The EV3's Ultrasonic (US) sensor is a proximity sensor, meaning that it measures the distance between itself and an object. An ultrasonic sensor measures distance by emitting a high-frequency sound that humans cannot hear. The US sensor then waits for the sound to bounce off of the object and return to the sensor. Using the time it took for the sound to travel away from and return to the sensor, the EV3 can estimate the distance between the sensor and the obstacle. This makes for an accurate sensor that is not susceptible to light interference. However, the surface of the object needs to be perpendicular to the sensor, otherwise the sound may not return to the sensor and the distance cannot be estimated. The US sensor is used in the Omnilander's autonomous program: