3 Aug
2004
3 Aug
'04
3:01 a.m.
Paul,
The system I was working on was rack-mount...on a submarine [NATO Walrus
Class - launched circa 1984]
I completely agree that there were other more accurate sources [of which
youy name a few. I should have been more precise in my statement.
I was comparing it to other board mountable electronic clock sources [since
we were discussing crystals].
The system in question actually connected to a highly accurate 1PPS External
source that was slaved to a radio clock under normal circumstances. However
our rack need to maintain time even in the event of external failures. It
was deemed that a <50mS error per day under error conditions was acceptable.
David
>>>>
>>>> I was involved in the development of soms Military
>>>> systems [1979-1983] that used a tempe0rature
>>>> stabilized crystal with 0.5ppm stability. To the
>>>> best of my knowledge this was a "state of the art"
>>>> implementation of automomous time keeping for 1979.
>>>>
>> 0.5 ppm for boxes that get carried around in military
>> trucks and bounced around in the field -- that's quite
>> good. For something that's sitting in a reasonably
>> controlled environment, that's not so great;
>> I think that 10^-8 would be considered state of
>> the art for OXCOs (1970s or not).
>>
>> For autonomous timekeeping independent of technology, the
>> state of the art was a second per year or so (that's 10^-8,
>> roughly) around the early 1900s. First with pendulum
>> clocks (Shortt clock), then around the 1940s or so crystal
>> clocks came in that could match this. And not too long
>> after that there came the rubidium (10^-10) and cesium
>> (10^-14) clocks. Some of that would be found in military
>> gear, I think (Rb at least, Cs somewhat less likely).
>> Consider GPS satellites, which have either or both built-in.
>>
>> paul
>>