# [ntp:questions] ntpd: time reset problem

Terje Mathisen Terje.Mathisen at tmsw.no
Wed Sep 16 07:19:42 UTC 2009

Unruh wrote:
>> 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 (for once) wrong here Unruh:

To make it simple we can assume that the Earth is standing still and
only Mars is moving (towards Earth):

ntpd on Mars sends a request at (local) time t1, it is received and
returned by Earth ot times t2, t3 (which are effectively the same), then
received back on Mars at t4, right? This is standard ntp after all!

However, due to the way Mars is getting closer all the time, the return
path happens to be ~60000 km shorter, so (t4-t3) will be 2 ms less than
(t2-t1).

This bias is something I'm sure Dave Mills' interplanetary version of
ntp knows about, and can therefore correct for.

> 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)

IMHO, this is simply wrong.

Terje

--
- <Terje.Mathisen at tmsw.no>
"almost all programming can be viewed as an exercise in caching"