[time-nuts] Is possible precise 1pps?
lists at rtty.us
Thu Mar 14 07:08:08 EDT 2013
NIST did several papers looking at GPS receivers. They found all sorts of strange issues on the timing outputs. Not all of them were easy to spot unless you had an ensemble of cesiums to compare to…
On Mar 14, 2013, at 1:02 AM, David <davidwhess at gmail.com> wrote:
> 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.
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