[time-nuts] synchronization for telescopes

[Attila Kinali]

On Thu, 5 May 2016 07:55:49 +1000
Michael Wouters <michaeljwouters at gmail.com> wrote:

> Has anyone thought out how you calibrate out the electronic delays in
> such a system ?
> My picture is that you bring a station close to your master so that
> you can physically measure the distance between the phase centres of
> the two antennas since I think that the phase centre of the antenna
> has to be reference point of each system. I am not a radio person: can
> the phase centre be defined (and kept constant in different
> environments) to within say 10 cm, particularly at 300 m-ish
> wavelengths ?

The standard way to do it is to put both receiver stations close to
eachother, maybe 2-10m distance. Then measure the time difference
between the stations using a TIC with high precision. Depending on
the level of precision you want to acheive, you have to account and
care for different error sources. One of them is the orientation of
the antenna. For good match you should use the same antennas with
the same orientation (relative to earth north/south). Then the "error"
induced by the phase center should be the same for all systems and
cancel out in the difference.


> Also, isn't the problem of calculating the delay between two stations
> more complicated than just knowing the separation of the two antennas?
> You need to know where the transmitter is (just like in GPS) to the
> same accuracy. You might need a network of stations to pin all this
> down properly.

Depends on what you do. If you want to transfer just frequency over
a link, then you don't need the distance (though you might want to
compensate for distance variations at some point). If you want to
transfer time too, then you need to know the exact distance, or use
a system that automagically can measure and compensate it.

This is also the main reason why highly accurate frequency transfer is
so much easier and goes to lover uncertainties than time transfer.


			Attila Kinali
-- 
Reading can seriously damage your ignorance.
		-- unknown