Patent GB-610935-A (GB)
Title: Improvements in or relating to the radio navigation of aircraft for blind landing
IPC: G01S1/02
Filed: 1946-04-10
Abstract: 610,935. Radio-navigation. SOC. FRANCAISE RADIO-ELECTRIQUE. April 10, 1946, No. 11072. Convention date, Dec. 17, 1943. [Class 40 (vii)] A radio-navigation system for the blind landing of aircraft comprises a pulse transmitter on the craft, the pulses from which trigger two responders, located either side of a desired course or group of courses, radiating return pulses of slightly differing radio frequencies, a receiver on the craft receiving the return pulses and transforming them into positive and negative voltage impulses respectively and applying them to a cathode-ray oscillograph, the time base of which is triggered by the original pulses transmitted. In a system for landing aircraft an azimuthal hyperbolic path may be followed to the airport by maintaining the spacing between the two indications, above and below the trace, equal and the particular one of the family of hyperbolae with the same foci, and thus the particular landing point, depends on the spacing of the indications, dependent on the difference in path lengths to the two responders, Fig. 1 (not shown). One responder may use a frequency F +f and the other F - f so that by using a frequency modulation-type receiver tuned to the frequency F on the craft positive or negative voltage outputs according to the partioular responder can be obtained for application to the vertical deflection plates of the oscillograph. The time base may be arranged so that each scan comprises ten successive and vertically-displaced traces across the tube, (Fig. 2, not shown), the position of the indications on this trace indicating the distance to the responders and thus to the landing ground so that the craft may be landed with the aid of an altimeter. Where it is desired to land down the path which is the perpendicular bisector of the line between the responders the two indications cancel each other, but any deviation to right or left produces short-deflections, positive and negative, at the ends of the overlapping portions of the indications which determine the position of the craft (Fig. 3, not shown). If the pulse repetition rate is low (10-20 per second) then, as the oscillograph trace occupies only about one ten-thousandth of a second after each pulse, the chance of mutual interference between a number of aircraft using the responders is very unlikely. At an airport having three parallel runways KLM spaced 300 metres apart, Fig. 4, the responders A and B are symmetrically placed on the airport boundary so that the outer runways become tangents to two hyperbolic approach paths. Responders A and B have directional aerials and their receivers have an operational threshold level so arranged that the pilot of an incoming craft knows when he crosses the line A, B because of the cessation of return signals. It is also arranged that the responder signals differ slightly in length, that successive horizontal traces across the tube each correspond to a path difference to the responders of 600 metres, and that the craft receiver is detuned slightly so that the longer (positive) impulses are large in amplitude than the negative ones. Figs. 5 a, b, and c, show the correct relative positions of the two indications for landing on paths K, M and L, respectively. The receiver on the craft may also include a second oscillograph giving range on an exponential time base obtained by charging a condenser through a fixed resistance. This time base may be split into two vertically-displaced horizontal traces by applying an equal and opposite voltage to the horizontal deflection plates at the instant the condenser reaches half its full charge and, at the same time, a vertical displacement voltage. Specification 577,824 is referred to.