Apparatus, to be carried on-board of an aircraft, for warning the aircraft of an obstacle on the earth's surface, the apparatus comprising a warning system arranged to: (i) receive first information corresponding to the obstacle's position and second information corresponding to the aircraft's horizontal and vertical position as a function of time; and (ii) calculate an aircraft trajectory for the aircraft using the second information; (iii) the apparatus further comprises storage means storing a database; (iv) generate a warning signal if the obstacle lies on or within a pre-determined vicinity the aircraft's horizontal trajectory and the difference between the height dimension of the obstacle and the altitude of the aircraft at the obstacle position of the aircraft trajectory is less than a pre-determined value; characterised in that (v) the apparatus comprises a position receiver arranged to receive the first information and pass it to the warning system, whereas the first information additionally comprises the obstacle's identity, (vi) whereas the warning system is arranged to derive the first information from an Automatic Identification System signal, (vii) the database contains the obstacle identities and corresponding height data; and (viii) the warning system is arranged to retrieve a height dimension of the obstacle from the database using the obstacle's identity as derived from the first information, wherein the warning system is arranged to receive third information derived from the Automatic Identification System signal corresponding to the obstacle's speed and direction and wherein the warning system is arranged to use the first and third information and the aircraft's horizontal trajectory to calculate whether the aircraft and the obstacle will coincide in horizontal position at a coincidence point, and to generate a warning signal only if the aircraft and obstacle are calculated so to coincide and the difference between the height dimension of the obstacle and the altitude of the aircraft at the coincidence point is less than a pre-determined value. (i) receive first information corresponding to the obstacle's position and second information corresponding to the aircraft's horizontal and vertical position as a function of time; and (ii) calculate an aircraft trajectory for the aircraft using the second information; (iii) the apparatus further comprises storage means storing a database; (iv) generate a warning signal if the obstacle lies on or within a pre-determined vicinity the aircraft's horizontal trajectory and the difference between the height dimension of the obstacle and the altitude of the aircraft at the obstacle position of the aircraft trajectory is less than a pre-determined value; characterised in that (v) the apparatus comprises a position receiver arranged to receive the first information and pass it to the warning system, whereas the first information additionally comprises the obstacle's identity, (vi) whereas the warning system is arranged to derive the first information from an Automatic Identification System signal, (vii) the database contains the obstacle identities and corresponding height data; and (viii) the warning system is arranged to retrieve a height dimension of the obstacle from the database using the obstacle's identity as derived from the first information, wherein the warning system is arranged to receive third information derived from the Automatic Identification System signal corresponding to the obstacle's speed and direction and wherein the warning system is arranged to use the first and third information and the aircraft's horizontal trajectory to calculate whether the aircraft and the obstacle will coincide in horizontal position at a coincidence point, and to generate a warning signal only if the aircraft and obstacle are calculated so to coincide and the difference between the height dimension of the obstacle and the altitude of the aircraft at the coincidence point is less than a pre-determined value.