[ntp:questions] ntpd: time reset problem

Unruh unruh-spam at physics.ubc.ca
Tue Sep 15 05:57:06 UTC 2009


E-Mail Sent to this address will be added to the BlackLists <Null at BlackList.Anitech-Systems.invalid> writes:

>Unruh wrote:
>> BlackLists writes:
>>> David Lord wrote:
>>>> Can you then suggest how it's at all possible for ntpd
>>>>  to be of any use on a mobile connection with such high
>>>>  latency?
>>
>>> Interplanetary Timekeeping ?
>>> <http://www.cis.udel.edu/~mills/ipin.html>
>>
>> IF the latency is known to be symmetric ( which it is
>>  on an interplanetary system) then large latency is no
>>  problem.

>I don't see how they are likely to have symmetric latency.

> e.g.
>  The earths orbital velocity is something like 67k mph?
>   mars orbital velocity is something like 54k mph?

>  If you were to send a message, and wait for a response,
>   the planets would by significantly closer or farther
>   apart when the response was sent?

The constant velocity does not matter. (Relativity). Only the
acceleration matters, and the accelerations are small. 
I am on mars. I can regard mars as at rest ( and in fact the time on
mars demands that I do so. I send a message to earth. It returns it
after much less than a second. The difference in distance in that time
is tiny (eg in a msec the relative motion of the earth is much less than
a Km, which means that the difference in travel time is less than
10usec.)

>   {Then again I know almost nothing about astrophysics.}


>> If it is not known to be symmetric and worse, both in
>>  and out latencies are variable, then it is pretty well
>>  impossible.
>>
>> Note that despite this, ntp is probably the best you
>>  can do. Ie, there are no other procedures which will
>>  give you better time with those kind of latencies.




More information about the questions mailing list