[time-nuts] time-nuts Digest, Vol 123, Issue 35

John Nordlie nordlie at aero.und.edu
Fri Oct 10 12:27:06 EDT 2014

FYI, COSMIC also measures precipitable water vapor via GPS ground stations too.  It's called Souminet:



Message: 2
Date: Fri, 10 Oct 2014 06:17:13 -0700
From: Jim Lux <jimlux at earthlink.net>
To: time-nuts at febo.com
Subject: Re: [time-nuts] Sun Outage
Message-ID: <5437DC59.9090102 at earthlink.net>
Content-Type: text/plain; charset=UTF-8; format=flowed

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.

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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