Abstract: The invention relates to apparatus for generating electrical signals representing the instantaneous position and attitude of an object with reference to a coordinate system. A pair of wave transmitters are mounted and controlled so that they follow any movement of the object and are arranged to transmit ultrasonic pulses to three planar transducers arranged to convert the received energy into signals representative of the instantaneous position and attitude of the two transmitters relative to the three transducers. The invention is particularly applicable to apparatus for the ultrasonic examination of bodies having non-planar surfaces and in this case the object referred to is the ultrasonic probe designed to transmit wave energy into the body. The probe may be mounted with one or two degrees of rotational freedom on a boom which is itself mounted with at least two degrees of rotational freedom on a vertical column.