Patent GB-904990-A (GB)
Title: Radio guidance systems
Inventor: SHAPCOTT SIDNEY EDWARD
IPC: G01S1/54
Filed: 1958-01-08
Abstract: 904,990. Radio direction-finding. AVIATION, MINISTER OF. Dec. 23. 1958 [Jan. 8, 1958], No. 820/58. Class 40 (7). A radio navigational system comprises at a transmitting station a directional transmitting aerial arranged to create a scan of its radiation pattern which is symmetrical, means for transmitting one radiofrequency pulse signal from the directional aerial for each regular angular increment of scan of the radiation pattern, and means for transmitting a distinctive radiofrequency signal at a datum angle of scan; and at a receiving station means for determining the difference in the number of pulse signals having an amplitude greater than a predetermined minimum received before and after receipt of the distinctive signal when the distinctive signal is received during the receipt of a regular succession of pulse signals. In the transmitter shown in Fig. 1 a directional aerial 1 is rotated on a vertical shaft 2 which carries a transparent disc 4 marked with short opaque radial lines at, say, every one-thousandth of a radian; a lamp 5 and photo-cell 6 sense the passage of each line past a slit, and the output of the photo-cell modulates a transmitter 7 so that it transmits a pulse through aerial 1 each time a line passes the slit. The output of the photo-cell is also fed to a counter 8, which is reset once each revolution of the shaft 2, so that the output of counter 8 represents the bearing of the aerial; this output is fed to a coincidence circuit 9 where it is compared with a datum bearing set up on a register 10. Thus transmitter 11 sends a pulse from omni-directional aerial 12 each time aerial 1 reaches the datum bearing. In one type of receiver, Fig. 2 (not shown), signals received from aerial 1 are passed through a receiver and a limiter (to eliminate pulses received from side lobes of aerial 1) to two parallel channels each comprising a gate, a counter, and a store. One gate is normally open and the other closed, and the state of the gates is reversed on receipt of the distinctive signal from aerial 12 on a separate receiver. Thus the difference in the count in the two stores gives the number of half-thousandths of a radian the craft is off the datum bearing and the sign of the difference indicates which direction. If the craft does not lie within the beam width from the datum bearing the difference will give a false indication, and this may be obviated by suppressing a gate circuit in the output whenever there is a time lag between receipt of pulses from aerials 1 and 12; in these circumstances a coarse indication of bearing may be obtained by measuring the time lag, the rate of rotation of aerial 1 being known. Fig. 3 shows an alternative receiver wherein pulses from aerial 1 (X pulses) are received in receiver 14 and passed through a limiter 30 to a pulse frequency discriminator 31 which passes only pulses of the recurrence rate emitted by aerial 1. The output of the discriminator is fed to a reversible counter 32; the first pulse of each sequence is lost in the discriminator so the counter is set to one before counting starts. Datum pulses (S pulses) from aerial 12 are received in receiver 26 and passed to a pulse width discriminator 34 which passes only pulses of the same width as are transmitted from aerial 12. An output from the discriminator is applied to the counter 32 so that the counter counts X pulses positively until an S pulse is received when it counts X pulses negatively; thus the counter gives the bearing relative to the datum bearing in half-thousandths of a radian provided the craft lies within a beam width of the datum bearing. An output from discriminator 34 is applied to a 312 microseconds delay unit 35 to provide S1 pulses which are in correct time relationship to the X pulses, the 312 microsecond delay compensating for the pulse lost in discriminator 31; another output is applied to a 0.1 seconds delay unit 36 to provide reset pulses R1. The S1 and R1 pulses are applied to a trigger 37 so that it is put on for about 0.1 second between an S1 and an R1 pulse. R1 pulses are also applied to waveform generator 33 which provides three outputs A, B and C; the A output lasts from X1 to X2, that is from the first pulse received from discriminator 31 to 350 microseconds after the last pulse; the B output lasts from X1 to R1, and the C output from X2 to R1. The output of trigger 37 is applied to the input of an inhibiting gate 38 and the output B is applied to the inhibiting input of this gate which then produces an output only if S1 precedes X1 and then for a period from S1 to X1. Similarly gate 39 produces an output X2 to S1 if X2 precedes S1. The output of gates 38, 39 are fed to triggers 40, 41, respectively, to light a neon sign if there is an output from one of them. The outputs of the gate are fed together to open a gate 42 and allow oscillations of a frequency 2f to pass from oscillator 43 to counter 44. The frequency 2f corresponds to one pulse for every thousandth of a radian of rotation of aerial 1. The counter 44 is also fed with pulses of frequency f for a time X1 to X2 through gate 45, and this part of the count gives the half beam width in thousandths of a radian. If neither of the neon lights is lit, S1 occurs between X1 and X2 and the bearing is taken from counter 32; if one of the neon lights is lit, the total number recorded in counter 44 will be an approximate indication of the number of thousandths of a radian the craft lies from the datum bearing. Counters 32 and 44 are reset and the trigger 40 or 41 put off by a reset pulse R2 derived from R1 by a 1-8 seconds delay unit 46, the recurrence period of S pulses being 2 seconds. Receivers 14, 26 are immobilized between R1 and R2 by a blanking pulse so that the system is only effective if the craft receives all pulses from aerial 1 within a period Œ0.1 seconds from receipt of the S pulse; with a beam width of 1 degree and rotation speed of once every two seconds, this means that the craft must be within Œ 17¢ degrees of the datum bearing. Specification 717,114 is referred to.