[ntp:questions] Stick to PPS, even if the prefer server fails

Harlan Stenn stenn at ntp.org
Sat Mar 28 08:48:35 UTC 2009


>>> In article <FOjzl.18914$Db2.8066 at edtnps83>, Bill Unruh <unruh at physics.ubc.ca> writes:

> Harlan Stenn <stenn at ntp.org> writes:
>> From one POV, it seems to me that each "instance" of a PPS source should
>> come with "health information" about that PPS instance.  This "health"
>> information should include the current expected precision of the PPS
>> signal.

Bill> Not sure what the health info would be. The health of PPS is either
Bill> great, (if the seconds is good) or attrocious (if the second is bad),
Bill> and the driver presumably does not know this or it would have
Bill> corrected the seconds (remember that this is like the date being a
Bill> month out, if your accuracy was a second).

There are at least two issues here.

In one case, there can be a GPS device that may deliver a PPS second but
that PPS is not really sync'd.  The GPS device in this case will usually
have this information ([do not] believe the PPS signal) in there.

In another case, the Stanford Research PRS10 rubidium frequency standard has
an RS-232 output port that has diagnostic info, including letting you know
if the device is in its "warming up" state, which means during those 6 or 7
minutes the pulse is not yet as accurate as it should be.

I suspect there are other, similar situations with other devices.

>> Some PPS devices are "stand-alone" while others are "combined" with, say,
>> a GPS signal.

>> For the former case, it makes sense to treat them as separate sources of
>> time.

>> For the latter, it makes sense to treat the PPS signal as an "adjunct"
>> device.

Bill> Well, I would say that the GPS is the adjunct device since the PPS is
Bill> far far more accurate (4 orders of magnitude).

That may be, but the smarts for the driver are in the GPS side of things,
not on the PPS side of things.  We'd probalby want the config file to say
things like "I'm an XYZ refclock that has a PPS signal", instead of "I'm a
PPS signal that comes from an XYZ device".

If the final code can handle this either way I don't care.  But I figure
there is a good chance it will not be that way.

>> See http://support.ntp.org/bin/view/Dev/GettingNtpdItsConfiguration for
>> more information.

>> The bottom line is we need to figure out what else needs to be done to
>> make the "state of affairs" with PPS sources even more generally useful.

Bill> Clearly the PPS needs something auxilliary to determine the
Bill> seconds. On the other hand, once that second lock is obtained it is
Bill> hard to lose it. The system clock on most computers is not going to
Bill> drift by 500,000 PPM (especially not most time servers).

For me, the issues for a PPS source is how much can we believe the PPS
signal arrives at the instant of the stroke of the second.

That is a separate issue from "about what time is it".

Bill> Now maybe I am not imaginative enough to think of what could go wrong.

I know I'm not.
-- 
Harlan Stenn <stenn at ntp.org>
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