Re: [chrony-users] Possible bug in PPS support

[Rob Janssen <rob@xxxxxxxxxxxxxxxxx>]

Bill Unruh wrote:
> If you really need 20usec, then relying on one gps is certainly a bad
> decision. You should have two or three machines all with independent gps
> sources so you could catch one of them going rogue, or quitting.
The GPSDOs we are using are 2-3 orders of magnitude better than that.
These are not your typical $50 modules, but professional GPSDO with OCXO
or better oscillator.
Monitoring of their accuracy is done by their owners, we only get the signal
via distribution amplifiers.  That is why we would prefer to have some additional
validation, like the PPS signal completely missing.
(which could also be caused by a mistakenly unplugged or cut cable, which
would never be detected by the GPSDO monitoring)

> You seem to be saying that having no time source whatsoever is better than
> having one which may be off by 20us? I think you need to set out the real
> conditions that you need in detail ("We need accuracy to 20us" could be
> because it was a number that some administrator with absolutely no idea of
> time came up with, or it could be a legal requirement, or it could be "we
> should be able to do that" kind of requirement) 

The time is used for a single-channel simulcast transmitter system. That is,
the same signal is transmitted from multiple locations on the same frequency
at the same time.  When this is not done within 20us at the same time, it will cause
severe distortion of the signal.  When we don't know we are within 20us, we
prefer to not transmit at all, so disable that particular transmitter.

I think I know better what is involved and what the limitations are than you do.
Also, I prefer to discuss with Miroslav, who concentrates on the problem under
discussion rather than casting doubt on everything.  Thank you for you input
until now.

Rob

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