[ntp:questions] Re: Good GPS for attic?
Richard B. Gilbert
rgilbert88 at comcast.net
Sat Feb 12 05:07:28 UTC 2005
David Schwartz wrote:
><web1000 at shaw.ca> wrote in message
>news:1108154117.744445.33070 at g14g2000cwa.googlegroups.com...
>>how about getting the atomic clock time via internet?
> Sadly, this doesn't really work. The problem is that to request the
>time, you have to send out a request and then wait for a reply. Unless the
>request takes exactly as long to get to the clock as the reply takes to get
>back, you have no idea what to do with the time when you receive it.
> Say you send out a request for the time and get a reply 30 milliseconds
>later. The time in the reply was correct sometime between when you received
>it and 30 milliseconds before that. You can 'guess' 15 milliseconds,
>assuming it took as long for the query to get to the clock as it took the
>reply to get to you, but this is no more than a guess. Your time can be as
>much as 15 milliseconds off.
It works as designed! In the case you describe, you know what time it
is plus/minus 15 milliseconds. If you configure four or five servers
you may get slightly better time. If you need accuracy greater than
+/- 10-15 milliseconds, you are correct in that you probably can't get
it from the internet. For many applications +/- 10-15 milliseconds is
For those that need accuracy greater than this, GPS clocks are probably
the way to go. Rubidium or Cesium clocks are also possibilities for
those with lots of money.
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