The paper presents a review of ultrasonic sensors. Attention is focused on the possibility of using such sensors to keep an unmanned aerial vehicle at a constant altitude above the surface. Such a task requires filtered and unfiltered signals, so that the ground level can be distinguished even when obstacles are present. Three selected sensors were tested. The possibility of connecting these sensors to external devices was also studied. The potential of the individual sensors and of a set of sensors is discussed. The study showed that the best solution is to use a set of three ultrasonic sensors. Alternatively, a sensor with both unfiltered and filtered measurement was proved to be an adequate option.