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On 9/9/2015 12:02 PM, Greg Rudzinski wrote:
> Both I and the boat's captain (who also was doing some sextant
> work) felt like we noticed two occasions when the GPS clock jumped ahead
> by a couple of seconds on a given day, and then fell back to what we
> expected the following day.

It is quite possible, especially with older GPS units, for them to jump
a second. The bulk of the the chip's power is used in the
nanosecond/location calculations. The time display is/was not a resource
priority. This is according to an engineer at Garmin.

I had made a video recording of exactly that happening with my old
Garmin 76. At 30 frames/second it actually skipped over an entire
second--more than once. The seconds prior to and after the jumps were
not on the money either.

My favorite method of comparing two or more sources is with a video
camera. First check the 30 fps (or whatever fps rate you prefer and the
camera supports, the higher the better) against Colorado with your
computer, rated timepiece, or audio ticks. Now place the control and
test subject(s) together at equal distance from the camera (so both are
in focus), and record. As a sanity check place the speaker of a "time
cube" near the microphone of the camera for full speed playback. By
advancing the resulting video a frame at a time, you can see the
relationship.

An unexpected secondary outcome of these experiments was the time it
took for a display to change over. The sub second timing of digital
number changes can depend on how any elements have to change from one
number to the next. Also, in general (lower end of consumer grade
devices) the larger the display the longer it takes to change numbers.
For example, an RC watch is noticeably faster than a desktop RC clock.

If indeed an RC watch is rigorously tested and is running a second slow
*after* a successful reset, back to the seller/manufacturer it goes.
Life is too short to tolerate broken tools or toys.


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