To determine the direction of incidence for a signal from an acoustic signal source using a directional microphone system, which has at least two microphones, two or more directional microphone signals are produced, each having a direction-dependent sensitivity distribution with a minimum in one direction. A cylindrical member is formed integrally with the cover and has, at its inner bottom surface, opposite a jet pump, a fuel receiving portion for receiving fuel introduced from an introducing pipe. The processor includes a memory which stores transform data for each image tile, the transform data relating both to the expected distortion and to the predetermined overlap between image tiles.