[geeks] ntp rant

[Dave McGuire]

On January 10, Greg A. Woods wrote:
> >   Thank heaven *everyone* doesn't have that attitude, otherwise
> > there'd be no stratum-1 servers.
> 
> Strictly speaking GPS alone does not a stratum-1 clock make.....  Sure
> it's but one hop away from a real clock, in theory, but that's one
> awfully big hop, even if you do have three satellites visibile in the
> constellation at all times.  (Not to mention that as I understand it
> there are still provisions for the GPS time base being scrambled a bit
> more than usual when necessary.)

  It's a big hop, but a very stable one.  Most of the jitter in a
GPS-based system doesn't occur between the satellites' oscillators and
the receiver, but between the receiver and the computer.

> What's really stupid is that half those GPS-based ones claiming to be
> stratum-1 are nowhere near so stable as to deserve the designation
> (incl. our own GPS-based ntp.planix.com!)

  Ahh yes, agreed 100%.

> >   Nah.  Good rubidium primary standards can be had for $1K or so, and
> > rubidium oscillator modules can be gotten for about $400.
> 
> I've never seen one so cheaply that anyone serious about time standards
> would ever settle for, but then most of the people I know who are so
> serious are those providing time signals for television or telecom,
> and/or are designing, building, or repairing the equipment to do so.  At
> least not one that's been calibrated and certified!  ;-)

  Well I've sold three Ball/Efratom rubidium oscillator modules for
$400-600 in the past year, and they're among the best available.  And
as I'm sure you're well aware, one doesn't calibrate primary
standards...that's why they're called "primary"! ;) *poke*

> "normal" in this case is someone running a serious network of several
> computers, and perhaps a small WAN as well.  Eg. me, maybe you, etc.

  The point I was trying to make was that some of us do lots of other
things as well, sometimes involving our computer stuff.

> If you or I are not on the net then the stability of our clocks at the
> time don't matter a hoot.  If you are on the net you can have your
> clocks at least as closely sync'ed to the real stratum-1 clocks as mine
> are without anything more than a decent server and a copy of ntpd.

  Well the point above applies here...one thing that I do occasionally
is profiling a frequency standard using a unix box with an accurate
clock.  A decent degree of relative stability is required...small
variations can be averaged out over time but larger ones make this
setup kinda useless.

  Not that I'm suggesting that everyone here does this sort of thing,
but I'll bet I'm not the only one.

> >   Personally, I built most of a cesium-based stratum-1 server because I
> > *could*.  I had all the analog hardware for running calibrations of
> > secondary frequency standards, so why not do a little extra work and
> > make a *quality* stratum-1 ntp server as well?
> 
> Now that I can appreciate, but that's a whole long ways away from what
> your average GPS-based stratum-1 server is, including ntp.planix.com.

  Agreed, again.

    -Dave

-- 
Dave McGuire
St. Petersburg, FL         "Less talk.  More synthohol." --Lt. Worf