How these weather satellite
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.
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.
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
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.
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
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
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"
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..
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.
HERE for the NOAA Polar
Orbiter information web site.
HERE for other NOAA Satellite status.
Click on the links
below to read a Daly Road Graphics article about polar orbiters from Satellite Times
Daly Road Graphics All rights reserved.