The recent article on the proposed technical developments of the global positioning system (Flight International, 15-21 April, P27) fails to reassure non-US users that all the technical and political questions have been answered satisfactorily.

It is implied that the USA will cease to degrade the signals available to civil users to 100m accuracy, otherwise there would be no point in providing additional signals to enable such users to calculate the ionospheric delay.

It is clear that the USA reserves the right to degrade the system in time of conflict, and the present administration can give no assurances about the circumstances in which this might be done, since they cannot commit their successors.

The article gives no information about other technical issues which are at least as important as ionospheric delay, such as the fact that there are not enough satellites in the constellation to provide adequate redundancy and that self-monitoring and warnings to the user of faults are inadequate. The wide-area augmentation system will alleviate some of these problems but can never be fully satisfactory because of its limited coverage.

Expensive improvements are needed. It is perhaps unreasonable to expect the US taxpayer to pay for them: so perhaps an international agency should be set up to provide a satellite positioning service.

A H Thomas

Epsom, Surrey, UK

Source: Flight International