[time-nuts] Is possible precise 1pps?

David davidwhess at gmail.com
Thu Mar 14 01:02:22 EDT 2013

On Wed, 13 Mar 2013 23:07:01 -0400, Michael Tharp
<gxti at partiallystapled.com> wrote:

>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.

I have watched the output pretty carefully on my repaired Racal-Dana
1992 reliably to the nanosecond and I have never seen a jump like that
and I doubt I would have missed it.  At least for the Garmin 18x, the
output pulse is definitely synchronous to the basic 16 MHz internal
clock which is expected and I can see the sawtooth error change
frequency as the GPS unit's temperature changes.  It actually makes a
pretty good temperature sensor.

My 1992 lacks GPIB so I can not use it for logging unfortunately and I
lack a better timebase for comparison purposes anyway.  Those are on
my list of things to take care of.

More information about the time-nuts mailing list