[time-nuts] GPS Filter

[jimlux]

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.