"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 <jim...@earthlink.net> wrote: >On 10/9/14, 10:16 PM, Andy wrote: >> Bob Stewart <b...@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. > > > > >_______________________________________________ >time-nuts mailing list -- time-nuts@febo.com >To unsubscribe, go to >https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >and follow the instructions there. -- Sent from my Motorola Droid Razr HD 4G LTE wireless tracker while I do other things. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
