[time-nuts] Leap Second quirk, Another hanging bridge

Hal Murray hmurray at megapathdsl.net
Tue Aug 12 19:29:05 EDT 2008

> Yep, that's to be expected, given the iterative solver they're using.
> Try turning on the ZDA message, apparently the $ sign is aligned to
> the start of the second ... if its implemented in your version of
> firmware. Since you're running a BU-353 you could have all kinds of
> broken things in your firmware (not impressed by Globalsat). 

Do you expect the ZDA message to be better synchronized than any of the 

Is the BU-353 any better or worse than any of the other units using the SiRF 
chips?  I've tried several different brands.  I can't tell them apart unless 
I look at the physical package.

> As an aside, how and what are you collecting? I'm starting to think
> about setting up different loggers to catch the leap second... 

I just plug them into ntpd, use the noselect option on the server line, and 
turn on ntpd's statistics.

> I've used the LVC w/ PPS as the reference clock... not sure that I
> trust the delay/jitter characteristics of USB enough to give me better
> time than a wrist-watch. 

USB isn't fundamentally evil.  It's polled, so you won't get great response 
to something like a PPS interrupt.  But the polling is handled automagically 
with modern hardware so It's not much worse (maybe better) than the typically 
interrupt batching that RS-232 chips do.

I think USB will be good enough if your target is a few milliseconds rather 
than a few microseconds.

These are my opinions, not necessarily my employer's.  I hate spam.

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