I like that we are thinking of a fix for this because it's confusing to (advanced) users. Another alternative would be to add a pre-arm check and then spit out a text message as to what the problem is.

@rmackay9 @tridge Just changed prearm_failure_reason() to return "no cores exist" instead of a null pointer. Leaves it to the user to figure out why the core pointer is null.

needs description on commits

As a side note, I think the message might be easier to understand if it was "EKF2 not initialised".

@rmackay9 Thanks, I chose "no cores" because that is the only information available by that point in the code (as far as I understand). I guess the original author thought "unhealthy" made it perfectly clear what was wrong :)

To be more useful, the message should obviously tell the user how to fix the problem, but I don't think either "no cores" or "not initialized" does that; though the latter is IMO even more vague.

But if the only reason the core pointer might be null is a lack of memory (or EKn_enable not set) then the message could change to indicate that. I assume it's not that simple, though.

@kd0aij, ok great. I think we should probably update the wiki page so that we list all the possible failures and possible solutions. I'm going to go ahead and merge this now. thanks!

OK, I'll look for relevant info in the wiki.
Are these the only pages discussing EKF configuration?
http://ardupilot.org/plane/docs/common-apm-navigation-extended-kalman-filter-overview.html?highlight=ekf
http://ardupilot.org/dev/docs/extended-kalman-filter.html#extended-kalman-filter

@kd0aij, this is the pre-arm failure message so the new messages should go here I think: http://ardupilot.org/copter/docs/common-prearm-safety-checks.html#common-prearm-safety-checks