Dear All,

Here is the reference about Allan Variance - a measure of random errors of 
very precise clocks. 

http://en.wikipedia.org/wiki/Allan_variance

I'll check to see if I can find other sources. 

Speaking of clocks - just for fun I wrote this in my Galapagos Journal:

"Robert FitzRoy, captain of the H.M.S. Beagle, carried 22 chronometers on 
board so that accurate longitude measurements could be made. Only 3 
instruments would have been required by the standard practice of the time. 
The British Admiralty refused to pay for so many chronometers but FitzRoy 
was determined to perform the most accurate survey possible. He paid for 
the chronometers himself and hired a person to look after their operation. 

I wonder how my GPS system compares with his survey and roughly calculate
that FitzRoy at the end of his voyage knew his East-West position to about
15.2 km without correction. My simple GPS gear is accurate to about 3
meters (least decimal) but I marvel at the technical achievement of that
earlier time. Think of it, 22 chronometers mounted on gimbals and
specially packed in a sawdust-filled lockbox stored in a special cabin.  
Not only were they knocked about on the open seas but also carried up and
down rivers in the tropics to establish various benchmarks. Only after 5
years of operation, having traveled all the way around the world, were the
clocks resynchronized with the standards back in Greenwich. It was
determined that they were only 33 seconds misaligned [footnote]. The
fractional error is quite small and is about 1 part in 4.8x106 - much
better than a modern quartz digital wristwatch. For comparison the
state-of-the-art as reported by NIST 04-Feb-10 is 1 second in 3.7x109 y
corresponding to 1.2 seconds over the age of the earth.

FitzRoy was as remarkable as Darwin was and the chronometers made by 
Thomas Earnshaw were quite ingenious. Both FitzRoy and Earnshaw deserve 
more credit."  

What's the Allan Variance of the most accurate atomic clock? 

-- 
Best regards,

        Frank

	Frank E. Taylor

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