The Air Navigation Order 2009

5.  In this Schedule—

(a) “Airborne collision avoidance system” means an aeroplane system which—

(i) conforms to requirements prescribed for the purpose;

(ii) is based on secondary surveillance radar transponder signals;

(iii) operates independently of ground based equipment; and

(iv) 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;

(b) “Automatic direction finding equipment” means radio navigation equipment which automatically indicates the bearing of any radio station transmitting the signals received by such equipment;

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

(d) “Mode A” means replying to an interrogation from secondary surveillance radar units on the surface to elicit transponder replies for identity and surveillance with identity provided in the form of a four digit identity code;

(e) “Mode C” means replying to an interrogation from secondary surveillance radar units on the surface to elicit transponder replies for automatic pressure-altitude transmission and surveillance;

(f) “Secondary surveillance radar equipment” means such type of radio equipment as may be notified as being capable of—

(i) replying to an interrogation from secondary surveillance radar units on the surface; and

(ii) being operated in accordance with such instructions as may be given to the aircraft by the appropriate air traffic control unit;

(g) “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.