Re: WiFi & NTP.

In article <20070831031051.597CD4500C@xxxxxxxxxxxx>,
oberman@xxxxxx (Kevin Oberman) wrote:


Kevin Oberman wrote:
From: "Richard B. Gilbert" <rgilbert88@xxxxxxxxxxx>
Date: Thu, 30 Aug 2007 10:45:01 -0400
Sender: questions-bounces+oberman=es.net@xxxxxxxxxxxxx


Maarten Wiltink wrote:

"Guy" <gdelavil@xxxxxxxxxxxxxxxxxx> wrote in message
news:1188474891.630591.247300@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx



Something important I forgot to mention is that my goal is not to
have the most precise absolute time but to have the smallest offset
as possible on a local network and using a WiFi connection.

So I declared one of my devices as an NTP server and the other as a
client (syncing to a single time source as a matter of fact).


The 'problem' with that is that you are then at the mercy of the
stability of your server. The stability of the crystal that ultimately
drives a local clock reference isn't great, it's quite temperature-
dependent.

On the other hand, it _is_ quite predictable - people have produced
graphs where it is clearly visible when the airconditioning in the
server room started up, or when doors and windows were opened.

The lesson as I read it is to make sure your server has a stable
environment. If the motherboard temperature is constant, so will be
the crystal speed. However, you are surrounded here by some rather
hardcore geeks, who read the lesson differently as requiring that you
have the One True Time, and stability will naturally follow from it.

Groetjes,
Maarten Wiltink



Let's hear it for "The One True Time"!


And which time is that? UTC, GMT, TAI, ...?

UTC!


I hate leap seconds!!!

Leap seconds do not affect my daily existence. My computer takes care
of them.

Sorry, but that is beyond the capability of computers. It takes astronomers
to
determine when one is needed and humans to teach the computers and other
equipment. Only your 'one true time', UTC has this problem. GMT continuously
adjusts (also ugly) and TAI makes no attempt to stay in sync with the earth.

Note that GPS has no concept of the leap second There is a mechanism for
handling this, but at some level, it requires human intervention. NTP is nice
because it means that YOU don't personally have to deal with them, but some
of
us do and it is a royal pain. After the last one it was over a month before
all of the CDMA and GPS systems I sync against to adjust their times, so I
had
to adjust my stratum 1s twice, once after the second was inserted and then,
one at a time as the carriers got around to them.

It is all made worse because POSIX does not do leap seconds. To do it right,
a system as to be able to run one 61 second minute and POSIX makes this
impossible, so there is no really accurate way of making most computers do
leap seconds. Assuming your stratum 1s are properly handling leap seconds,
NTP has to drift the time, so it is off by quite a bit for quite a while.

The core requirements for POSIX time is to support file timestamps, and
thus file versioning such as that done by make. This has to work in a
totally isolated computer, one with no access to GPS or phone lines or
radios or to the astronomical data required for UTC or UT1. So POSIX
time is basically a form of TAI, at least in theory, although the
standard is silent on the connection.

In practice, many people set their GPS receivers to emit UTC (versus GPS
System Time), and let their computers follow that time using NTP. The
only problem with this is the gyrations subsequent to a leap second.

Where these gyrations cannot be tolerated, one approach is to set the
GPS receivers to emit GPS System Time, and make UTC as needed where
needed in application code, the bulk of the system using GPS System Time
(which is explicitly a form of TAI).

Joe Gwinn
.

Relevant Pages

  • Re: NY Times Op Ed on Discontinuation of Leap Seconds
    ... > in-my-lifetime consequences will be of giving up leap seconds. ... > of satellites and GPS receivers. ... > redoing the whole GPS system. ... If this is the case then assuming a need for leap-second adjustments ...
    (alt.horology)
  • Re: Daylight Savings Time - How to Set?
    ... If the rate at which the earth rotates ... > The GPS system does not have the leap seconds and is now out of time by ... GPS time differs from UT by 13 seconds, if I remember correctly, because, as ... you say leap seconds were not taken into account, ...
    (alt.os.linux.suse)
  • Re: NY Times Op Ed on Discontinuation of Leap Seconds
    ... offset to compute UTC from GPS time. ... there are other reasons for omitting leap seconds - not having to ... Since the scientists are all known to be Godless Communists, ...
    (alt.horology)
  • Re: GPS vs ADF
    ... Use of GPS in lieu of ADF and DME ... Waypoints, fixes, intersections, and facility locations to be used for these ... Charted requirements for ADF and/or DME can be met using the GPS system, ...
    (rec.aviation.ifr)
  • Re: Extra leap seconds due to earthquake?
    ... >> On the GPS front, I think that GPS internal system time (UTC as at ... >> satellites) is now 13 seconds ahead of today's UTC, ... any news on when the next leap second will be added? ... > It is expected that Celestial Pole offsets related to the IAU1980 Nutation ...
    (sci.geo.satellite-nav)
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