[time-nuts] PTTI 2012, part 2

Bob Camp lists at rtty.us
Tue Dec 4 12:37:26 UTC 2012


I went through a similar process quite a while ago. The dimensions of the actual fountain can be quite small. One could make one the size of a shoe box and still have it perform quite well. 


On Dec 3, 2012, at 2:07 PM, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:

> --------
> In message <AB5B0278225B4BD483382A39E6834203 at pc52>, "Tom Van Baak" writes:
>> - USNO rubidium fountains
>> While many national labs have developed cesium fountains (for
>> accuracy), USNO has been gradually building rubidium fountain clocks
>> (for stability) and 4 of them are now fully operational. The ADEV
>> of these clocks gets well under 1e-16. The paper will have all the
>> details but the note I made was that with 20 months of data, the
>> stability was near 5e-17 at tau 4 months. That's 100x better than
>> a commercial cesium standard; better than all of USNO's other 70
>> cesium clocks and 15 H-masers combined. Yes, I've added "rubidium
>> fountain" to my automated eBay searches.
> I happened to miss a turn (or something...) and stumbled into the
> building where they keep those fountains when I visited USNO some
> months ago.  WhatI found most remarkable about them were how compact
> they were, I still expected fountains to be room size, but these
> were rack-sized.
> I asked what the material cost would be and if a competent amateur
> would be able to do something like that, and the clear message was
> that the single biggest problem was the vacuum for a vessel that
> size (when you can't use ferromagmnetic materials) and getting
> the optical bench calibrated.  "Apart from that it's just some
> plumbing"
>> - Loran/UrsaNav
>> CW instead of very low duty cycle Loran pulses would improve S/N [...]
> Actually, it probably will not.
> The one smart thing about the LORAN signal is S/N, which means that
> LORAN for timing purposes is incredible insensitive to noise and
> at the same time, incredible transmitter power economy.
> The one caveat is that the GRI has to be a good number, preferably
> a four (or more!) digit prime number.
> (You need to grok moduls-arithmetic to really appreciate this, but
> its the magnitude of the prime factors of the product of the GRI
> and the disturbing CW which counts:  The smaller the are, the harder
> it is to filter the CW-RFI out.)
> This is why Europe switched to 4-digit GRIs and almost totally solved
> CW-RFI by doing so, and why the Russian Chayka at GRI 8000 is totally
> useless near anything resembling a transmitter.
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe    
> Never attribute to malice what can adequately be explained by incompetence.
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