I think what Gardener is referring to is the abysmal GPS coverage at the Azores.
Impossible..The satellite constellation orbits the earth at 12000km and so are always changing location and so there is no way that one place can have "abysmal GPS coverage"....You may get the very odd occurrence when the geometry of the satellites may be bad (Hence Dilution of precision - DOP, for example PDOP and HDOP) - perhaps one has gone down for example, but there are so many now (30 something?) that even if this did occur then it would only be for a brief time (order of minutes) and you should still get 3D nav accurate to 100m. The great thing is that the GPS receiver tells you this too! UNlike an NDB which the only way to work out when it is not working is to keep indenting it all the time.