Patent NL-75996-C (NL)
IPC: G01S13/76
Abstract: 649,322. Radio navigation ; radio relaying systems. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. July 30, 1946, No. 22734. Convention date, July 31, 1945. [Classes 40 (v) and 40 (vii)] In a radio navigation system the P.P.I. view of craft (ships or aircraft) within the service area of a ground located P.P.I. system is radiated as a signal by a ground located transmitter and displayed on a television receiver carried by each craft. As shown in Fig. 1, the view on the long persistence screen 26 of a cathoderay tube forming part of a P.P.I. system including a rotatable directive aerial 10, a pulse transmitter 11, T-R box 12 and pulse receivers 13, 13a, is scanned by a conventional television camera 28 together with a chart 34 of the surrounding territory and the resulting television signals are transmitted on frequency f3 by the transmitter 33. To receive these signals each craft (e.g. aircraft A/C1, A/C2) is provided with a television receiver 41, Fig. 3, operating on the frequency f3 and the received television signals are displayed on the screen 50 of cathode-ray tube 48 a device coupling the deflection yoke 49 to a directional gyro being provided for maintaining a desired orientation of the received P.P.I. display. Identification of the echo which corresponds to a particular aircraft may be effected by providing each aircraft with a responder, such as 42, comprising a pulse receiver 57 delay network 58 and pulse transmitter 59 (operating on frequency f2) the output from which is increased by closing a switch 55 so that the brightness of the particular echo is increased. In an alternative identification method demodulated pulses from the responder receiver 57 are applied to an integration circuit 62 and the resulting pulse (e.g. pulse 56, which is the integrated energy of the group of pulses received by 57 as the P.P.I. beam sweeps past of the aircraft) is applied to the grid of cathode-ray tube 48 to increase the brightness of the echo. If it is desired to operate the P.P.I. receiver at such a low gain that a rotating strobe line (e.g. due to noise signals) is not visible, means are provided to introduce a strobe line that is always visible regardless of the receiver gain setting. As shown in Fig. 5, a rotating strobe line 98, obtained by illuminating a slit 101 in a disc which is rotated in synchronism with the deflecting yoke 17, is projected on to the chart 34 and television signals representative of this display generated by a camera 96 and mixed in 97 with signals representative of the P.P.I. display generated by camera 28 are radiated by the television transmitter 33. The system is preferably operated with relative amplitudes of modulation so that the strobe line only produces a visible trace on the screen of the airborne television receiver when the identification pulse 56 is operative, the P.P.I. display producing maximum and the chart an intermediate degree of modulation. If the P.P.I. tube screen is made of phosphorescent material having a persistence of one or two minutes the resulting " trail " produced by the echo signals will indicate the direction of movement of the objects producing these echos. If the aircraft are not equipped with responders (operating on frequency f2) the echo signals produced by reflection from the surface of the aircraft may be utilized by providing the P.P.I. system with a receiver operating on the frequency of the pulse transmitter (e.g. f1). In an embodiment of the invention in which an aircraft flying at a certain altitude receives at any one time a picture containing only aircraft flying at the same level, or at any one of other predetermined levels, the P.P.I. ground installation is equipped with a plurality of television receiver-transmitter systems, each system of which operates on frequencies assigned to a particular level, the receivers and transmitters each being tuned to different carrier frequencies and the frequencies of the responder transmitter and the television receiver carried by the aircraft are adjusted automatically (by means of an altitude operated tuning mechanism such as 91, 93, 94, Fig. 3) to the assigned frequency as the altitude at which the aircraft is flying varies (Fig. 4, not shown).