How these weather satellite images are acquired...

Every 90 minutes a low earth orbiter skims the atmosphere and scans the earth below.

 

       The antenna used to receive the images at the Ojai Earth Station is at left.   It is called a turnstile, because of its shape.  The two bottom bars are reflectors, the uppermost are active elements.  Unlike most antennas, which aim toward the horizon, this one looks upward.

       There are two sets of pickup antennas because the signal from above is circularly polarized.  The axis of polarization spins as the signal comes down.  (Visualize the manner in which an airplane propeller spins as it moves forward through space. ) Since most man-made interference is polarized in one direction, this technique minimizes outside signal incursion in the same way that Polaroid sunglasses cancel out reflected glare. 

     Inside the PVC plastic pipe is 300-ohm TV twin lead,  with it's length cut to resonate at the satellite broadcast frequency of 137.5 MHz.  You can build one of these antennas for less than fifty dollars. 

     A popular alternative design is called a quadrifilar or QFH.  It is more efficient, but more difficult to construct. Likewise the double-cross, or DCA.  

     The signal is pre-amplified at the antenna base before it is fed to a modified police scanner.  A cable-TV amplifier from Radio Shack works fine.  This is necessary because the satellite broadcasts with about the same power ( 5 watts) as an old Citizens Band radio.  In other words, not very much.

 

 
          We are fortunate in the Ojai Valley, because the east-west hills act like a bowl reflector for the satellites, which travel north-south.  It's a miniature version of the Arecebo space radio telescope.  We get reception even when the satellite is under the horizon.  On land, such transmitting power is limited to thirty miles or so.  From space, we've received good images from Oregon  to south of the Baja Peninsula.

        PCs are formidable radio noise generators.  We ground ours to a water pipe, use RF filter chokes on the keyboard and mouse wires, run the incoming audio through an isolation transformer, and most importantly, keep the antenna and the radio as far from the PC as is practical.  Radio noise is still a problem. Burglar lights, for example, spit static once per second even during daylight hours.

     The old Bearcat police scanner pictured above has had its IF filter bandwidth modified from the public utility frequency deviation of 7.5 kHz to the standard used by the orbiters, 40 kHz.  This is a job you probably can't do on your own, so get out your checkbook.  Fortunately, the modified radio still picks up ham, NOAA weather, and police broadcasts afterward.  

 


     Audio from the scanner is then fed to a sound card in a PC, analyzed, and the image extracted using software available for free download. There are several versions, such as WxSat v2.57. We currently use WXtoImg.

     The color image that is produced by this software is processed using Photoshop, a date-time stamp is added, and then it is uploaded via FTP to this website on an (almost) daily basis.

     The image above is an unprocessed example of what is sent.  The image is built up gradually, as a slow scan, left-to-right, top-to-bottom, at one line per second.  The leftmost band is a gray scale spectrum, used for brightness calibration.  Then comes digital data about the status of the onboard instruments, fuel, and other data such as locations of discovered emergency distress transmitters.  Then a band showing minute markers to pinpoint the satellite's location.  

     The leftmost picture frame is visible light.  Rightmost is an infra-red image.  When inverted, this displays hotter objects as brighter.  It works at night, and you can use it to determine the temperature of a lake, for example.
 

 


     If the satellite is headed south to north, the images will appear to be upside down, but the clever software flips it rightside-up automagically.

     Sometimes we can combine the two frames and use the data to artificially colorize the image.  This is prettier, and by compressing more data into one image, the effects of temporary interference, such as static, are minimized.
 

 

 
      You can hear (but not use) the signal for yourself on any unmodified scanner by tuning to 137.5Mhz and being patient.  Each satellite will pass directly overhead twice daily.

     A free program called  Orbitron  will display the present and predict the positions of weather or other satellites.  The best shot of the day occurs with the sun directly overhead, around noon, give or take an hour. As the two frames (visible and infra-red) are sent line by line, it makes a distinctive "tick-tock" sound.

    Click on the icon  to hear a sample of a weather satellite as recorded at the Ojai, California ground station.
    Click on the icon hear a sample of a short-wave radio weather FAX transmission..

     Images from the latest generation of geostationary satellites contain much more information, but are also much more difficult for a home user to directly capture or interpret.  Since these images are available on the Internet, you may wonder why anyone would spend the time and trouble to get their own from the source.  As the old Steamship Line slogan put it, Getting There is Half the Fun.

     A project like this is technically challenging, but still doable, and not very expensive.  In the process, I've learned a great deal about the laws of physics: Keplerian elements, radio properties, signal processing, the weather, and so on.  And it yields something fascinating to look at every day.  It's fascinating because it's fresh.  You are seeing what's happening right now, as the angels and astronauts see it.

Click HERE for the NOAA Polar Orbiter information web site.

Click HERE for other NOAA Satellite status.

Click on the links below to read a Daly Road Graphics article about polar orbiters from Satellite Times magazine.

 

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