LIST OF DEFINITIONS The following definitions are intended to clarify specialised terms used in this Document. Altimetry system error (ASE). The difference between the altitude indicated on the altimeter display, assuming a correct altimeter barometric setting, and the pressure altitude corresponding to the undisturbed ambient pressure. Altimetry system error distribution. The distribution of an aggregate altimetry system error. Assigned altitude deviation (AAD). The difference between the transponded Mode C altitude and the assigned altitude/flight level. Automatic altitude-keeping device. Any equipment that is designed to automatically control the aircraft to a referenced pressure altitude. Collision risk. The expected number of midair aircraft accidents, in a prescribed volume of airspace, fora specific number of flight hours, due to loss of planned separation. (Note one collision is considered to produce two accidents.