[ntp:questions] Adjusting PPS offset

unruh unruh at wormhole.physics.ubc.ca
Sun Sep 4 22:23:42 UTC 2011


On 2011-09-04, Chris Albertson <albertson.chris at gmail.com> wrote:
> 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.

I am sorry, but since gps used the absolute time to determine your
position on the earth, if the gps works at all, it had better have time
that is good to a few 10s of ns. Otherwise it cannot place you in the
correct place on the earth's surface. So yes, there is a huge reason to
assume that the gps time IS better or more accurate than an Internet
pool server. 

>
> 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.

Always a good idea. 
However, why in the world would the gps receiver give you a pulse per
second? Could you please tell us of some gps receivers with PPS which do not
deliver their pulse on the UTC second? 

(and do not tell me about receivers which deliver 5 pulses per second).

> (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.

Sorry, that is not gps, that is your incompetence.

> (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)

Nuts. It would need a LONG cable to do worse than ntp does. (Maybe about
1000 ft.)

>
> 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".

No you cannot, because you do not know what the assymetry in that path
is. 

> 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.

Your other servers have a path assymetry which is the same for all (
implying that the asymetry is local)

>
> 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

Yes, you need a gps receiver:-)

>
> Chris Albertson
> Redondo Beach, California




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