Re: [time-nuts] Measuring sidereal/solar time? Re: A Leap Second is coming
To improve the accuracy, I would integrate several measurements. There is no reason a sampled measurement at only one time needs to be made. On Fri, 30 Dec 2016 09:18:17 -0800, you wrote: >Hi Anders: > >That's something I've thought about for decades using an optical system. A >few years ago I looked at it again and >found that astronomical "seeing" limits the accuracy. So the accuracy >achieved by a spaceborne "Stellar compass" will >be much better than a ground based observation. A radio based observation >might work since the atmosphere would not be >a factor. >http://www.prc68.com/I/StellarTime.shtml ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Measuring sidereal/solar time? Re: A Leap Second is coming
Hi: Maybe this could be done with GPS or higher frequencies so the angular resolution would be better? -- Have Fun, Brooke Clarke http://www.PRC68.com http://www.end2partygovernment.com/2012Issues.html The lesser of evils is still evil. Original Message Brooke, The problem in radio ground observation can be resolution accuracy, but there's also a good transmission into far infrared wavelengths, which could require smaller dishes to get stellar images. The problem of far IR is the cost of right filters/sensor, which are a bit difficult to find. Radio objects, on the other hand, can be solved using an interferometer: LOFAR interferometers work at frequencies higher than 10MHz, frequencies totally transparent to the atmosphere and easily computable even by consumer PCs. There is some work done with common PCs using two RTL-SDR dongles and two satellite dishes. see http://www.sbrac.org/files/DTP_RX.pdf Best Regards, Ilia. On 12/30/16 17:18, Brooke Clarke wrote: Hi Anders: That's something I've thought about for decades using an optical system. A few years ago I looked at it again and found that astronomical "seeing" limits the accuracy. So the accuracy achieved by a spaceborne "Stellar compass" will be much better than a ground based observation. A radio based observation might work since the atmosphere would not be a factor. http://www.prc68.com/I/StellarTime.shtml ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Measuring sidereal/solar time? Re: A Leap Second is coming
On 12/30/16 9:53 AM, Ilia Platone wrote: Brooke, The problem in radio ground observation can be resolution accuracy, but there's also a good transmission into far infrared wavelengths, which could require smaller dishes to get stellar images. The problem of far IR is the cost of right filters/sensor, which are a bit difficult to find. Radio objects, on the other hand, can be solved using an interferometer: LOFAR interferometers work at frequencies higher than 10MHz, frequencies totally transparent to the atmosphere and easily computable even by consumer PCs. There is some work done with common PCs using two RTL-SDR dongles and two satellite dishes. the earth's ionosphere is hardly perfectly transparent at frequencies below, say, 10 GHz. The effect is small at GPS L-band frequencies around 1.5 GHz, but still large enough that you need to either make measurements at two frequencies (so you can calculate the effect) or use other data, if you want accurate "sub-meter precision" data. At HF, the effect is huge: during daytime, you might not even be able to see the signals you're looking for, either from D-layer absorbption or F-layer reflection/refraction. The real challenge at HF (e.g. LOFAR) is that it's not just a time of flight thing, because the propagation is not in a straight line: the anisotropic ionosphere bends the rays: and even better, the bend depends on the polarization. For GPS, the signal is CP, and the effect is small, so they typically look at it as an overall propagation speed effect. At HF, the effect is so large that's not really valid. The ionosphere is also only stable on a time scale of <1 second: that is, on a HF skywave path (and by inference, on a HF "through ionosphere" path), signals are pretty much decorrelated at time scales greater than 3 seconds as clumps of ionization move around. This is the fundamental accuracy limit on things like the ARRL Frequency Measuring Test (FMT). It's true that you can do the interferometry easily on a PC, but taking out the ionosphere effect is tough, unless you carefully choose observing time and avoid high solar activity events, etc. I suppose one can do some sort of inversion process on measured data from known sources at multiple frequencies to infer the ionospheric structure, but this is a *hard* problem. If you want to use RF interferometry, I'd go higher: maybe Ku-band- cheap electronics and dishes available. There's some water vapor attenuation, and I'm sure that changes the propagation speed a bit too, but it's measureable with radiometry, you can easily tell whether there are clouds in the path with a pretty simple Ku-band radiometer. You'd want to throw out days when there's rain. I don't know if there's any useful celestial sources at Ku-band. DSN uses bright quasars as pointing & timing reference when doing Delta Doppler One way Ranging (Delta DOR) but on the other hand, they're also using cryogenic receivers with 34 meter apertures- something not available to the casual (or even dedicated) amateur. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Measuring sidereal/solar time? Re: A Leap Second is coming
Brooke, The problem in radio ground observation can be resolution accuracy, but there's also a good transmission into far infrared wavelengths, which could require smaller dishes to get stellar images. The problem of far IR is the cost of right filters/sensor, which are a bit difficult to find. Radio objects, on the other hand, can be solved using an interferometer: LOFAR interferometers work at frequencies higher than 10MHz, frequencies totally transparent to the atmosphere and easily computable even by consumer PCs. There is some work done with common PCs using two RTL-SDR dongles and two satellite dishes. see http://www.sbrac.org/files/DTP_RX.pdf Best Regards, Ilia. On 12/30/16 17:18, Brooke Clarke wrote: Hi Anders: That's something I've thought about for decades using an optical system. A few years ago I looked at it again and found that astronomical "seeing" limits the accuracy. So the accuracy achieved by a spaceborne "Stellar compass" will be much better than a ground based observation. A radio based observation might work since the atmosphere would not be a factor. http://www.prc68.com/I/StellarTime.shtml -- Ilia Platone via Ferrara 54 47841 Cattolica (RN), Italy Cell +39 349 1075999 ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Measuring sidereal/solar time? Re: A Leap Second is coming
Hi Anders: That's something I've thought about for decades using an optical system. A few years ago I looked at it again and found that astronomical "seeing" limits the accuracy. So the accuracy achieved by a spaceborne "Stellar compass" will be much better than a ground based observation. A radio based observation might work since the atmosphere would not be a factor. http://www.prc68.com/I/StellarTime.shtml -- Have Fun, Brooke Clarke http://www.PRC68.com http://www.end2partygovernment.com/2012Issues.html The lesser of evils is still evil. Original Message out of curiosity, are there any amateur/semi-pro experiments that can measure the length of the solar or sidereal day to sub-millisecond resolution? To reproduce data like this: https://upload.wikimedia.org/wikipedia/commons/5/5b/Deviation_of_day_length_from_SI_day.svg Something in the sky that goes "ping" every day - detected with a pointing accuracy of < 1ms/24h or <0.01 arc-seconds (!?). Or perhaps two satellite-dishes pointed at the sun and noise-correlation/interferometry?? Anders ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Measuring sidereal/solar time? Re: A Leap Second is coming
I don't think we could call it "amateur/semi-pro" but the millisecond pulsar J0437-4715 would be perfect for this. Bright and precise. Only for southern hemisphere people though. :-) Jim Palfreyman On 30 December 2016 at 19:59, Anders Wallin wrote: > out of curiosity, are there any amateur/semi-pro experiments that can > measure the length of the solar or sidereal day to sub-millisecond > resolution? > To reproduce data like this: > https://upload.wikimedia.org/wikipedia/commons/5/5b/ > Deviation_of_day_length_from_SI_day.svg > > Something in the sky that goes "ping" every day - detected with a pointing > accuracy of < 1ms/24h or <0.01 arc-seconds (!?). Or perhaps two > satellite-dishes pointed at the sun and noise-correlation/interferometry?? > > Anders > ___ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/time-nuts > and follow the instructions there. > ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
[time-nuts] Measuring sidereal/solar time? Re: A Leap Second is coming
out of curiosity, are there any amateur/semi-pro experiments that can measure the length of the solar or sidereal day to sub-millisecond resolution? To reproduce data like this: https://upload.wikimedia.org/wikipedia/commons/5/5b/Deviation_of_day_length_from_SI_day.svg Something in the sky that goes "ping" every day - detected with a pointing accuracy of < 1ms/24h or <0.01 arc-seconds (!?). Or perhaps two satellite-dishes pointed at the sun and noise-correlation/interferometry?? Anders ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
