From an examination of the outline requirements given by Mr. Davies, it is apparent that pressure measuring techniques can be used in all stages of a flight although it must be acknowledged that some of the tolerances he has called for cannot be met. The advantage of the pressure technique is that it uses the Earth's atmosphere which is experienced by all airborne vehicles and so provides a common datum for height or altitude measurement. It should be noted that wrong impressions can be got from the present pattern of accepting altitude or height in feet as a fixed physical measurement. A pressure sensor measures the pressure and, as the millibar is a widely accepted unit of pressure in aeronautics, it would really be more sensible to express aircraft altitude and separation values in millibars.