ArduPlane: add climb before turn to AUTOLAND

[Hwurzburg]

Alternate to #29135

After test flying using a TX GCS to change climb to altitudes, I agree with Tim that this is easier to use then options in #29135...and it actually uses less flash so I am closing #29135

Now fully terrain following capable

Thanks to Pete Hall for help

Updated autotest for terrain based mission out behind mountain with terrain following autoland return

[timtuxworth]

I think that's it just _m on values in meters.

[IamPete1]

Could we reuse RTL_CLIMB_MIN?

[Hwurzburg]

@tridge need help in getting glide_slope from point of climb to alt -> base loiter point to follow terrain...it always just use above home alt as it proceeds to the loiter-to-alt point....help would be appreciated

[Hwurzburg]

@tridge got it all working now.....full terrain support

[timtuxworth]

It seems to me that just because TERRAIN_ENABLE is true might not mean you want to use it for AUTOLAND. I guess it could be added to the TERRAIN_FOLLOW bitmask in a follow up PR.

[tridge]

i'm surprised you didn't need to subtract home alt here, I will need to look at this under gdb

[Hwurzburg]

set_target_altitude_location takes care of all that...the relative_alt bit is cleared when we convert the land alt to ABSOLUTE for later use...
since only the terrain alt flag is set when terrain following is enabled,

ardupilot/ArduPlane/altitude.cpp

Line 236 in bac78b5

if (!loc.relative_alt) {

removes the home alt

[IamPete1]

I suspect this is key to how why works. It sets the TECS target to a huge altitude which is enough to get the vehicle to do a max rate climb. That is the only purpose of the target altitude. The key thing is that this target is not used for the comparison of when the climb is done.

The problem is that without terrain, or if your at sea level then the target altitude may be lower than the threshold alt and the climb will never complete.

We could just set INT32_MAX here and the result would be the same. Although I would not recommend that.

[Hwurzburg]

without terrain it climbs to current amsl + climb alt in cm....so that's correct, right?
how do you suggest I set the max climb , terrain based?