I can't give you chapter and verse, but I do know that the GPS constellation is monitored by a 'mission control'. I used to know where this was but can't remember now.
So each satellite is regularly checked for errors, health, clock accuracy, orbital accuracy, etc. and corrections are sent to each satellite via a data link if required.
I don't know about the ionospheric delay, but you've said yourself that it can be corrected for by comparing the L1 and L2 signals, so presumably the satellite doesn't need to 'get involved'?
Last edited by Uplinker; 9th Jun 2013 at 12:39.