Almost Accurrate Clock

[jbn]

     The Heath WWV receiver is not just counting cycles of a waveform;
WWV is actually sending the time and date in a suitable format.  So
this is an absolute reference, not a relative one.
     For those of you who do not know what WWV is, a brief explaination is
in order.  WWV is a radio station operated by the US Government National
Bureau of Standards.  WWV broadcasts various time signals on different
frequencies.  The time broadcast is controlled directly by multiple
cross-checked cesium-beam atomic clocks at the WWV site in Boulder, Colorado.
These are in turn checked against the United States Master Clock at the
U.S. Naval Observatory in Washington (composed of over a dozen cesium-beam
atomic clocks cross-checking each other).  Cumulative error in WWV signals
is under 300 microseconds per century.
     WWV time is defined relative to the Earth's rotation, and the rotation
of the earth is slowing down;  the Naval Observatory measures this and 
every few years WWV inserts a ``leap second'' to compensate.  When this 
occurs, the time sent by WWV jumps one second exactly.  These adjustments are 
made only at the beginning of a year.  (Exactly at the beginning of a year, 
0000Z UT, 1 January).
     Receivers which obtain WWV signals on bands reflected off the ionosphere
(these are the short wave bands) are subject to some drift due to ionospheric 
height variations; this drift is measured in tens of milliseconds but the
long-term drift is zero.  There are also satellite signals which can be used
for timekeeping and navigation; these are also very tightly controlled.
     Unless you have some very exotic timekeeping application requiring
millisecond-level syncronization at distant points, the Heath clock is
fine.