There have been a number of systems that have used ultrasound in an attempt to support the independent travel of blind people. In simple terms, such systems generally operate by sending out a pulse of ultrasound. Eventually the pulse is reflected from a solid object in the path of the pulse. The time between the outgoing pulse being transmitted and its echo being received corresponds to the distance between the transmitter and the object. This information is then relayed to the user in some audible way.
Most practical systems are rather more complex.
They use multiple transmitters to derive a more detailed view of their environment. The former provides users with a large number of environmental cues encoded in audio information. The Sonic Pathfinder provides fewer audio cues with the goal of avoiding information overload. Both systems provide information on a regular basis. Despite a wide range of these systems being available the take up by the user community has been low.