[ntp:questions] Re: Good GPS for attic?

Danny Mayer mayer at gis.net
Sun Feb 13 01:17:50 UTC 2005

At 07:53 PM 2/11/2005, 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?
> >
> > Matt
>     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.


Dave Mills is much smarter than that. He originally wrote the code some
25 years ago when the internet was young and unreliable (relatively).
One of the fundamental assumptions was that the time to send and the
time to receive would probably NOT be symmetic. In fact it wasn't
even assumed that if you sent a packet it would arrive or if it arrived
you would receive the packet or that it would return on the same path
that the original packet used.

The algorithms are that good.


>     DS
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