Daly Road Graphics Weather Satellite Image FAQ page

	How the weather images are acquired... (Click on the thumbnails to see larger images.) *Every 90 minutes a low earth orbiter skims the atmosphere and scans the earth below.* Click HERE for the NOAA Polar Orbiter information web site.
	The antenna used to receive the images at the Ojai Earth Station is at left. It is called a turnstile, because of its shape. It was constructed from PVC pipe. 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 because the signal from above is circularly polarized. Inside the plastic pipe is 300-ohm TV twin lead, cut to resonate at the satellite broadcast frequency of 137.5 MHz. The signal is amplified once before it is fed to a modified police scanner. A cable-TV signal amplifier from Radio Shack for $20 works just fine as a preamp.
	The scanner has had its IF filter bandwidth modified from the police frequency deviation of 7.5 kHz to the standard used by the orbiters, 40 kHz. It still picks up police, weather, and ham calls. Audio from the scanner is then fed to a sound card in a PC, analyzed, and the image extracted. That image is then reprocessed as a .jpg, labeled, and uploaded to this website as a public service.
	The image at left is the raw copy of what comes down. It 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 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. Sometimes we can combine the two frames and use the data to artificially colorize the image.
	You can hear (but not use) the signal for yourself on any unmodified scanner by tuning to 137.5 and being patient. Each satellite will pass directly overhead twice daily. The best shot of the day occurs with the sun directly overhead, around 11 AM, 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. The satellites also transmit a high-bandwidth, high-detail image, but capturing it requires a remotely-controlled antenna and a high-capacity PC. Images from the latest generation of geostationary satellites contain much more information, but are also much more difficult for a home user to capture or interpret. Click on the icon at left to hear a brief sample of a weather satellite as recorded at the Ojai, California ground station.