GNS:
Not exactly an expert on this subject but:
TSOs are an umbrella document that usually reference technical documents produced as guidance by such organisations such as RTCA, EUROCAE etc (committees for offshore safety have also been active in the production of technical standards for survival suits, lifejackets etc. that will be accepted as JTSOs).
The requirement for the application of TSO is contained in requirements; JAR-OPS is one such and makes statements such as "...minimum performance standards are those prescribed in the applicable JTSO (the JAA equivalent to TSOs - TSOs are also acceptable), unless different performance standards are prescribed in the operational or airworthiness codes". This applies to all instruments, equipment, communication and navigational equipment etc.
To be part of approved equipment GPS must have been approved - both the functional and build standard and the fit.
The question that was posed by SASless exemplifies the quandry of the modern world where technology moves faster than the mind of the regulator (or his processes). This is exacerbated by the requirement (as stated in an earlier post) for technical assessment of operational requirements and hazard analyses.
As far as I am aware, the regulations for navigation aids operates in the area of 'deal with what you understand'. Thus most regulations prescribe the aircraft equipment and say nothing about the ability to receive the signal (as you pointed out with your perceptive comment on VORs). The fit for IFR offshore operations is a good example of anacronistic regulations in some States.
What would be better is an objective regulation that deals with these issues (state the objective for the navigation equipment and permit alternative methods of compliance). FAR 135.165 appears to deal with that quite well as does JAR-OPS. ICAO is in this respect objective but does require that, if the equipment that is being used for navigation fails, alternatives are available.
We have to solve the problems of GPS signal and equipment reliability. As stated earlier, redundancy at the box level (GPS + INS in a single box) is an answer - particularly for safety critical applications. (The ongoing debate is more about continuity and security of signal than aircraft equipment.)
The problem that operators face is the need to have helicopters equipped for multi-purpose activities (it is after all a (nearly) once-and-for-all choice).
Last edited by Another KOS; 14th Jun 2003 at 20:39.