Patent GB-1100071-A (GB)
Title: Aircraft landing approach coupler desensitization means
IPC: H03G3/20
Filed: 1966-02-24
Abstract: 1,100,071. Radio navigation. BENDIX CORPORATION. 24 Feb., 1966 [4 March, 1965], No. 8182/66. Heading H4D. [Also in Division G3] Relates to aircraft apparatus in an instrument landing system utilizing glide path and localizer beacons the signals from which, at the outputs of corresponding receivers in the aircraft, are fed to known " approach couplers " which are connected to an automatic pilot or a flight director. In order to compensate for effective increase in " beam error gain " as the aircraft proceeds towards the runway, the receiver outputs can be reduced in accordance with some function related to the distance of the aircraft from the runway; account must be taken of terrain height variations and according to the invention the receiver outputs are controlled by a "desensitization " device in accordance with the output of a radio altimeter, the control being such that it is effective when the radio altimeter output is decreasing and at least partially ineffective when that output is increasing. In one embodiment, Fig. 1, the desensitization device is electronic and comprises two sections 62, 72 in series; the input of the first section is a positive D.C. voltage H RA  proportional to height above the terrain and the output signal e 3  of the second section controls a variable gain amplifier 14, 34 in the receiver outputs via a manually operable switch 112 used for initial setting-up purposes. The first section includes a diode clamp circuit 100 : as voltage H RA  is reduced during a normal approach over flat terrain the clamp circuit operates such that the output signal e 2  lags, and is proportional to, voltage H RA , but as the aircraft passes over a valley the output of the first section is prevented from following the increase in voltage H RA . In the second section a circuit incorporating a biased diode 84 and a capacitor 92 follows reductions in aircraft height only when within preset rate limits so as to reduce the effect of undesired height reductions due to hills. A comparator 132 initiates a visual or audible warning when voltage e 2  fails to follow reductions in voltage H RA  within predetermined limits. In the second embodiment, Fig. 2, the desensitization device 250 is electro-mechanical and incorporates potentiometers 252, 254, 256 controlled by a two-phase servo-motor 277; the D.C. voltage H RA  from altimeter 52 is converted to A.C. in modulator 51 and thence fed to a phase sensitive detector 264. Motor 277 operates conventionally when signal H RA  is diminishing. With switches 257, 286 in their lower positions the difference between signal H RA  and the output of potentiometer 256 is applied via detector 264 to drive motor 277 and hence potentiometers 252, 254, 256. When signal H RA  increases no voltage exists at the input to servo amplifier 272 and operation of motor 277 is inhibited. Tachometer feed-back 305, position feedback 256 and the maximum speed of motor 277 are selected to give a desirable time lag and altitude rate limit which motor 277 can follow. Prior to initiation of the landing approach switches 257, 286 are placed in the condition illustrated so as to condition the apparatus to the beginning of an approach.