It has recently come to my attention that EGNOS reception by my GPS receiver is not that reliable.  (In crude terms, EGNOS is a separate signal from the GPS / GLONAS signal which it is claimed to reduce the error of location from circa 7m to circa 3m.)  I have had several occasions where the signal appears and then disappears from a stationary location which is not obstructed by a southerly located hill.  A simple trawl of the internet does not give any clues as to why this might be.  Any suggestions? 

The other questions which arises in my mind is can I determine whether the GPS data output has been subjected to EGNOS correction other than by looking at the satellite display on my receiver?  (On my Garmin eTrex 20, it shows up as a D on each GPS satellite which is subjected to an EGNOS correction and also shows the EGNOS satellites,  numbers 33, 37 or 39.)  The gpx file does not appear to give any clues.

It is only a guess, but the current security threat level may have something to do with it. But like I say it's only a guess.

with regards to your second paragraph, it may be possible to analyse the NMEA data string from the GPS receiver to determine the use, or not of egnos correction.