[time-nuts] Sun Outage

Didier Juges shalimr9 at gmail.com
Sat Oct 11 07:50:32 EDT 2014


"Just to put that in perspective, we're measuring a few degrees of phase 
shift in a 32 GHz signal on a path that is over a billion km long."

Now this is fully qualified nuttiness :)

Didier KO4BB


On October 10, 2014 8:17:13 AM CDT, Jim Lux <jimlux at earthlink.net> wrote:
>On 10/9/14, 10:16 PM, Andy wrote:
>> Bob Stewart <bob at evoria.net> wrote:
>
>>
>> It occurred to me that one could use satellite signals as a
>meteorological
>> instrument to measure the water density in the atmosphere above you. 
>I
>> wonder if the NWS does that.
>>
>
>WHy yes they do: that's what weather radar is. It detects the 
>reflections from the rain drops or ice crystals in the storms. These 
>days, it's doppler radar, so not only do you get the density of the 
>return but whether it is moving towards or away from the radar.  If 
>multiple radars in different places cover the same volume, you can get 
>full X-Y motion.
>
>
>On a more time-nutty note, they also use the small variations in GPS 
>signal propagation to do this kind of measurement.  COSMIC (and soon to
>
>be launched COSMIC-2) measure GPS signals passing through the
>atmosphere 
>from satellite to satellite- grazing the earth's surface, and by 
>measuring the phase and amplitude variations (because you know the 
>underlying GPS signal is locked to an atomic standard), you can infer 
>the properties of the atmosphere at various elevations.
>
>Such radio occultation measurements are the 3rd or 4th most useful 
>measurement in feeding the numerical models that are used for weather 
>prediction.
>
>
>On an even more gnat's eyelash time measurement note:
>We use radiometers (basically a sensitive power meter) to measure water
>
>vapor content (and, incidentally, cloud cover) at the DSN stations, to 
>remove some of the variation in the measurements of propagation delay
>to 
>and from spacecraft.  By carefully gnawing away at all sources of
>error, 
>we can measure the round trip light time with accuracies of 1E-14 (1000
>
>second tau), which is how we can measure range to something at Saturn
>to 
>a few cm, and radial velocity (range rate) to a few mm/sec.
>
>Just to put that in perspective, we're measuring a few degrees of phase
>
>shift in a 32 GHz signal on a path that is over a billion km long.
>
>
>http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/18497/1/99-1986.pdf, 
>page 7, shows some radiometer data from a 13.402 GHz radiometer I built
>
>installed in Las Cruces, NM.  It was easy to tell when it was overcast 
>or clear: clear is cold, because you're seeing sky; overcast is warm, 
>because you're seeing the reflection of the ground, and the warm water 
>in the clouds.
>
>
>
>
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