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Ultrasonic sensors âare based on the measurement of the properties of acoustic waves with frequencies above the human audible range,â often at roughly 40 kHz . They typically operate by generating a high-frequency pulse of sound, and then receiving and evaluating the properties of the echo pulse. Three different properties of the received echo pulse may be evaluated, for different sensing purposes.
Remote sensors collect data by detecting the energy that is reflected from Earth. These sensors can be on satellites or mounted on aircraft. Remote sensors can be either passive or active. Passive sensors respond to external stimuli. They record natural energy that is reflected or emitted from the Earth's surface. The most common source of radiation detected by passive sensors is reflected sunlight. In contrast, active sensors use internal stimuli to collect data about Earth.