Some interesting comments above about GPS "dropping out" this is not an issue as the LNAV will revert to DME/DME or VOR/DME but i would allways back up the LNAV with raw data.
As for GPS failure just how often will it "drop out" ?
I have been flying in europe with IFR aproved GPS units for about 10 years now and in that time have had the system "flag" twice and then for less that 3 min ,i think that to much anti GPS rubish is put out by the crystal palace at gatwick in the past but i am told that a GPS aproach is soon to be aproved at LGW because that some of the american airlines dont have ADF fitted and so if the ILS is off line would have to divert.
Comments please !.