I sure hope the CAA would not be so lame as to act on such a limited report...... I’ll admit it’s a pretty report. But I’d say their conclusions are HIGHLY questionable.... One phenomenon, one helicopter, one blade construction, only a few receiver designs... and after all that data , which really only shows the effect on the signal – NOT THE PERFORMANCE, they go ahead and draw sweeping conclusions.
By their own admission, one receiver showed no appreciable change in accuracies, and the other – they couldn’t get access to the data to see the real effect. And then, based on this, they essentially recommend dropping the navigation provisions of TSO-c129 certified systems for helicopter operations?!?!? Give me a break!!!!
The report admits this is not a likely a case of inaccurate data from your GPS... but mostly an availability issue: That in some cases, you might not get the predicted RAIM accuracy when you actually get there. And because of that, it’s unsafe !?!?!
Even this loss of availability is a conclusion – not supported by data in itself. And even they say that this loss of perfomance (which is not really quantified) depends not only on the rotor masking, but also on the receiver design (and I'm sure it depends on even more than that.)
Well, if we’re going to do that, let’s not just pick on helicopters, and let's not just pick onthe rotor masking phenomenon. Let’s look at all phenomenon that can potentially effect GPS reception.... There’s weather phenomenon, installation variations, multi-tail configurations, airframe in certain pitch-up maneuvers, antenna cable length, number splitters, location of bends in the cable, corrosion in the cable and connectors over time, ice accumulation on the antenna. Lot's of reasons can affect the GPS reception performance. At least in a helicopter, the obstructions tend to move. In a fixed wing, in certain constellations and maneuvers, you can find yourself blocked from a satellite. They live with their problems - we live with ours.
Let’s remember what predicted RAIM is.... it’s whether the satellite constellation visible at the time over the location will have the geometry to support the accuracy for the phase of flight (either enroute, terminal, or approach) We’re talking accuracy requirements in the hundreds of meters – and the effect cited in the report was what??? 2m? Even this was for which geometries of satellites??? Also to my knowledge, predicted RAIM is established for the worst case accuracy when the US-DOD has SA cranked up to maximum. There’s lots of slop built in. Even so, that says a lot for what the report didn’t examine. And that’s why we depend on the real RAIM determined by the receiver, and not just the predicted.
You might as well say that since weather doesn’t always match what’s predicted, flight in general is unsafe and all air traffic should be stopped.