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    Re: NIST website time accuracy
    From: Douglas Denny
    Date: 2010 Aug 5, 11:36 -0700


    In my opinion you definitely do NOT trust anything to do with computer timed signals - supposedly corrected or not. They are too likely to be corrupt. Also when you have corrected computer time, the accuracy then depends on the computer internal crystal 'clock' .. which is hopeless as you have found: one second in one hour drift is probably quite good for a computer crystal ......

    The only way for general use: i.e. for simple, perfectly accurate to a fraction of a millisecond or better at all times, and absolute time standard timings anywhere in the world is to use the radio time signals. Any simple small communications receiver will do the job.
    Using: WWV in the Pacific or USA. Here in Europe you need to go to RWM (Moscow) on 4996 KHz or 9996 KHz which gives the same kind of WWV signal. Or there are other time signals in the VLF band at 60KHz and 72 KHz here in Europe.

    Loran on 100 Khz was another good source of very accurate time signals but is going to be switched-off soon.

    These signals are only subject to a propagation delay from the transmitter to receiver which you can predict approximately (actually for practical purposes - accurately) knowing the distance from the transmitter (one micro second for every 300 Metres); but there is only a slight phase change of a few cycles one way then the other over a few seconds depending on the propagation/ionosphere in practical terms.

    This method of using radio time signals was the only way of comparing observatory clocks between countries on a more or less continuous basis until GPS. The only other way was to take a rubidium or Caesium time standard physically from one to the other.

    If you want ultimate absolute time standards available, the only way is to use the second time pulses from a GPS receiver which are accurate to about 200 nanoseconds.
    This is the way time standard observatories check their clocks against each other these days. The National Physical Laboratory at Teddington keeps a bank of GPS receivers for this purpose. They also check the accuracy of their clocks in relative terms with atomic time sources of course.

    Douglas Denny.
    Chichester, England.
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