AM radio in Italy (what is left) is from 525KHz to 1715KHz and the TV broadcast is now all digital, using QPSK or QAM COFDM modulation: not so easy to use as a synchronization signal.
On Wed, May 4, 2016 at 7:42 PM, Chris Albertson <[email protected]> wrote: > 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 <[email protected]> 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 < >>> [email protected]> >>> 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 >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > > > > -- > > Chris Albertson > Redondo Beach, California > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
