The Air Navigation Order 2000

4  In this Schedule:

(1) ‘Automatic direction finding equipment’ means radio navigation equipment which automatically indicates the bearing of any radio station transmitting the signals received by such equipment;

(2) ‘VHF omni-range equipment’ means radio navigation equipment capable of giving visual indications of bearings of the aircraft by means of signals received from very high frequency omni-directional radio ranges;

(3) ‘Distance measuring equipment’ means radio equipment capable of providing a continuous indication of the aircraft’s distance from the appropriate aeronautical radio stations;

(4) ‘Secondary surveillance radar equipment’ means such type of radio equipment as may be notified as being capable of (a) replying to an interrogation from secondary surveillance radar units on the surface and (b) being operated in accordance with such instructions as may be given to the aircraft by the appropriate air traffic control unit;

(5) ‘Airborne collision avoidance system’ means an aeroplane system which conforms to requirements prescribed for the purpose; is based on secondary surveillance radar transponder signals; operates independently of ground based equipment and which is designed to provide advice and appropriate avoidance manoeuvres to the pilot in relation to other aeroplanes which are equipped with secondary surveillance radar and are in undue proximity.