GB990138A - Improvements in or relating to communications systems - Google Patents

Abstract

990,138. Signalling systems; radio direction finding. H. P. HUTCHINSON, and P. R. ARENDT. May 8, 1961 [May 13, 1960], No. 36041/64. Divided out of 990,134. Headings H4D and H4L. In a method of transmitting intelligence by radio from a first body to another in space moving with a substantial radial velocity relative to the first, the frequency of the transmitted signal is controlled so that it differs from a nominal frequency by the amount of the Doppler shift of the signal transmitted from the first body and received at the other. Energy is received from the other body and the optimum direction of transmission from the first body is continuously maintained by varying the direction of transmission according to the direction from which this energy is received. A suitable method of adjusting the frequencies transmitted from the first body, which may be an earth station, so that after they have been affected by Doppler shift they lie within the pass band receivable by the other body, which may be a space station, is as shown in Fig. 5. A frequency f 2 transmitted by the space station is received at frequency f 1 at the earth station, where it is heterodyned with f 2 so that a signal frequency f 1 -f 2 corresponding to the Doppler shift is derived. This is fed to an inductor 94. Similarly, the transmitter frequency f 3 is heterodyned with a frequency f 4 which is the same as thw space station receiver frequency and the difference frequency fed to an inductor 95. The two signals are rectified by the diodes 92, 93 respectively and the rectified currents flow in opposite direction through the resistor 91. Any difference actuates a recording potentiometer 96 to adjust a variable master oscillator 118 so as to adjust the transmitter frequency at 116 to make the correct allowance for Doppler shift. A way of adjusting the transmitting antennµ at a transmission station by reference to the signal received from the space station is shown in Fig. 2. The signal picked up by the rotating receiving aerial is demodulated and used to hold on the relay 49, normally keeping the switch C1 closed. The switch opens as the antenna passes through a null, so indicating the direction of the space station. An arm 54 is driven in synchronism with the receiving aerial and when this contacts a brush 53 on a ring 52 driven in synchronizm with the transmitting aerial, the relay 50 is actuated, closing the switch C2 to keep the relay permanently held on and switching on the relay 51. This releases the brake and engages the clutch of the motor 59 which drives the transmitting aerial 56. The relay 51 has a long persistence hold so as not to release if the relay 49 momentarily is switched off. To allow for fluctuating ionospheric effects &c., it is advantageous if the transmitter at an earth station transmits on a range of frequencies (see Fig. 1). The space station then responds to the signal which it receives most strongly. Since the space station itself will also have a multi-channel transmitter, the carrier frequency of the signal which the space station transmits is adjusted by reference to the frequency received most strongly to be approximately the same as that frequency. Assuming reciprocity to hold, this will be the most suitable transmission frequency. The optimum frequency band for the transmission of communications signals in the systems described is 500-4,000 mc(s. while for tracking and location purposes the band is narrower, 650-1,050 mc/s., centred at about 850 mc/s.