<p><pre><code>    &gt; atomic clocks in general is that they have very good
    &gt; short term stability, but not-so-great long-term
    &gt; stability
</code></pre>
Well... The normal number to look at is the &quot;Allan Variance&quot;. Over a timescale of days (100&#x27;000 seconds) a good rubidium will achieve about 10^-12, which is 10ns. A hydrogen maser will have 10^-13 or 10^-14 over even longer timescales. This is pretty great long-term stability in my book. A good, but undisciplined, quartz oscillator will have 10^-12 over timescales of hours.<p><a href="http:&#x2F;&#x2F;ivs.nict.go.jp&#x2F;mirror&#x2F;publications&#x2F;gm2002&#x2F;takahei&#x2F;img8.gif" rel="nofollow">http:&#x2F;&#x2F;ivs.nict.go.jp&#x2F;mirror&#x2F;publications&#x2F;gm2002&#x2F;takahei&#x2F;img...</a> <a href="http:&#x2F;&#x2F;www.ke5fx.com&#x2F;rb.htm" rel="nofollow">http:&#x2F;&#x2F;www.ke5fx.com&#x2F;rb.htm</a> <a href="http:&#x2F;&#x2F;www.thinksrs.com&#x2F;assets&#x2F;instr&#x2F;PRS10&#x2F;PRS10diag2LG.gif" rel="nofollow">http:&#x2F;&#x2F;www.thinksrs.com&#x2F;assets&#x2F;instr&#x2F;PRS10&#x2F;PRS10diag2LG.gif</a><p>By definition, anything steered by GPS will have infinite precision for infinitely long time-scales, as GPS time and UTC time ultimately are steered such that they are within a certain tolerance of each other.<p><a href="http:&#x2F;&#x2F;www.leapsecond.com&#x2F;pages&#x2F;tbolt-tc&#x2F;" rel="nofollow">http:&#x2F;&#x2F;www.leapsecond.com&#x2F;pages&#x2F;tbolt-tc&#x2F;</a><p>And UTC time <i>is</i> the pretty elaborate average of ensembles of atomic clocks, hence atomic clocks produce the international reference time there is.<p>You can, of course, improve the long-term stability of <i>any</i> clock by locking it to a GPS (or other means of time-signal distribution). And for a good Caseium that might only require you to look at the phase, and slightly nudge a potentiometer (magnetic correction field adjustment) on the front by a tiny amount every week. But  atomic clocks in general are pretty darn impressive.<p><i>UPDATE</i>: This presentaiton has all the interesting plots combined, it seems: <a href="http:&#x2F;&#x2F;www.hipster.net&#x2F;ShadNygren_Atomic_Clocks_for_Amateur_Radio_Astronomy.ppt" rel="nofollow">http:&#x2F;&#x2F;www.hipster.net&#x2F;ShadNygren_Atomic_Clocks_for_Amateur_...</a> (pages 45 and following).