"Gill Windobserver 70 (sonic) anemometer"@en . . "TOOL1213" . "A solid state, heated ultrasonic anemometer. Designed for use within the aviation industry and for more extreme weather conditions. It measures the times taken for an ultrasonic pulse of sound to travel from the North transducer to the South transducer, and compares it with the time for a pulse to travel from S to N transducer. Likewise times are compared between West and East, and E and W transducer. The wind speed and direction (and the speed of sound) can then be calculated fromthe differences in the times of flight on each axis. This calculation is independent of factors such as temperature. It uses 150 Watts of electrical heating in the anemometer head to prevent icing. Wind speed accuracy is +/2 2% at 12 m/s. Wind direction accuracy is +/2 degrees at 12 m/s." . .