Patent GB-644364-A (GB)
Title: Blind approach or landing radio system for aircraft
IPC: G01S1/02
Filed: 1947-01-28
Abstract: 644,364. Radio navigation. WEIGHT ON, D., and PYE, Ltd. Jan. 28, 1947, No. 2570. [Class 40 (vii)] The invention relates to path-defining radio beacon systems of the type in which a transmitter modulated by a low frequency tone is connected to an aerial system in such a way as to radiate a signal modulated in opposite phase to right and left of a course line and a receiver on a craft indicates the phase of the received modulation with respect to a reference signal (which may be in phase with the modulation to one side of the course) transmitted on a subcarrier or by a separate transmitter. In one such system the low frequency tone generator 10 is coupled through balanced modulator 15 to feed outer aerials 16 and 17 with carrier suppressed signals of opposite modulation phase. Generator 10 also modulates a subcarrier in reference signal generator 14 which is used to modulate the carrier signal radiated from central aerial 13. According to the present invention the signal radiated from aerial 13 is directly modulated with the low frequency tone from generator 10 through 90 degrees phase shifter 11 so that a modulation signal in phase quadrature with the reference signal is received on-course and the phase of the modulation signal on one side of the course line differs from that on the other side by less than 180 degrees. The same receiver may be used as with the known system described above. In such a receiver the reference signal is obtained from the separated sub-carrier and applied across points 30, 31 of the indicator circuit and the directly detected low frequency tone across points 32 and 33. The sum and difference of the applied signals are fed respectively each to one of the rectifiers 37 and the difference in current flow in the two circuits is indicated by meter 36. The centre-tap of resistor 38, also connected across the two circuits, is connected through the coil of warning relay 39 to earth so that the relay is controlled by the sum of the circuit currents. When the indicator is used in the known system there is no modulation when the receiver is on the course line and thus no input at 32, 33 and the rectifiers pass the same current due to the reference modulation. The meter 36 then reads zero and relay 39 is adjusted to be operated so that it will release and give a warning if the reference signal fails. It will not, however, give any indication if the modulation radiated by aerials 16 and 17 fails giving an " on-course " indication at any position. Off-course the input at 32-33 will either be in-phase or out-of-phase with the reference signal producing a corresponding unbalance between the rectifier circuits and an indication on meter 36. When the system of the present invention is used however a modulation signal in phase quadrature with the reference signal is received on the course line and although the currents flowing in the two rectifier circuits are still equal providing zero meter indication the current through relay 39 is dependent on both the reference input and the phase quadrature input (at 32, 33) and the relay itself may be adjusted to release and give a warning on failure of either of these.