Correlation Protected Instrument Landing System: An account of a new ILS system originating from RAE Farnborough that is expected to combine the advantages of the conventional system with greater accuracy and freedom from interference

THE guidance aid installed at most of the world's major airports is the Instrument Landing System which is currently being developed to cater for all phases of all weather landing programmes. For this application, the system has to achieve a guaranteed performance of less than one failure in ten million landings, thus inferring high equipment reliability and integrity. The VHF ground system at present installed consists of a localiser to provide azimuth guidance, a glide path transmitter to provide pitch guidance and marker beacons to give position information at distances of about 5 miles and one mile from touchdown. The guidance signals are formed by modulating two overlapping beams of different frequencies such that when the aircraft is on the extension of the runway centre line, equal modulations are received. This is termed zero difference in depth of modulation contour. The glide path signal is formed in a similar way with the zero contour defining a descent path angle of 3 degrees to the runway touchdown point. In the aircraft, the ILS receivers separate the demodulated tones by filters, and they are then rectified and compared differentially.