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

David L. Mills mills at udel.edu
Sun Feb 13 02:00:57 UTC 2005


Danny,

You are too kind. Fact is, one of the dirty rotten things that cannot be 
totally resolved is errors due to systematic delay differences on the 
outbound and return legs. I have a dusty old dissertation here that 
proves that beyond a shadow. Even with a mesh of interlocking servers, 
there is always one less equation than necessary for a deterministic 
solution. However, you can, and the huff-'n-puff filter does, compensate 
for highly assymetric dalays if there is some time during the day/night 
when the delays are substantially equal.

Dave

Danny Mayer wrote:
> 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.
> 
> 
> David,
> 
> 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.
> 
> Danny
> 
>>     DS
>>
>>
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> 
> 



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