[Aavso-photometry] GPS or internet time?

[Wolfgang Renz]

Hi Yenal

> I am tying to set my computers clock as accurate as possible. Until
> now, I was using internet's several time servers which sync computer
> clock using XP's date and time properties. Last night, I compared the
> Autostar's time (which was set by GPS automatically) and computer's
> time and I saw that they differ 5sec.
> Why is there that much of difference?
> Which one should I trust?

If you don't have memory usage issues, its probably best to set up
a local time server background process.
But for most folks a simple time client like e.g. "AboutTime" will
do the job sufficiently. These ones one can also easily exit when
they are not needed.
With such software one can usually sync the clock every x minutes. So
even if the computer clock runs away (e.g. during booting or due to lost
time interrupts during higher priority tasks or I/O), one can bring the
clock back every x minutes and limit the differences to a minimum.

Most computers have a BIOS CMOS clock that is powered by a battery
or a capacitor and hold the time while the computer is off. During boot-
ing this clock is read to set the computer clock in memory that uses time
tick interrupts to follow the time. If these time tick interrupts get interrupted
or overlayed by higher priority interrupts, they are lost and the clock will
run away in time.

Does your clock show a constant time offset ?
Or does it run away with time ?

You do use Atomic Clock Sync, right ?
How often does Atomic Clock Sync actually sync the computer clock ?
If the clock runs away and you sync just seldom (e.g. just during boot time),
it might not be sufficiently often.
Can you set there how often to sync the clock ?
Does it consider the ping round trip time ?

It would be interesting to get to know if other Meade LX90/LX200
GPS/ACF users observe the same issue. Then it could be also due
to the Meade GPS unit or the Autostar (they might loose clock ticks
too while being bussy with more complex tasks).

Clear skies
 Wolfgang

-- 
Wolfgang Renz, Karlsruhe, Germany


To read more on time issues in general just take a look at the former
discussions on the AAVSO lists:
http://www.google.com/search?q=site%3Awww.aavso.org+Internet+Time+Sync
http://www.google.com/search?q=site%3Awww.aavso.org+Atomic+Clock+Synchronization
http://www.google.com/search?q=site%3Awww.aavso.org+Time+Sync+Software
http://www.google.com/search?q=site%3Awww.aavso.org+Clock+Control
http://www.google.com/search?q=site%3Awww.aavso.org+Time+Loss+on+Camera
http://www.google.com/search?q=site%3Awww.aavso.org+SNTP
http://www.google.com/search?q=site%3Awww.aavso.org+Clock+Time
http://www.google.com/search?q=site%3Awww.aavso.org+Computer+Time