I'm not sure I would call a rubidium oscillator an "atomic clock".<p>This, on the other hand, certainly qualifies:<p><a href="https://coverclock.blogspot.com/2017/05/my-stratum-0-atomic-clock_9.html" rel="nofollow">https://coverclock.blogspot.com/2017/05/my-stratum-0-atomic-...</a><p>The CSAC module is somewhat more expensive, but still cheaper than a Swiss watch :-)<p>That said, just because you have an atomic frequency reference doesn't mean you have a clock. You still need to compensate for gravitational time dilation (because of General Relativity, time flows faster as your altitude increases and it's actually measurable using atomic clocks) to get TAI atomic time, and then factor in leap seconds for UTC.