[Tfug] A sense of time

[Bexley Hall]

--- Louis Taber <ltaber at gmail.com> wrote:

> Time again,
> 
> I couldn't locate the documentation, but I seem to
> remember that you can
> slew the clock to set it to a new time on some UNIX
> type systems.  So the
> time at any given instant is moving toward the new
> setting and not making a
> finite jump.

Yes.  But, the hack not only ensures the transition
is "graceful" but it also tries to avoid letting time
run backwards.

>  With ntp (Network Time Protocol) it is
> not really an issue.
> ntp in general is a MUCH better way to go than
> trying to set the time
> yourself.
> 
> If you really want to know what time it is get a GPS
> receiver, not an
> "Atomic Clock".  An "Atomic Clock" looks at a single
> 60kHz signal from Fort
> Collins CO.  This signal is usually not adjusted for
> the signals time of

Correct.  And propagation delays vary over land,
water, etc.

But, you can still use any such precision reference
to create a *frequency* locked loop -- i.e. your
time per unit time will be "perfect" ... but, may be
off from the *actual* time by a fixed (but very small)
amount.  Usually, people (consumers) are more
interested in how well a device "keeps time", not
how *accurate* that time is (i.e. if it is always
off by 2 minutes, they don't care.  But, if it
gaisn or loses time OVER TIME, then it "needs
attention").

> flight.  A GPS receiver figures out where it is
> specifically  by looking at
> the various time of flights.  If you have access to
> L2 it also compensates
> for the lensing effects of the atmosphere. 
> Internally a GPS receiver will
> know what time it is to about 10-20ns.  About the
> time it takes light to
> cross a medium size room.  A system with a GPS
> receiver connected to it with
> a PPS (pulse per second) and serial connection can
> be set up as a stratum 1
> ntp server.  If this is on a fast local area
> network, all of the connected
> systems can be quite accurate as well.  At least
> good enough to snipe on



       
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