Sending out sound waves and listening for their echo is a great way to detect obstacles in water – submarines can detect objects many kilometres away. But sound doesn’t travel nearly so well in air. Bats can only detect objects up to 20 metres away, falling to around two metres in poor conditions. Light is less affected by atmospheric conditions, which is why self-driving cars use LIDAR (‘light detection and ranging’), bouncing infrared laser light off objects in order to detect them.
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