Re: Which clock to trust?

Kevin Michael McGrath (kmmcg@execpc.com)
Sat, 12 Sep 1998 19:08:47 -0500

The phone number for tWWV in Ft. Collins, CO is 303 499-7111.

Kevin McGrath


Frank Reed wrote:

> I recently acquired a Garmin GPS-III receiver which I had hoped would
> provide a precise indication of UTC (in addition to its primary function as
> a navigation tool in back country hiking) for timing satellite flashes.
> Previously I have been using Radio Shack's "Radio Controlled Clock" which I
> believe uses the 60 kHz radio signal from WWVB to calibrate itself.
>
> A shortcoming of the WWVB receiver/clock is that I don't know how recently
> it has updated itself from WWVB.  It has an "in/out of calibration"
> indicator, but it shows "out-of-calibration" only if it has failed to
> update itself within the past 96 hours.  I have measured the free-running
> clock drift to be nearly 0.2 seconds per day.  Therefore, it could be in
> error by the better part of one second even when it shows itself to be
> "in-calibration".
>
> I had hoped that I could use the the GPS receiver to check the calibration
> of the WWVB receiver/clock.  When in calibration, I expected the GPS
> receiver and the WWVB receiver/clock to appear to be perfectly synchronized
> as I watch them count seconds.  They don't!  Yesterday morning (Friday,
> 9/11/98) the WWVB radio clock was faster than the GPS receiver display by
> about 0.8 seconds.  At that time the WWVB clock had updated itself from
> WWVB within the previous 12 hours, as evidenced by an "in-calibration"
> display after showing "out-of-calibration" the evening before.  The GPS
> receiver had been turned on and showed satellite lock on six satellites
> less than five minutes prior to comparing its time with the WWVB receiver
> clock.  BTW, the GPS receiver's internal clock drifts by nearly ten seconds
> per day when the unit is turned off.
>
> Today the WWVB receiver/clock leads the GPS receiver by about 0.6 seconds.
>
> Unfortunately I don't have easy access to a WWV 5/10/15 MHz receiver to
> cast a deciding vote :-(
>
> Can anyone shed some light on this?  I have asked Garmin's Tech Support
> people about this but have not yet received a reply.
>
> -- Frank Reed
> Scottsdale, Arizona, USA  111.898W  33.484 N  1227 feet