WWV vs. The Naval Observatory Clock -- Who's Right ?(LONG)

[barry]

root at petro.UUCP (Petty System God) writes:
>  [ confusion as to who is correct deleted... ]
>
> ...the Naval Observatory was reporting time about 12 seconds behind WWV.
> To confirm the observation I called back the Naval Observatory with cu and
> let it report while WWV was reporting over the shortwave -- sure enough when
> WWV hit 124900 the Naval Observatory was reporting about 124848.
> 
> OK, I give up, who's right ?
> 
> ----
> Gilbert B. Andreen  -  bruce at petro.UUCP 

	They both are.  By Act of Congress the following relationships
	are true  F != 1 / T  and  T != 1/F.

	In its great wisdom Congress has defined as part of each 
	organization's mission the following.  That the Bureau of Standards
	be responsible for keeping the frequency standard, and the
	Naval Observatory is responsible for keeping the correct time.

	Of course the scientists at both organizations understand the
	physics and mathematics of the situation.  Something that
	Congress does not (what does reality have to do with Congress
	anyway, have you ever watched C-SPAN? [0.5 :-) ] ) They
	achieve their mission the following way.  The NBS maintains
	radio stations WWV WWVH and WWVB.  These time broadcasts are
	synchonized to the Master Atomic Clock in Boulder CO.  An
	Atomic Clock BTW is a container of Cessium Gases which has the
	proportions to maintain resonance.  This is a very accurate
	frequency source for long intervals, but things like air 
	pressure variations (such as people talking in the room)
	make minute changes in the cavity dimensions which will affect
	frequency stability.  This gives a very poor short term
	frequency standard.  The output of the Cessium Cavity is
	phase-locked to a 10 Mhz crystal oscilator which has good short
	term accuracy, but poor long term accuracy.  This 10 Mhz
	oscilator then drives a rather conventional clock that is
	broadcast on WWV/WWVH/WWVB.  Each of these clocks are
	synchronized, with propagation delay accounted for for both
	the voice and data portions.  WWVB sends out time in BCD on
	60Khz (via power drops of 10dB) WWV and WWVH send out not
	only the time in voice (Male for WWV Female for WWVH) but
	also in BCD via a 100Hz sub-carrier.  In addition to the
	time the current UT1 and UT2 in milliseconds are broadcast.

	Once a year, (I believe in the spring)  at the Naval Observatory 
	a special telescope with a pinpoint aperature is uncovered.  
	When a particular star lines up exactly with this telescope a 
	signal is sent to the NBS over a dedicated circuit.  This circuit 
	has a known path length, and an exact propigation delay.  The
	signal marks midnight of the siderial year.  The propigation
	delay is added to the signal and the Master Time Clock is sync'ed
	to X miliseconds after midnight (where X is the propigation delay)
	The current values of UT1 and UT2 are updated, and WWV is
	now set by the Naval Observatory.  Note that the Master Atomic
	Clock is not a clock but a very accurate (1 part in 10 ** -12)
	"crystal" oscilator.  The Master Time Clock is a digitally 
	sync'ed tape deck (or so I've been told).

	The Naval Observatory in Wahington DC has an Atomic Clock 
	sync'ed to the NBS Master Atomic Clock in Boulder CO.  So what
	happend to the 12 Seconds, well propigation delay on the
	phone line added some to that, then there was the time sharing
	system or whatever you access at the Naval Observatory, its 
	processing time, and other things.  Still 12 seconds seem to
	be excessive.  The correct answer acording to Congress is the 
	Naval Observatory, but the real answer is WWV is right.  If
	you want to load the correct time into your system, you may
	want to do what I do.  Heathkit makes a clock that recieves the
	WWV signals in BCD and decodes them.  The clock has an RS-232
	connector, and so I just read it when I want to set the time.

	I have no other connection with Heath other than a long time
	user.  Note this clock is somewhat expensive for personal
	use, but it is neat to have around, when the power goes out
	and comes back, I have the correct time in 4 minutes. Also,
	the NBS has a bit that they send for Daylight Savings in
	effect, so that is also covered.  The accuracy is +/- 250
	miliseconds.  You can save a little money by building it
	yourself.  It will be a challenge to a 1st time kit builder,
	but if you go slow, and don't rush you'll do fine. Hope 
	this helps.
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Barry Berg			UUCP:    barry at tanstaafl.edgar.mn.org
-- Too much of the world is run on the theory you don't need road
   manners if you are a five-ton truck. --

	#include <std.disclaimer>
 			-30-  {sometimes older ;-) }