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Blind people can navigate corridors using ultrasonic mobility aids. A research question of interest is whether they track the wall of the corridor, the free space or a combination of the two. To study this and related questions, we set up a wall tracking experiment to collect echo data as a mono-aural sensor was moved parallel to a wall. This involved an investigation of the components of the echo and the geometry that produced them. We developed feature extractors to enable the detection of multiple objects from echoes. The results indicate that more useful information is contained in echo components than previously thought. They also demonstrate that accurate wall following is possible.