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Re: GPS ERROR ? Kep's



Thanks Tom for straightening out the quake / time issue for me.
Well done I say.

Joe  K0VTY
Amsat # 860
Amsat Area Coordinator NE
================================
On Fri, 21 Jan 2005 00:29:13 -0500 "Tom Clark" <w3iwi@toad.net> writes:
> OK -- let me weigh in here with the definitive answer. The effects of 
> the
> earthquake on the earth are really hard to see!
> 
> The "official" source for UT1 (the position of the earth relative to 
> the
> fixed stars) is Very Long Baseline Interferometry (VLBI -- see
> http://ivscc.gsfc.nasa.gov/). Some of you may know that I developed 
> the VLBI
> network that makes the observations before I retired from NASA (My 
> group can
> be seen at http://lupus.gsfc.nasa.gov/). It was my colleagues who 
> published
> the material cited in the NASA press release seen at
> http://www.nasa.gov/home/hqnews/2005/jan/HQ_05011_earthquake.html. 
> The real
> publication was in last week's American Geophysical Journal's weekly
> newspaper called EOS (I can supply a PDF version for anyone who is 
> seriously
> interested).
> 
> If you want to see the actual quick-response data, it is published 
> by the US
> Naval Observatory in Bulletin A at http://maia.usno.navy.mil/. In
> particular, take a look at the plot in
> http://maia.usno.navy.mil/bullaprobt.gif and note that the 
> earthquake,
> Dec.26 was on MJD 53365. You might also be interested to note the 
> changes in
> X- and Y-pole position in http://maia.usno.navy.mil/bullaprobx.gif 
> and
> http://maia.usno.navy.mil/bullaproby.gif (On these two plots, 1
> milliarcsecond is about 3 cm at the surface of the earth. The +X 
> axis passes
> thru the Greenwich meridian and the Indian Ocean earthquake lay 
> right on the
> +Y axis at 90d East).
> 
> The reason UT1 varies with time is that the rotating earth must 
> conserve
> angular momentum. On time scales of seasons, the principal driver is
> variations in atmospheric angular momentum (AAM) due to global wind 
> fields.
> On the plot, you will see the result plotted as AAM of integrating 
> global
> climate models. On a longer time scale we have effects of oceanic
> circulation changes, and on even a longer time scale (like 10's to 
> 1000s of
> years) we have changes in the circulation of the molten rock under 
> our feet.
> These very long scale effects are undoubtedly coupled to the dynamo 
> that
> makes the earth's magnetic field, and these flows are known to 
> change enough
> to reverse the earth's magnetic field on time scales of hundreds of
> thousands of years. Also on long time scales, we have effects from 
> the tidal
> dragging of the moon on the earth. Aeons ago, the day was only 18 
> hours and
> the moon has dragged it down to 24. To compensate for this in the 
> earth-moon
> system, the moon is slowly receding from the earth (at a speed, if I 
> recall
> correctly ~ 1 cm/year). A brief description of UT1 variability can 
> be found
> at http://www.iers.org/iers/earth/rotation/ut1lod/ut1lod.html. To 
> see that
> UT1 (i.e. the time read out by your sundial) has varied over a ~3 
> year
> period, see 
> http://www.iers.org/iers/earth/rotation/ut1lod/figure1.html;
> corresponding to this plot, the actual variability of the length of 
> each day
> is seen in 
> http://www.iers.org/iers/earth/rotation/ut1lod/figure2.html.
> 
> Except for effects of precession and nutation, the earth's spin axis 
> is
> fixed in space. However the crust of the earth shifts with respect 
> to the
> inertial axis. The X & Y pole (and hence your apparent latitude) 
> wander by a
> few meters, typically over time scales of ~13 months (called the 
> Chandler
> wobble, see http://science.howstuffworks.com/question442.htm and 
> listen to
> this July 200 report at 
> http://www.npr.org/ramfiles/me/20000718.me.04.ram).
> 
> Hope this helped -- 73 de Tom, W3IWI
> ----
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