[time-nuts] synchronization for telescopes

Chris Albertson albertson.chris at gmail.com
Wed May 4 17:42:30 UTC 2016


How to increase the SNR.  Remember you are doing this in post processing
not in real time.  So for each even that needs to be trimmed yo have access
to literally millions of cycles of RF data both in the past and future
relative to the event.  The signal cane near the noise floor of your
receiver but it won't be.  These a stations with tens of kilowatts of power
and yo get to use any number of them simultaneously if you like.

HDD space might be a problem but you don't need to continuously record the
data.  you only need X milliseconds per second.  X is what every you need
for good statistics.  You assume your local oscillator (a $25 crystal) does
not drift much over 100 milliseconds.

Basically what you'd really like to do is have a two channel recorder one
for your data and one for a continuously broadcast "timecode"  Then in post
processing you create the time tag for each event by interpolating the time
code.  All telescope listen to and record the same broadcast time code.  In
post you remove the time of flight delay from broadcaster to each telescope.

The question then is what to use as a time code.  You just need a
transmitter that is common to everyone.  GPS can work.  GPS might be best
because the receivers are dirt cheap because they are mass produced.
But if you can sample a "free" rf signal in quadrature you can recover the
phase very accurately if you have a 100,000 samples of it.  Take those
100,000 samples 10 times per second and you only have a 8MB/sec data rate.
Those are made-up numbers.  I don't thing SNR is a problem as you are using
an autocorrelation function to time aligned large data blocks not working
in real time on each sample

On Wed, May 4, 2016 at 11:52 AM, Ilia Platone <info at iliaplatone.com> 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.
> A problem found was how to increase SNR: do you have a solution for this?
> If possible this method would be the best, since longer baselines could be
> made. The distance from the carrier source is not a problem since we'd use
> a GPS module at each telescope. Also the software part is not a problem too.
> Good the relative timestamp also, as it saves HDD space.
>
> Regards,
> Ilia.
>
> On 05/04/16 15:28, Chris Albertson wrote:
>
>> One more comment.   It seems to me time-raging events is hard because you
>> need many very good clocks that tracks absolute time.
>>
>> If you redefine the problem to be "determine the time difference between
>> to
>> events that occurred a couple nights ago it might be much easier.  This
>> does not need to be done in real  time
>>
>> On Wed, May 4, 2016 at 8:25 AM, Chris Albertson <
>> albertson.chris at gmail.com>
>> wrote:
>>
>> Maybe there is some simpler way to synchronize the telescopes.   Do they
>>> even need to know the absolute time?  I think only relative time maters.
>>>
>>> For that all they need is some kind of a signal that all the telescope
>>> can
>>> "see".   Could they use an FM or TV broadcast station?  They could sample
>>> and record the signal at a very high sample rate (maybe 4X the career
>>> frequency) and record their data at the same time.  each telescope would
>>> need to know its distance to the broadcast antenna.
>>>
>>> The idea is to make the hardware cheaper and simpler and put all the
>>> "work" on the post processing software developers.
>>>
>>> For this purpose, measuring the time difference of photons detected at
>>> different locations, I don't think the broadcast career needs to be
>>> exceptionally stable.  In post all you do to slide the recorded signal
>>> until a best match is found.  So we do need a modulated carrier.  We also
>>> have LOTS of data to use to compute the time alignment because you do it
>>> later, we'd have billions of samples so it should be immune to noise
>>>
>>>
>>>
>>>
>>
> --
> Ilia Platone
> via Ferrara 54
> 47841
> Cattolica (RN), Italy
> Cell +39 349 1075999
>
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-- 

Chris Albertson
Redondo Beach, California



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