[GPS_Standard] Latest GPS Standard is alive. Mostly

[Dave Platt]

Bob Bownes wrote:
> Finished up my GPS standard over the weekend. Still struggling with
> getting it to lock and stay locked however. It seems to think the
> oscillator is drifting quite a bit with temperature, however, my
> counters seem to indicate otherwise.
> 
> Not quite sure where to go from here.

Some additional data would be helpful, I think.

*  What make/model of GPS are you using?

*  How good a lock on your location does it have?  Is it in
   "position hold" mode, or is it trying to track a (presumably
   moving) receiver location?

*  What are your FLL parameters?

For what it's worth - I fired up my own standard about six
weeks ago (after several years of procrastination in building
it).  It's not done yet, but I've got "first light" on the
build, with the ability to achieve a lock and some initial
statistics and evaluation of its behavior.  I've noted several
things:

*  Achieving an initial lock is a bit of a mixed bag.  With the
   FLL set to a relatively short averaging period (e.g. 8 or 10
   cycles) and a relatively low unlock threshold, it indicated
   "lock achieved" within a few minutes, but then continued to
   adjust in the same direction after almost every averaging
   period for quite some time... it took a day or so to reach
   the point at which it was either not adjusting, or was
   "dithering" between + and - adjustments fairly consistently.

*  With the FLL parameters adjusted for "OK, we're just about
   there" operation (8- or 16-minute averaging period, different
   thresholds) the system does tend to make a fine adjustment every
   few periods, apparently in response to temperature shift (I can
   see a regular daily cycle).  From what I can see of the amount of
   frequency-count error which triggers these changes, the frequency
   error wouldn't show up on any of my counters.  I did compare the
   system with a friend's rubidium standard, and the disagreement
   seems to be between 1/30 Hz at worst and about 1/300 Hz at best.

*  I'd love to see some "adaptive parameter adjustment" logic added
   to the controller firmware, so that it could (e.g.) increase the
   averaging period dynamically when the system is stable, and reduce
   it (or adjust the control voltage with finer resolution other than
   just coarse/fine) when the system is drifting or off frequency.
   Perhaps some sort of proportional-integral (P-I) control logic
   could be used?