[ntp:questions] Single GPS/PPS time source gets marked as a falseticker

Chris Albertson albertson.chris at gmail.com
Thu Jul 14 16:16:02 UTC 2011


On Thu, Jul 14, 2011 at 7:04 AM, unruh <unruh at wormhole.physics.ubc.ca> wrote:

> Why? That native binary will also suffer from large latency (the serial link
> is slow no matter what is being sent over it) and probably also fairly
> large jitter. And it suffers from the huge downside of your having to
> learn a whole new language.

Those ASCII characters have to be assembled from internal binary data.
 Binary to ASCII conversion is not a fixed time process.  for example
the software might convert "3" faster then "57".   And then some
GPSes, I don't know about yours don't transmit leading zeroes so that
sometimes the NEMA sentance is longer of shorter depending on the data
values.

Finally ASCII is MORE BYTES.  so yes both are sent at 1200 bps but
less data are sent if it does not need to be in ASCII and you avoid
the conversion time.  For example a floating point value is four bytes
in binary and 8 or 9 in ASCII and therefor take half the time to send
and zero time to convert.

About the PPS being "dead on".  How did you compare the PPS to the UTC
second?  I think you meant the period was one second as close as you
could see by eye on the screen but the phase was unknown.    At some
point you will need to figure out the "fudge" number of the PPS
otherwise your NTP time could be off.

I'm not surprized that this is hard.  You are setting up a stratum 1
NTP server in the ocean using some low-end equipment.  That is outside
of what people normally do and you'd expect to do some problem
solving.

-- 

Chris Albertson
Redondo Beach, California



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