Bats can find their way in the dark using echolocation, so why can't drones? Researchers have managed just that. They've developed a system that lets a drone navigate using just four microphones and a speaker. It relies on the familiar concept of measuring distance by generating echoes, but uses an algorithm based on communicative algebra that 'echosorts' to determine which distances represent given objects. It won't produce "ghost walls" that leave the drone afraid to move.