[time-nuts] PTTI 2012, part 2

Poul-Henning Kamp phk at phk.freebsd.dk
Mon Dec 3 19:07:33 UTC 2012

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

>- 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.

More information about the time-nuts mailing list