PC time control (was Re: Obs for Feb. 12 and Skymed 3 Flare)

From: Marco Langbroek (marco.langbroek@wanadoo.nl)
Date: Sun Feb 13 2011 - 15:54:58 UTC

  • Next message: Kevin Fetter: "Feb 13 obs"

    Op 13-2-2011 15:28, Mark Robinson schreef:
    > On 14/02/11 00:28, Marco Langbroek wrote:
    >> Op 13-2-2011 02:24, Bill Arnold schreef:
    >>
    >>> This observation was timed with a digital watch that was calibrated with time.gov just before I went out to watch. The time was stated to be accurate to within 0.2 seconds. My watch was in sync with the time.gov page.  The watch was calibrated again when I came back in from outside, after about 20 minutes, time remained the same.
    >>
    >> I would strongly advise against using webbased time services for time
    >> calibration. Even if the time server itself is correct, internal processes in
    >> your PC and your ISP's network introduce time lags.
    >>
    >> Time signals obtained over the internet *never* can be accurate.
    >
    > Nothing is ever completely accurate, however NTP manages to hold my machines
    > within a few ms.
    
    Unfortunately, variable protocols running on your PC and your operating system 
    assigning variable priorities to different instances of software running on your 
    pc, introduce variable delays *internally* in your pc in distributing time 
    signals to software or your display. For this reason it is never wise to use the 
    pc clock for accurate time keeping when you need subsecond accuracy.
    
    It is exactly for this reason that for example observers of asteroid 
    occultations use a GPS time inserter that inserts the time-mark into the video 
    stream *before* it enters the pc. Practical results show that otherwise the 
    time-keeping is simply not reliable enough.
    
    My own PC adjusts its internal clock each 5 minutes (using Dimension4, not the 
    native Windows thingy), yet I can often see clear discrepancies between the pc 
    clock changing seconds on the screen compared to a DCF77 clock.
    
    - Marco
    
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