Abstract: 13. In a control system by means of which a dirigible craft may be controlled, means on said craft for detecting electric fields through which said craft may be moving and for producing three signals indicative of orthogonal components of the electric field detected thereby; first and second computing networks each comprising input means and output means; and means connecting said detecting and signal producing means to said networks so the first and second of said signals are applied to the input means of one of said networks and so that the second and third of said signals are applied to the input means of the other of said networks, said networks being characterized by producing output signals at the output means thereof indicative of the product of magnitude of the smaller of the signals applied thereto and a trigonometric function of the phase angle between the signals applied thereto.