Re: [Ntp] Antw: [EXT] Re: The bump, or why NTP v5 must specify impulse response

[Hal Murray <[e-mail]>]

There are 2 parts to PPS.

The first is part of the normal refclock tangle.  The time accuracy of the 
serial text from a typical GPS is crappy.  Most OS-es provide a method of 
grabbing a time stamp at interrupt time from something like a PPS connected to 
a modem control signal.  That gets much better timing.  The complication in 
this area is that the PPS doesn't tell you which second it is associated with. 
 You need something like the typical NMEA sentences to tell you that.  So the 
PPS gives you the bits to the right of the decimal point and something else 
gives you the bits to the left of the decimal point.

The other part is that many kernels have an option to do-it-all in the kernel. 
 (It's probably not configured in the kernel shipped by your distro.  You have 
to build your own.)  ntpd (or whatever) sets a flag and the kernel does the 
rest.  Normally, ntpd would get the time close enough, turn on kernel PPS 
mode, then watch the serial stream and disable the kernel mode if the serial 
stream said that GPS stopped working.  This generally works much better than 
not using the kernel mode.

I'll come up with some graphs if people want real data.

I don't know why the kernel mode works better.  My guess is that the PLL 
parameters are tuned "better", but I don't see any reason why that code 
couldn't be move to user mode.  Trying that hasn't gotten to the top of my 
list.

------

To answer your question:  Yes, you should be able to distribute a PPS to 
multiple systems.  It's simpler if you distribute the serial signal with it.  
You can make a splitter.  It's just a couple of RS-232 level shifter chips.  I 
don't know of any commercial version.

Some GPS gear just sends the serial text every second once you get it 
configured correctly.  Some needs to be poked every second.  If you run the 
same software on 2 systems connected via a splitter, one of the pokes falls 
into the bit bucket.  That works fine as long as the system doing the poking 
is running but the non-poking system won't get any serial data if the other 
system crashes.

-- 
These are my opinions.  I hate spam.