Sir - The success of the first trials of the DASA/Collins Category III global-positioning-system (GPS) landing aid (Flight International, 2-8 August, P6) indicates that the applicability of the GPS to virtually all navigation and landing tasks will not be limited by engineering snags - apart from those of jamming and others.
The capabilities of the GPS, augmented where necessary by differential techniques, are such that it is being used, or proposed for use, all over the world in many civil and military applications - in fact it is rapidly becoming indispensable. The signals from space, however, are provided at the expense of the US taxpayer. Can we be sure that US Congress will for ever be content that the world should thus be provided with a free resource, particularly one with such powerful capabilities?
It is surely quite possible that, in some years' time, following political changes or pressures, Congress will insist on means being found of recovering from other nations some of the cost of maintaining the GPS - principally that of replacing two or three satellites every year. This might involve further degradation of the C/A signals and a requirement for all users to carry a decoding device to enable that degradation to be removed.
Until this question of the long-term availability of the GPS is resolved, no one should adopt a navigation or landing system, which relies entirely upon it. An alternative of similar accuracy, although of lesser capability in other ways, should always be provided.
A H THOMAS
Epsom, Surrey, UK
Source: Flight International