Anti-aircraft units from Fort Dorset, Wellington, working from suitable hilltops, carried out operations in the detection of an aeroplane which, working from Rongotai Aerodrome, circled above Wellington suburbs. The system under which the work was carried through is illustrated by these two pictures. Each of the three units consisted of a sound indicator (shown right) and searchlight (above). The sound indicator is operated by a detail of three, two of whom with stethoscopes to their ears, direct the “trumpets” to the aeroplane, the position of maximum sound indicating the correct aim. With a telephone transmitter connected direct to the operator of the searchlight, the third member of the indicator party, looking along a special sighting attachment which moves with the- “trumpets” directs the movement of the searchlight’s beam until it coincides with the aim of the sound indicator and picks up the moving aeroplane.

Nelson Evening Mail, Volume LXX, 7 October 1936, Page 4

Anti-aircraft units from Fort Dorset, Wellington, working from suitable hilltops, carried out operations in the detection of an aeroplane which, working from Rongotai Aerodrome, circled above Wellington suburbs. The system under which the work was carried through is illustrated by these two pictures. Each of the three units consisted of a sound indicator (shown right) and searchlight (above). The sound indicator is operated by a detail of three, two of whom with stethoscopes to their ears, direct the “trumpets” to the aeroplane, the position of maximum sound indicating the correct aim. With a telephone transmitter connected direct to the operator of the searchlight, the third member of the indicator party, looking along a special sighting attachment which moves with the- “trumpets” directs the movement of the searchlight’s beam until it coincides with the aim of the sound indicator and picks up the moving aeroplane.