Sir - The report by the UK National Air Traffic Services (Flight International, 4-10 December, P6) indicates that the global-positioning system (GPS) is unreliable for sole-means navigation. I believe that it should be used with another aid, preferably an inertial-navigation system.
The questions of reliability and the political problems of ownership and control throw doubt on the wisdom of replacing the instrument-landing system with the GPS. Outages, whatever the causes, will have a critical effect in the landing phase. The GPS should be used, in conjunction with a microwave-landing system only to provide an accurate time reference.
The clocks which are installed in GPS receivers are run independently and are also corrected by satellite signals. They will continue to run through outage periods which occur with little loss of accuracy.
The time reference thus obtained could be used for range determination, eliminating the need for precision distance-measuring equipment. The microwave-landing system would provide measurements of the azimuth and elevation angles of the aircraft and would be augmented by the addition of a timed signal for ranging.
A H THOMAS
Epsom, Surrey, UK
Source: Flight International
