# [ntp:questions] ntpd: time reset problem

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Tue Sep 15 18:36:18 UTC 2009

```Unruh wrote:
> BlackLists 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.)

IIRC, the round trip time approaches 40 min when they are
farthest apart?  With the worst case velocity difference
approaching 121K mph (54KmpS)?  Thats approaching 65km
difference in distance between when a message is sent,
and the response is sent?

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