NavList:

David,  Thanks for the link.  The last 20 years of my flying was in North America and only the last 6 years did we use GPS.  It was only required at a few, small airports.  We only needed to check on the avaibality of WAAS if we were going to one of them. My world - wide flying was before GPS and even INS so we needed to use cel-nav.  I did not know about the other systems around the world.  Thanks again.  John H.

John.  The main reason for the slow acceptance of GNSS even for en-route flying was integrity rather than accuracy.  I.e. the ability of the user to identify a failing satellite immediately rather than having to wait until it came into view of a ground station.  This was solved by the introduction of RAIM (Receiver Autonomous Integrity Monitoring).  With four pseudoranges, a RAIM receiver can get a fix, with five it can spot something is wrong, and with a minimum of six pseudoranges it can identify which is in error and discard it (although in practice geometry means that more ranges might be required).  When I finally retired from groundschool teaching 11 years ago, there was a requirement to check on websites like AUGER in Europe that there were no ‘RAIM Holes’ for the intended GNSS monitored route flying.  I.e. places where the satellite ephemeris predicted you might not receive at least five useable pseudoranges.  I would imagine theres something similar now with WAAS/EGNOS GNSS approaches.  DaveP