[time-nuts] synchronization for telescopes

Michael Wouters michaeljwouters at gmail.com
Wed May 4 21:55:49 UTC 2016


On Thu, May 5, 2016 at 4:53 AM, jimlux <jimlux at earthlink.net> wrote:
> On 5/4/16 11:52 AM, Ilia Platone wrote:
>>
>> You got it, however: It only matters relative time. Start and Stop times
>> will be known, and that is solved.
>> Someone has proposed using TV or other broadcasting carrier as reference
>> clock: this can be another very cheap solution. There are many AM
>> stations near the places we chosen, and these can be used.
>
>
> AM stations are nice, because it's ground wave, so the propagation is
> reasonably stable, but the frequency is on the order of 1 MHz (and not very
> stable in a time-nuts sense).  if your two sites are 2km apart, in the worst
> case, that's a time difference for your AM signal of 6 microseconds.  I'm
> not sure what the phase noise on an AM transmitter is at 160kHz out from the
> carrier, but it could be pretty bad and not affect the audio quality on
> someone's car radio.

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 ?

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.


Cheers
Michael

> TV intended for simulcasting is MUCH better (because the digital receiver
> needs to see the multiple arriving signals as if they were multipath
> scattered versions of a single signal).
>
> If you have line of sight to a TV station, I would think this could be quite
> good, and it's up around 100 MHz.  The period is 10ns (or 10,000 ns, 1E7
> ps).  You want 100ps kind of timing, that's measuring the phase of the TV
> signal to 1 part in 100,000, which is challenging, but not impossible.
>



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