On 3/7/11 9:37 PM, Tom Van Baak wrote:
I think it's simple, at least in the nice/common cases. If the antenna
geometry has a point that everything swivels around, consider that to
the the location of the antenna. I think that covers the typical
alt-az mount:
http://en.wikipedia.org/wiki/Altazimuth_mount
The point is where those two axes intersect. Now just fudge the coax
delay to correct for the time/distance from the real antenna location
to that point. That's the before location coax delay (in there with
the ionospheric delay) rather than the post GPS antenna-to-box delay.
Of course, it gets a bit more complicated than that if you want to
track several satellites in real time. That probably takes an antenna
per satelite. But again, VLBI geeks have been doing that sort of math
for ages.
I've always wondered if someone would do this. Place a bunch
of mini az-el mounts in a 3x3 or 4x4 grid 10 cm apart and let
each 9 or 16 antennas pick a unique SV to listen to. After all,
the receiver knows exactly where each SV is and so can point
right to them. Seems it might also be a way to reduce multi-path
as well since each antenna can then be much, much narrower.
I don't know about the RF side. But phase delays and the offset
within the grid can be handled in software. Or let each antenna
have it's own receiver, sharing a common clock.
It would be very fun to watch this antenna in time-lapse. For a
wonderful demonstration of the GPS constellation, imagine on
a misty evening emitting a laser beam out of each antenna.
This was partly inspired by photos PHK posted many years
ago of an array of M12 receivers.
Without the complexity (or beauty) of an array of az-el mounts (a mini
VLA or SKA) you can do almost the same with a phased array. Several
people (including some at JPL) have done stuff with various sized arrays
of what look like standard hockeypuck antennas.
I can try to dig up the references if anyone is interested.
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