In your comment on Michael Stannard's letter (Flight International, 2-8 July) you correctly state that jamming of GPS could affect a far wider area than the jamming of a single conventional radio navigation aid. Of equal importance is the fact that the GPS signals are extremely weak - 30db or 1,000 times weaker than the thermal noise in the receiver.
It is only because the signals from each satellite are known that they can be detected in the midst of this noise - position is determined by the time of receipt of the signals, not by their content. This means that a jammer need only emit a correspondingly weak signal to be effective, making such devices readily portable and easily deployed. To jam a conventional aid would require a jamming signal perhaps 40 db or 10,000 times stronger.
The report of the FAA's current proposals (Flight International, 18-24 June) implies that the microwave landing system (MLS) has been abandoned. While perhaps no bad thing it should surely be possible to devise a simplified MLS which would be a cheaper adjunct to the GPS than the present ILS.
A H Thomas
Surrey, UK
Source: Flight International
