HOW IT WORKS BY IAN J. TWOMBLY VISIBILITY SENSOR A MACHINE THAT MIMICS THE EYES Receiver WHAT DOES THREE-QUARTERS OF A CUBIC FOOT OF AIR have to do with visibility? It's the volume of air an automated weather station uses to calculate visibility. Visibility should be an easy concept. It's what we can see. But the FAA makes things complicated. There's flight visibility, reported visibility, human-gathered, and machine-gathered. At an airport without a control tower, weather reporting is done through an automated weather observing station or an automated surface observing station. These automated weather observation system (AWOS) and automated surface observation system (ASOS) units gather data automatically and transmit it via a radio frequency. The sensor system-which includes a transmitter, receiver, and processor-determines visibility. The transmitter sends out a cone of light twice per second. The light then scatters among the particulate in the air. The receiver senses how much light has scattered, and the processor then determines-based on preexisting formulas-what the visibility is. The processor computes a one-minute visibility average. Transmitter Sample area STEVE KARP THEY THINK OF EVERYTHING Engineers who created the visibility system seemed to have thought up all kinds of interesting tricks to make sure it is as reliable as possible. Light sensor-Since the ambient light changes during the night, the system incorporates a day/night sensor that tells the processor when to account for darker conditions. Sensor protection-The sensors are hooded and pointed down to protect them from birds and precipitation. Temperature-A temperature sensor in the processor senses when it drops below 40 degrees Fahrenheit, at which point the sensors and the computer are heated. The system has nine heaters in all. Processor WWW.AOPA.ORG/FT 17http://WWW.AOPA.ORG/FT