[time-nuts] The name "Totally Accurate Clock"

Jim Lux James.P.Lux at jpl.nasa.gov
Tue Aug 5 19:12:35 EDT 2008


At 02:10 PM 8/5/2008, Tom Clark, K3IO wrote:
>    Rob noted:
>
>Good point. How was the position of the so called "Totally Accurate Clock"
>obtained. Strange name though - no such thing as a totally accurate clock...
>
>  :-)
>
>Rob
>
>    Here is the story -- and some of the details are in my contributions on
>    [1]http://gpstime.com:
>    For many, many moons, I headed up NASA's program that developed Very
>    Long Baseline Interferometry (VLBI) as a tool to make various
>    geophysical measurements. The measurements yielded up
>      * Accurate vector determination of the motions of the earth's
>        tectonic plates ("continental drift", now known to ~20 microns/yr).
>      * Very accurate measures of the rotation of the earth with respect to
>        the celestial reference frame, aka UT1 (in effect "sundial time"),
>        now observed at levels of a few tens of usec on a daily basis. VLBI
>        is the "official" BIPM and USNO source of UT1, including the info
>        on which we have a leap-second pending.
>      * Accurate positions of the earth's rotation pole (polar motion,
>        nutation & precession) and of a few hundred extragalactic radio
>        sources at the sub-milliarcsecond level.
>      * And some nifty astronomy too!

And, such things as one way DeltaDOR measurements for navigation of 
deep space probes, and straight interferometry for measuring the 
position of the Lunar Rover. (the paper after yours at the 1972 PTTI 
conference)
Other nifty applications at:
http://www.sciencemag.org/cgi/reprint/sci;178/4061/607.pdf by 
Counselman, et al.

Also used to do stuff like measure the winds on Titan and Venus. (but 
that's the astronomy Tom referred to)




>    Each VLBI station around the world has a Hydrogen Maser as its local
>    clock (Jim Palfreyman's note concerned the VLBI station near Hobart,
>    Tasmania) serving as both the stable  short-term phase reference for
>    microwave observations, as the timing clocks for data bits written onto
>    tape (now replaced with disk arrays), and as the still longer term
>    reference to tie between UT1 and UTC. If you look at my VLBI timing
>    tutorials on [2]http://gpstime.com you will see info on VLBI's timing
>    requirements.

For DeltaDOR they use a quasar as the (almost)simultaneous reference.


I would have thought there would be a easy to find JPL reference on 
this (since we actually do it here, on the recent Phoenix landing, 
for instance), but it would appear that ESA has the nice description.
http://www.esa.int/esapub/bulletin/bulletin128/bul128i_madde.pdf

Here's a very simple description
http://www2.jpl.nasa.gov/basics/bsf13-1.html


Jim





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