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

[Hal Murray]

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

> Yes. In SiRF receivers ZDA is specifically aligned to the second. I
> just checked my copy of the manual I got with my BU-353, and Table 1-1
> NMEA Output Messages says "ZDA     PPS timing message (synchronized to
> PPS)." RMC is produced after the navigation solution is computed and
> that does not happen in constant time.

My documentation for ZDA says "Each message is output within a few hundred ms 
after the 1 PPS pulse..."  I was hoping for something (much) better than 
that.  I don't actually care about the offset, I can correct for a constant.  
It's the jitter/wander that I'd like to avoid.

I finally got time to try it.  The released ntpd doesn't support ZDA, but I 
hacked in some code.

ZDA isn't any better than RMC.  That's just an eyeball of a graph of the 
peerstats offset, not a formal mathematical analysis.  It's still got roughly 
100 ms of wander/jitter, probably more if I wait longer to collect more data. 
 I don't see anything good enough to be worth any effort to investigate more.

Another problem is that ZDA doesn't have a status field to tell you if the 
time is invalid because it isn't seeing enough satellites.

It's also got the leap second bug.



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