[time-nuts] Sun Outage
jimlux at earthlink.net
Fri Oct 10 09:17:13 EDT 2014
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
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.
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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