Paul_Sengupta wrote:I don't think anyone's saying that GPS is inaccurate, it's more that it's easy to interfere with or jam and if that happens you have no way of flying an accurate missed approach.
Unlikely to happen in the UK of course, but there have been occurences of aircraft in the USA carrying out instrument letdowns in hilly or mountainous areas and losing the GPS signal as multiple satellites become shadowed by terrain. As Paul says, the GPS signal received is inherently weak as it comes so far so it doesn't take a lot to interfere with it especially as it will get weaker the lower you go.
Although the way it is encoded makes it much easier to detect and process than a VOR or NDB signal.
I would be interested to see any actual example of terrain masking of a GPS signal in the approach area. A quick search finds very few examples of loss of signal not associated with user error or NOTAMed issues. (Interestingly, I did find an example of a CAT aircraft that rolled before allowing its GPS to lock on and filed an ASR as a result of the DME/DME position being not sufficiently accurate to fly their RNAV departure.)
IMHO the CAA position on GPS has been shockingly Head in Sand for many years and even still seems to require an overwhelming pile of paper to allow people to officially move from using the NDB (which is manifestly inaccurate) to using certified GPS, which either works, or doesn't work and knows/Annunciates this fact. As a point of interest, in one of the few NDB approaches I have flown outside training, I flew it 'perfectly' (needle dead on track) and realized (when my GPS showed the NDB passing underneath me and the needle still pointing forward) that my ADF had chosen those few moments to have its needle drive motor fail (and that I could have flown any heading and would have been on track)!. The ADF system provides no indication of such a failure until one had reason to turn and see the needle not move.