Re: Which clock to trust?

Frank Reed (f-reed@metabien.com)
Sun, 13 Sep 1998 11:30:53 -0700

Yesterday I 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.
  ...
> ...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. 
  ...
>I have asked Garmin's Tech Support people about this ...

I received a prompt response from Garmin even though it was Saturday.  This
is what they wrote:

----------------------------------------------

"Thank you for your inquiry. 

"The display on the GPS III is updated once per second, so the accuracy of
the time display on the GPS III, when locked onto satellites, will be to
within +/- 1 second of UTC.

"Thank you and best Regards, 

"Bill Stone 
Product Support Manager 
GARMIN International"

-------------------------------------------------------

Thanks to all who responded to my query!

At this point, I believe the Radio Shack Radio Controlled Clock will have
the smaller average time error, provided it shows itself to be "in
calibration," although its error could be nearly one second due to its
oscillator drift since calibration.

It's fairly common for my Radio Controlled Clock to show itself out of
calibration, indicating that it has failed to receive the WWVB signal in
the past 96 hours.  Does anyone have information on how to get a better
antenna to improve reception?

- Frank


-- Frank Reed
Scottsdale, Arizona, USA  111.898W  33.484 N  1227 feet