Sir - Mr Montel's proposal for differential instrument landing systems (ILS) (Flight International, Letters, 14-20 December, 1994, P44) does not address another principal disadvantage of the ILS - its inability to be used to determine the position of the aircraft other than within a narrow angle close to the glideslope.
Although the wide coverage of the microwave-landing system is not so necessary for curved approaches as it once was - in view of the availability of area navigation and, in particular, the global-positioning system (GPS) - it is nevertheless valuable in easing the transition between navigation in the terminal-manoeuvring area (TMA) and final landing phase. As it provides an independent measure of the aircraft's position, the MLS can monitor the performance of the area-navigation system and allow the aircraft to fly through any disturbances to the GPS.
The ILS is much less effective in this role in combination with the GPS and, indeed, as Mr Montel's proposal indicates, itself requires protection against disturbances.
A combination of a modernised MLS and GPS to provide timing for the ranging function will provide the highest possible integrity and allow a seamless hand-over from en route or TMA navigation, with mutual monitoring of accuracy at the point of transition.
A H THOMAS
Epsom, Surrey, UK
Source: Flight International
