[ntp:questions] PPS and NMEA

Chris Albertson albertson.chris at gmail.com
Mon Sep 10 17:09:46 UTC 2012

I am surprized no one, including myself was said this using precises and
technical terms.  In can be stated in two short sentences:

   - The purpose of NEMA is to "number the seconds"
   - The purpose of PPS is to define the start of the UTC second.

I tried to say this above in a previous email but when I re-read it I see
it is not clear.   In the telephone analogy the voice recording "numbers
the seconds" and the "beep" defines the start of that second.  One could
imagine a voice time system that lacks the "beep" and it would be good
enough for most people, you'd still get to work on time.

A GPS' NMEA output is an ASCII serial message that says the current time
and location and by the NMEA standard it is required to be output "during
the second to which it applies".  (The quote is from the NEMA specification
and is very close, but is from memory)  This means it can be almost a full
second "off".  But typically it is not so bad as that.    The PPS on the
other hand is a simple pulse and the raising edge of the pulse is specified
to occur right on the start of the UTC second.  Of course there is some
error.  The amount of error depends on the care you put into selecting
parts and your budget.  But much care and $100 will buy you an error of
only a few tens on nano seconds.  NEMA alone will typically get you  a few
tens on milli seconds.  Literally a million times worse.   That said,
nanosecond level accuracy is wasted on NTP, microseconds are good enough
fro driving NTP.   But perhaps you have other timing equipment driven by
the same GPS reciever?

I hope this is now clear.

Chris Albertson
Redondo Beach, California

More information about the questions mailing list