[ntp:questions] Re: gps; why pps?

David J Taylor david-taylor at blueyonder.co.not-this-bit.nor-this-part.uk
Wed Mar 29 07:24:17 UTC 2006

Chris Richmond - MD6-FDC ~ wrote:
> In article <j5eWf.43591$wl.43175 at text.news.blueyonder.co.uk>,
> "David J Taylor"
> <david-taylor at blueyonder.co.not-this-bit.nor-this-part.uk> writes:
>> I use both - serial for time and PPS for precision.  If you only use
>> PPS, how are you going to know the time?
> David,  why is one set up as a fudge verses both being servers
> in your config?
> Chris

Extract from my config:

#       The GPS receiver on COM1 at 4800 baud
#       mode 1 = use $GPRMC statements
#       time1 = trimming offset
#       flag3 1 = enable Kernel PPS discipline
server    mode 1       prefer
fudge    time1 0.000  flag3 1  refid PPS

The fudge command allows me to adjust certain parameters of the specified 
server.  In this case, I alter:

- the reference ID to the string "PPS"
- flag 3 to the value 1, enabling the kernel PPS discipline
- the time offset to 0

The latter adjustment does nothing, but had my pulses been the opposite 
polarity to the ones I actually have, the NTP software might have locked 
to the trailing edge of the pulse 200ms after the leading edge, and rather 
than change that in hardware I could have set time1 to +0.200 (or 
whatever) to get the correct time.

Does that help?  I'll add it to the Web page.


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