GB990134A - Improvements in satellite and space communications systems - Google Patents

Abstract

990,134. Radio signalling transponder apparatus; automatic frequency control systems. H. P. HUTCHINSON, and P. R. ARENDT. May 8, 1961 [May 13, 1960], No. 16741/61. Headings H3A and H4L. In a method of communicating by radio between at least two bodies in motion with respect to each other, hereafter considered to be the earth and a space station S which includes an antenna, intelligence is transmitted simultaneously via at least two separate radio waves from at least two geographically widely separated stations A, B on the earth, the separation of these stations from one another being sufficient in relation to the distance of the space station that the antenna can have a null orientation with respect to at most one of the radio waves. The response at the space station is arranged to be only to the strongest of the several signals received from the separated stations. Information to be transmitted from earth to a space station, either for use there or for retransmission to a further earth location, is fed to a modulator 3 at one station A related to the particular location and also via a channel 9 to the modulator 11 of a further station B related to that location. The information is used to modulate different carrier frequencies fta and ftb at the two stations, the modulated frequencies then being transmitted to the space station from the antennµ 5 and 13. It may be necessary to include apparatus at the stations A and B to correct for time differences between the singlas received at the space station because of differences in the signal path lengths between the stations A and B and the space station. The transmitting antennµ at the stations A and B are adjusted to the optimum transmitting azimuth and elevation by referring to direction finders 6 and 14 which track the space station and respond to signals transmitted from it. A system for coupling the direction finders to the antenna is explained (see below). At the space station a multichannel receiver demodulates the received signals and feeds them to level monitors, whereby the strongest signal is selected. This signal may be utilized to actuate apparatus in the space station, or may be used to modulate a further carrier frequency and retransmitted to a further earth location having at least two separated stations A<SP>1</SP>, B<SP>1</SP>. At these stations the received signals are demodulated in receivers similar to 21, 25 for stations A, B and passed to a signal combiner (similar to 22) prior to being fed to the location output. To allow for fluctuating ionospheric effects &c. it is advantageous if the transmitters at all stations such as A, B, transmit on a range of frequencies (see Fig. 3). The space station then responds (in a manner similar to that outlined above) to the signal which it receives most strongly. Since the space station itself will also have a multi-channel transmitter, the carrierfrequency 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 500-1,000 mc/s. while for tracking and location purposes the band is narrower, 650-1,050 mc/s., centred at about 850 mc/s. It may be necessary to make allowance in the system for Doppler shift frequencies caused by the relative motion between the space and earth stations. A suitable method of adjusting the frequencies transmitted from the earth stations so that after they have been affected by Doppler shift they lie within the pass-band receivable by the space station as shown in Fig. 6. 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 is heterodyned with a frequency f 4 which is the same as the space station receiver frequency and the difference frequency is fed to an inductor 95. The two signals are rectified by the diodes 92, 93 respectively and the rectified currents flow in opposite directions 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 Dopper 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. 4. 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 synchronism 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 log persistence held so as not to release if the relay 49 momentarily is switched off. Specifications 990,135, 990,136, 990,137, 990,138 and 990,139 are referred to.