[ntp:questions] ntpd: time reset problem

Unruh unruh-spam at physics.ubc.ca
Wed Sep 16 01:00:38 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:
>>> 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?

So what? The signal goes from Mars to earth-- 20 min. On earth the clock
timestamps the receipt and the sending of the ntp packet. -- typically .00001 sec.
between those events. During that time the earth moves say 5 m. 
Then going back the packet
takes 20min-5/3*10^8= 20min-.000000016 sec. Ie, the outward and inward
time delays are the same to .016usec. Who cares what the earth does
before or after it receives the packet? That the earth happens to be
60000 km closer to mars when the packet arrives at mars is irrelevant.

You are probably trying to analyse things from the viewpoint of the
earth. That is called the "synchronization" effect in special
relativity. Yes, the midpoint of the receive/send time on earth and the
midpoint of the send/receive time on mars are mot the same according to
the earth bound observer, but they are to the Mars observer which is
what counts. (In special relativity things which are synchronized to one
observer, or not to a moving observer)

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