[ntp:questions] ntpd: time reset problem

David Mills mills at udel.edu
Thu Sep 17 03:14:03 UTC 2009


Terje,

Funny you should ask. Interplanetary timekeeping is covered in Chapters 
17 and 18 in the second edition of das Buch. However, one of the first 
things to learn in space is that where you are is much more important 
than what time it is.

To compare clocks on the Earth and Mars surfaces you have to transform 
proper time on the planetary surface to coordinate time at the solar 
system barycenter. To do that, you have to account for general 
relativity effects of planetary gravity (redshift) and surface velocity 
(time dilation) in the planetary-centric inertial frame. This is 
complicated by the facts that Earth bulges at the equator and Mars is 
something like a pear revolving on its side. For Earth a clock at the 
planetary center runs about 60 microseconds per day fast relative to one 
at sea level.

Then you have to transfer coordinate time at the center of the Earth to 
coordinate time at the solar system barycenter. Accounting for gravity 
and orbit variations this results in difference of approximately 17 
milliseconds over the Earth orbit.

Not done yet. Knowing the detailed geometry and motion of the surface 
state vector of both planets, you can use an iterated procedure 
described in the book to determine the light time and time of arrival of 
a ray from Earth. The NASA/JPL Deep Space Network can determine a 
position on Earth to within 2 cm and range to a spacecraft within 10 ns 
using radiometric ranging and lots of averaging. Using means described 
in the book, the spacecraft clock can be set to a nominal accuracy of 15 ns.

Using current techniques the spacecraft clock can be set within a few 
milliseconds via the telemetry stream. To get the ultimate accuracy 
would require minor changes at the DSN and spacecraft electronics. Given 
the current financial state and the slippage for the Mars Science 
Laboratory mission of two years, I doubt JPL is in the mood to do that 
anytime soon. However, the technology might be first deployed for 
missions to the Moon polar regions and used for vehicle navigation.

Disclaimer. You might consider this advance publicity for the book, 
which is about ready for the publisher.

Dave

Terje Mathisen wrote:

>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
>
>




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