Right now, the Reach will report NSATS=0 and DOP=0 if it is not receiving measurements from the base. This means that it will fail GPS prearm checks described here:
http://ardupilot.org/copter/docs/parameters.html#ek2-gps-check-gps-preflight-check
if it isn’t being used in RTK mode or if the base station measurements stop for whatever reason.
I imagine there are GPS FS checks during flight as well that rely on NSATS and DOP being reasonable values (not zero). I haven’t verified this yet though.
I know the navigation solution will degrade since there is a check on GPS’s NSATS to determine if the GPS’s velocity can be used:
http://ardupilot.org/copter/docs/parameters.html#ahrs-gps-minsats-ahrs-gps-minimum-satellites
Here is a discussion on the NSATS and DOP issue:
If the Reach would report the correct value for NSATS and DOP for 3D Fixes (non-RTK), I would have more confidence that it could be used as the primary GPS.
Another issue is that the Reach’s compass isn’t exposed so you’ll have to provide an external compass or use the compass from the non-Reach secondary GPS.