Your contributor (‘Airmail’, September) is to be commended for suggesting a method for providing features similar to a transponder but without the equipment overheads. Unfortunately, the way that mobile phone technology works is that the handset needs to be in coverage of a terrestrial base station, so airborne reliability will be significantly affected. Over the sea or unpopulated areas, and even above an altitude of only a few thousand feet, the base station signal might not reach the handset. Even if the handset can receive the base station, the handset signal may not reach the base station.
There may be limited situations where the use of a mobile phone would work but, on the whole, the current coverage areas would be too restricting to use it as an effective SSR replacement. Martin Jewell (telecommunications systems architect), London