METHOD OF RADIO TRANSMISSION OF SIGNALS UNAFFECTED BY INTERFERENCE

1281331 Radio signalling L BOLKOW 20 Sept 1962 [21 Sept 1961] 35920/62 Heading H4L A method of radio transmission of signals, unaffected by interference, comprises generating at the transmission station a wide band noise of approximately constant power ratio per frequency interval, from which at least two frequency bands are selected, of which, in order to generate a signal content, one frequency band is stopped and replaced by a frequency band of the same power distribution correlated with the other frequency band, so that the noise to be emitted to the receiving station also has an approximately constant power ratio per frequency interval. The method may be applied to transmitting command signals to a guided missile. In the transmitter shown in Fig. 3 a noise generator 1 supplies noise, extending for example from 10 to 25 Mc/s, to a filter 2 which rejects two spaced bands centred on frequencies f 1 and f 3 and having a width of 4 Mc/s for example; the rejected bands pass to terminals of order transmitters 9 and 10, and the noise, less the rejected bands, is passed on to a filter 3. The frequency band centred on frequency f 2 , which is midway between f 1 and f 3 , is taken wattless from unit 2 and fed to the converter 5 where it is transposed to frequency bands f 1 and f 3 by a signal from a local oscillator 6 operating at frequency f 2 -f 1 . The outputs from converter 5 are fed direct and through phase inverters 13 and 14 to terminals of the order transmitters 9 and 10. Each order transmitter has three positions and passes on to unit 3 either the original band f 1 or f 3 abstracted by unit 2 (zero signal) or a transposed band f 2 (positive signal) or a transposed and phase-inverted band f 2 (negative signal); in each case the output of unit 3 is a noise spectrum covering the whole range of noise generator 1, and this output is passed to the transmitter output stage 4 and the aerial. At the receiver, Fig. 4 (not shown), the three bands f 1 , f 2 and f 3 are separated by a filter, and bands f 1 and f 3 are transposed to frequency f 2 by a local oscillator and mixers. The local oscillator must be in phase with that at the transmitter, and in the case of missile control the synchronism can be established before launching. The transposed bands f 1 and f 3 are then compared with band f 2 in two correlator units, which may comprise simple multipliers and low-pass filters, to recover the two command signals chosen by units 9 and 10. In a modification, Figs. 9