US Patent 8135503 Ground proximity sensor

An unmanned aerial vehicle comprises a housing, a rotor that is rotated to propel the housing, a pressure sensor that generates a signal indicative of an air pressure proximate a bottom surface of the housing, and a processor configured to determine, based on the signal, when an increase in air pressure proximate the bottom surface is greater than or equal to a threshold value associated with the ground effect of the rotor, wherein the processor controls the rotor to cease rotating or decrease rotational speed to land the unmanned aerial vehicle upon determining that the increase in pressure is greater than or equal to the threshold value.