a question i have though as a result of reading this thread, i gave a little more attention to the satellite's that i pick up, when the #51 shows up, its there for ten or 30 minutes at a time then disappears. it reappears but i'm wondering why, does it get bumped by satellites that give me a better signal?
I have a 76C which I believe behaves differently from the x series units but I think you are right that if another WAAS satellite is stronger it may bump 51. Also remember that 51 is still in test phase so the signal may be turned off at times.
in some cases, the slot that #51 always shows up in (far right %100 of the time if there) when 51 is not there, the slot is blank displaying no satellite at all. and there is no satellite number there either, just the two blank underscore symbols.
When you have WAAS enabled the GPS reserves a channel or channels for the WAAS satellites. On the 76C it starts with two channels but once it has a strong lock it drops one of the reserved channels so it can be used by regular satellites. IIRC the x series only uses one channel for WAAS