[ale] Re: WWVB to computer

[John Mills]

Christopher -

I enthusiastically second Mike's recommendation.

I looked into this many(!) years before GPS for a satellite ground station 
that had to get time references from various international standards 
bureaus, including WWV (but not WWVB). Our standards lab at S-A _did_ 
synch from WWVB. There are several factors that complicate using WWV[B], 
render it expensive, and/or limit its accuracy.

You don't say anything about the accuracy you need, but ...

First, the accuracy: "shortwave" WWV gives you a "beep" tone every minute,
which can be detected with an accuracy around 60 msec (to drive - say - a
counter or similar application). Not even close to GPS accuracy! This
turned out to be our only global option c.1976, and the 60 msec. was a
limiting factor for some orbital computations: good enough, just.

WWV signals also include modulation to drive time-code generators to much
better accuracy, but this requires a special demod and the time-code
generator to follow it.  I have also been told (but did not verify) this
approach is not reliable very far from Colorado Springs!

WWVB _can_ be demodulated to very good accuracy over the continental US,
but you have to phase-lock your time generator to the 60KHz _carrier_. S-A
used a large, multi-turn loop antenna on the roof, a high-grade PLL
filter, and (again) one or more time-code generators and/or converters. 
Rate information comes off the 60Khz and (IIRC) phase demodulation gives 
you an encoded absolute time.

Systems I delivered more recently had a remote "hockey-puck" GPS antenna,
and a receiver and decoding logic that plugged into the time-code
generator. You can't believe what a relief, economy, and improvement this
was!

If you only need a serial or USB computer interface (and accuracy), you
may find a "consumer oriented" GPS that has a remote antenna.

(Interesting side note: GPS time may carry a fixed offset from UTC - I
don't remember exactly how that works, but it bit us when we were
recording time and position from a simple GPS into a laptop computer in a
small aircraft, and trying to correlate it with our radar track of the
same airplane, using time derived from USNO. #8-)

NIST used to have an excellent, free paperback reference on timekeeping.  
I expect Google on 'time-code generator' or a visit to [www.usno.navy.mil]
would give you a lot of data to sort through. If not, drop me a line and
I'll see how many old vendors have survived or adapted to GPS.

Finally, have you checked into 'ntp' referencing? I think it even works
effectively over a dialup, by using multiple round trips and [optionally]
multiple servers to sync-up. The full ntp software download also includes
a server you can run on your LAN that is corrected for offset and average
drift rate each time you re-sync. Again, USNO (and an interested faculty
member at U.Del. -- follow USNO's links) have a wealth of info.

Just some thoughts.

 - Mills

On Tue, 18 Dec 2007, Christopher Fowler wrote:

> On Tue, 2007-12-18 at 13:09 -0500, Michael H. Warfield wrote:
> > In this day and age of cheap GPS modules, I wouldn't even consider
> > going the WWVB route. 
> 
> GPS propagation inside Quality Services in Suwanee?  Feasible?