Which clock to trust?

Frank Reed (f-reed@metabien.com)
Sat, 12 Sep 1998 14:07:12 -0700

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

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