[ntp:questions] ntp-dev: PPS is a falseticker?

Rob nomail at example.com
Sun Jun 15 06:29:48 UTC 2014


Paul <tik-tok at bodosom.net> wrote:
> On Sat, Jun 14, 2014 at 2:03 PM, Rob <nomail at example.com> wrote:
>
>>
>> Everyone seems to think that GPS equates NMEA.  Wrong.
>> ...
>> It apparently assumes anyone who has a PPS signal also has a
>> device that provides date and time information, which is wrong.
>> ...
>> But of course the assumptions of the main author have been wrong
>>
>
> nomail at example.com thinks you can run NTP as you imagine it should work
> rather than reading the documentation ... wrong.
>
> You have to be able to number the seconds.  If you have any source of
> date/time you can number the seconds.
> These include:
> The network.
> A supported clock type that provides date+time.
> Your TOD clock.
> A clock on the wall.
>
> Since you're connected to the network your problem is solved.  You will
> need to make some effort to understand how NTP works though.

Maybe you can help me?   The following is in the documentation:

------------------------------------------------------------
As the select algorithm scans the associations for selectable candidates,
the modem driver and local driver are segregated for later, but only if
not designated a prefer peer. If so designated, the driver is included
among the candidate population. In addition, if orphan parents are found,
the parent with the lowest metric is segregated for later; the others
are discarded. For this purpose the metric is defined as the four-octet
IPv4 address or the first four octets of the hashed IPv6 address. The
resulting candidates, including any prefer peers found, are processed
by the select algorithm to produce a possibly empty set of truechimers.

As previously noted, the cluster algorithm casts out outliers, leaving
the survivor list for later processing. The survivor list is then sorted
by increasing root distance and the first entry temporarily designated
the system peer. At this point the following contributors to the system
clock discipline may be available:

    (potential) system peer, if there are survivors;
    orphan parent, if present;
    local driver, if present;
    modem driver, if present;
    prefer peer, if present;
    PPS driver, if present.

The mitigation algorithm proceeds in three steps in turn.

1.  If there are no survivors, the modem driver becomes the only survivor
    if there is one. If not, the local driver becomes the only survivor if
    there is one. If not, the orphan parent becomes the only survivor if
    there is one. If the number of survivors at this point is less than
    the minsane option of the tos command, the algorithm is terminated
    and the system variables remain unchanged. Note that minsane is by
    default 1, but can be set at any value including 0.
2.  If the prefer peer is among the survivors, it becomes the system
    peer and its offset and jitter are inherited by the corresponding
    system variables. Otherwise, the combine algorithm computes these
    variables from the survivor population.
3.  If there is a PPS driver and the system clock offset at this point is
    less than 0.4 s, and if there is a prefer peer among the survivors
    or if the PPS peer is designated as a prefer peer, the PPS driver
    becomes the system peer and its offset and jitter are inherited by the
    system variables, thus overriding any variables already computed. Note
    that a PPS driver is present only if PPS signals are actually being
    received and enabled by the associated driver.

If none of the above is the case, the data are disregarded and the system
variables remain as they are.
------------------------------------------------------------

After reading this, especially the sentence after 3., 3rd line, I
believed I could mark the PPS as the prefer peer and not the other
sources, and it would work.

But it doesn't.   I need to make ALL SIX time sources prefer to make
it work.  (or at least the five NTP sources)   Can you please explain?

I don't mind if the situation is complex but that does not mean the
documentation has to be written like this.



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