A guided underwater object (GUO) (100) comprises steering means, an
on-board guidance system (200) for controlling the steering means, and a
hydrophone unit or units (104A, 104B, 104C, 104D, 104E, 104F, 104G, 104H)
arranged to pass signals induced therein by an acoustic guiding beam to
the on-board guidance system to effect directional control of the
self-propelled underwater object. The GUO is guided by a remote guidance
system characterized by a plurality of transmitter arrays (402A, 402B,
402C, 402D; 412A, 412B, 412C, 412D; 422P, 422Q, 422R) arranged to
transmit a plurality of contiguous acoustic beams (302A, 302B, 302C,
302D) each having a different frequency or frequency chirp (or other
acoustic signal coding) and enclosing a guidance corridor of lower sound
level (`null`) (304) extending approximately in the direction of
propagation of the acoustic beams and defining a flight path of the
object. The GUO requires no on-board search sensors, no communications
link with the guidance system and is of low cost.