Abstract: This invention concerns a system to protect active or passive electronic equipment from radar detection. In other words equipment which transmits or receives microwave radiation, for example a radar or telecommunications transmitter or receiver.For this purpose, the transmitting surface of the equipment is covered by a device which can be controlled to apply a phase shift of approximately .pi./2 to the incident wave; this enables a microwave incident on the equipment to be modulated in phase. The frequency spectrum of the wave reflected by the equipment is modified and spread thus making it more difficult to detect the equipment. To avoid any perturbation of the operation of the equipment, an inverse modulation is applied to signals transmitted and/or received by the equipment itself.

Abstract: An air-navigational beacon, designed to simulate an imaginary circular ben with a multiplicity of pairs of diametrically opposite dipoles excited in cyclic succession, comprises an array of stationary dipole pairs disposed along lines parallel to a given direction. The spacing of the dipoles of each pair varies from one pair to another according to a sinusoidal law in their order of energization. The two dipoles of a pair are energized in relative quadrature and with a signal amplitude proportional to their spacing.

Abstract: In a system for use in helicopter navigation antennas are carried at the e ends of each of the blades of the rotor of the helicopter. Radio waves transmitted by a beacon at a known location on the ground are received by the antennas and phase modulated by their rotation. On the rotor the signals are amplified and mixed with oscillations from local oscillators and are detected with the aid of reference signals from the local oscillators. By means of slip rings and contact brushes the signals are introduced into the body of the helicopter where they are first multiplied with each other and then filtered so as to produce phase-modulated signals with a carrier suppressed which are supplied to an electroluminescent diode illuminating a rotating-disc light modulator with a light-transmitting function which is analogous to the phase modulation of the signals received by the antennas. The light beam passing through the disc is integrated in a vidicon and displayed on a screen.

Abstract: High-frequency waves transmitted by a rotating antenna array at a ground tion are intercepted by an airborne receiver for photoelectric evaluation by one or more discs whose light transmissivity, by translumination or reflection, varies periodically according to a sinusoidal law in one diametral direction while being constant in another diametral direction orthogonal thereto. The disc or discs are rotated at a speed corresponding to that of the transmitting antenna array, under the control of periodic synchronizing pulses, and are illuminated by a flux modulated by an electrical signal derived from the incoming wave, this signal containing a phase component dependent on two spatial coordinates such as elevational and azimuthal angles determining the relative position of the ground transmitter and the airborne receiver.