I.L.S. transmitter monitors for automatic blind landing

The safety requirements of an automatic blind landing system imply that the ground I.L.S. transmitters must have a high `integrity¿, a high degree of accuracy and reliability. This, in turn, implies that there must be monitors at the transmitters which can measure the main parameters of the radiated signal and react fast to any fault condition. Two types of monitor are available, near-field monitors taking their signals from field probes and internal monitors using signals from the aerial feed cables. A study of internal monitors applied to existing localizers was made as part of the R.A.E. I.L.S. feasibility study and showed that these monitors could provide accurate performance measurements and also that the localizers did achieve the required standard of stability. The integrity of the I.L.S. beacons depends largely on the way in which monitors are used; one possible monitor configuration for blind landing involves four monitors arranged to give good continuity of service and a very high probability that faults will be detected. One monitor measures the performance of the stand-by transmitter and the other three measure the radiated signals; if there is disagreement between these three the action is determined by `majority vote¿