[ntp:questions] Adjusting PPS offset

Chris Albertson albertson.chris at gmail.com
Sun Sep 4 18:15:18 UTC 2011


On Sat, Sep 3, 2011 at 10:27 PM, unruh <unruh at wormhole.physics.ubc.ca> wrote:
> On 2011-09-04, Chris Albertson <albertson.chris at gmail.com> wrote:
>> On Sat, Sep 3, 2011 at 4:34 PM, A C <agcarver+ntp at acarver.net> wrote:
>>
>>> What I meant was what method is the best way to determine the value of
>>> time1.
>>
>>
>> That is what I meant above when I said, setting it is easy.  Knowing
>> what to set it the is the hard part.  The simplest way is to connect
>> with a few pool servers then "Fudge" your GPS' time for best match to
>> the Internet servers.
>
> Since the pool servers are a few orders of magnitude worse than PPS,
> that would not be a vary useful way.

There are two kinds of accuracy  One is what I think NTP calls
"jitter". This is the randomness about the "tick".   The other is the
absolute phase of the PPS.    A directly connected GPS will have very
low jitter but there is zero reason to assume its absolute phase is
"better" or more accurate then an Internet pool server.    In fact on
a newly connected GPS I'd suspect the phase is not correct, not until
I measure it.

There are a few reasons a GPS might have the phase as wrong as 1/2 second.
(1) Your GPS unit was not designed to have PPS fall on the exact UTC
second.  I think many navigation GPS are like this.  Read the user
manual.  Many don't even have this in their specification.  These
units claim to have a pure every second but make no claim about WHEN
current the second to pulse occurs..  READ the user manual carfully.
(2) You may have the polarity backwards.  Remember that RS232 uses
different convention for the control pins as from the serial data
pins.  One uses positive voltage for "1" the other is negative.  If
you get this wrong the phase of the PPS will be "off" by whatever the
pulse width is.
(3) smaller error comes from delays in the cables.  But this is likely
less than NTP can measure although important in precision timing
(where nanoseconds count)

As for internet servers beiing "worse" than your GPS.  I disagree.
Those severs are likely using GPSes.   What's worse is only the
communications path.  There is more jitter in the path from the server
than from a local GPS.    But we can average over a day or so and
determine the average difference in phase. GPS calls this "offset".
If you run your GPS and several Internet servers for a day or so NTP
will compute a good "offset number".

If multiple servers all agree on an offset from your GPS you have to
make a decision:  Is your GPS correct and all the other servers are
wrong by the same amount or is your system the one that is offset from
true UTC time.

The pool servers likely will not all have identical offset.  The
"spread" will limit how accuracy you will be able to use this method.
 The best you can do is adjust your NTP server so is sees on average
zero offset with most other servers but you wil never get to exactly
zero.  It will bounce around.    As I sad above in another email.  You
need a trusted clock.  This can be done but some specialized hardware

Chris Albertson
Redondo Beach, California



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