically reduced by looking only at the WAAS bird(s).
John WA4WDL
--------------------------------------------------
From: "J. Forster" <j...@quik.com>
Sent: Friday, October 08, 2010 1:27 PM
To: <bro...@pacific.net>; "Discussion of precise time and frequency
measurement" <time-nuts@febo.com>
Subject: Re: [time-nuts] GPS backup for the stationary time and
frequencyuser
I've been half following this thread and can't make out the reason for a
less than hemispheric antenna pattern. GPS needs several birds to lock up,
and if you look at a single bird, Dopplar will make the signal useless as
a frequency reference.
Best,
-John
=============
Hi Jim:
I've got a spare Ku band satellite dish and would like to use it for GPS.
In an ideal application the GPS antenna would be mounted in the normal
manner and above it would be a sub-reflector aimed at the Ku dish.
That way the antenna might pickup sats near the horizon directly and
from a narrow part of the sky by means of the dish.
The dish might be aimed at a WAAS GPS sat.
I've heard that you can just use the TV dish with a normal GPS antenna,
and it gas gain even though the polarization is reversed.
Have Fun,
Brooke Clarke
http://www.PRC68.com
jimlux wrote:
Bill Janssen wrote:
Magnus Danielson wrote:
On 10/08/2010 03:35 AM, jmfranke wrote:
When I said the feed would work, I was meaning it would work if LHC.
The illustrations and text imply you could just place a normal GPS
receiver at the feed location, but the polarization would be wrong.
Which was what I reacted on...
I am by no means a practical antenna expert, and the EM-theory is a
bit fuzzy on the edges, but I do distinctly recall that signal is
RHC and reflections becomes LHC so an antenna with RHC orientation
will provide some first-degree damping of the LHC reflections. For
this antenna setup the intended RHC signal is reflected and should
become LHC... just as the interference... so it relies on the
antenna gain of the dish to out-perform the other reflections for
the half-space receiver that a normal GPS antenna is. The choke ring
for a dish head has a distinct different pattern (forming an inner
cone rather than flat space).
So, a normal antenna would kind of work since the antenna gain would
overcome the poor LHC supression of a simple RHC antenna... yay.
If an LHC antenna was used instead... now we are talking.
Cheers,
Magnus
So a dish reflector and a sub reflector and the GPS receiver at the
dish would work? What is that
configuration called? I can't remember at this early hour.
Depends on the relative curvatures and focal points:
Cassegrain if the subreflector convex.
Gregorian if the subreflector is concave parabolic.
Dragonian if the subreflector is concave hyperbolic
IEEE Ant and Prop Magazine a few years back had a series of articles
on designing them all.
All of them can be done offset or coaxial
Any would conceivably work.. It's all about what your pattern looks
like, what sort of efficiency you need, any mechanical constraints, etc.
_______________________________________________
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
--
Have Fun,
Brooke Clarke
http://www.PRC68.com
_______________________________________________
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
_______________________________________________
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
_______________________________________________
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
