The development of these systems proceeded by recognising that although the ILS would be the source of the guidance, the ILS itself contains lateral and vertical elements that have rather different characteristics. In particular, the vertical element (glideslope) originates from the projected touchdown point of the approach, i.e. typically 1000 ft from the beginning of the runway, while the lateral element (localiser) originates from beyond the far end. The transmitted glideslope therefore becomes irrelevant soon after the aircraft has reached the runway threshold, and in fact the aircraft has of course to enter its landing mode and reduce its vertical velocity quite a long time before it passes the glideslope transmitter. The inaccuracies in the basic ILS could be seen in that it was suitable for use down to 200 ft. only (Cat I), and similarly no autopilot was suitable for or approved for use below this height.
Posted on: Sun, 01 Sep 2013 14:38:21 +0000
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