170,
It is probably correct that the most potent of combinations is that of the GPS and INS - at least for the most critical portion of flight, the instrument approach (where the horizon prevents other signals from being received). The INS - updated by the GPS - would provide the guidance signal; if the GPS signal were to fail, there would be sufficient accuracy left in the system to complete the approach or go-around.
Strange then that these installations are few and far between - which points to cost or technical problems. The only (almost) helicopter which I know that is equipped with one of these is the Bell X-15.
Jim