[time-nuts] Is possible precise 1pps?

Michael Tharp gxti at partiallystapled.com
Wed Mar 13 23:07:01 EDT 2013

On 03/13/2013 09:05 PM, David wrote:
> This brings up something that I have wondered about for a while.
> The Garmin GPS18x (and many other receivers) specify the PPS output as
> within 1uS but does that mean it wanders around over say 12 or 24
> hours within 1uS of GPS Time or does it mean something else?  I can
> easily see the granularity of the PPS output but that is obviously not
> what the specification refers to.

If it's anything like the ancient Motorola receivers I purchased off 
fleabay on the mistaken assumption that they were UT+, it might have a 
1uS phase skip every 20s or so. In other words, after a random number of 
seconds the PPS (and all subsequent PPS) will be exactly 1uS early. The 
idea being that it loosely tracks UTC/GPS, but jumps whenever it gets 
too far away. Totally useless for timing but at least they were cheap.

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