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douglas.denny@btopenworld.com wrote:
> I was told that the only way of doing it with sufficient accuracy pre-GPS 
was to physically take a portable caesium standard (actually not that easily 
portable and weighing quite a lot) by aircraft to the facility required.  The 
chap who's responsibility was to take it would book an extra seat on the 
aircraft next to him for the time standard in the name of Mr. C.S Clock  or 
something equally silly.
Ah, yes, but then you have to compensate it for relativistic effects due 
to the standard's motion.  I remember when "portable" atomic clocks 
first became available (late 1950s or early 1960s) Hewlett Packard (now 
Agilent) bought one of them a first-class ticket on a series of flights 
that circled the globe and discovered that it was off from its brother 
that stayed on the ground by exactly the amount predicted by the Theory 
of Relativity.

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