[time-nuts] How do I know my GPS stabilizedoscillator isworking?

[Tom Van Baak]

Magnus,

> Well, in your above measuring case with 34 missing pulses, you *DO* have a
> problem if you are going to do Allan Deviation (ADEV), Modified Allan
Deviation
> (MDEV) or Time Deviation (TDEV). Replacing the start and stop as you did
above
> will destory that information. The gap will cause a measurement error.

Right, unless otherwise corrected, the missing data
may mess up statistics. But you can't correct for the
missing data unless you know the data is missing.
Hence the emphasis on a data collection method
that can detect gaps, should they occur.

Note in that example yesterday, the 34 second data
gap exists regardless of which start/stop method you
choose. The only difference is that one way you get
visible notice that a gap exists and the other way
you'll never know.

Timestamps -- I should mention the other way to
detect gaps in GPS 1PPS data is to timestamp each
measurement. I use an RS-232 logging tool that
prepends a MJD timestamp to each serial data line
as it read. The log for the GPS18 looks like this:

    53893.093681 8.9755  us
    53893.093693 8.9368  us
    53893.093704 8.8986  us
    53893.093716 8.9214  us
    53893.093728 8.8829  us
    53893.094131 34.0000088063 s
    53893.094143 8.8310 us
    53893.094155 8.7332 us

Using this method one can detect gaps in the data
simply by looking at the deltas in successive time
stamps. And then it doesn't matter which channel
is start/stop.


> For ADEV/MDEV/TDEV measurements you also want to compensate out driftrate
> before doing the actual calculation. The frequency error is not a problem
for
> the calculation, but the frequency error can be a problem for some, but
not
> all, counters.

I was under the impression that these statistics were
invariant with respect to phase and frequency offset
(but not frequency drift).

Can you elaborate on the counter problem?

/tvb