Support RTL_ALTITUDE when using RTL_AUTOLAND

[Maarrk]

Feature request

We have a fixed-wing plane which has a complicated landing sequence, which forces us to always land in AUTO mode using RTL_AUTOLAND parameter. However, this means that RTL_ALTITUDE is ignored, since the vehicle always proceeds to the first waypoint after MAV_CMD_DO_LAND_START. I also need a way to support "return at current altitude", set when parameter is equal to -1.
The landing sequence contains MAV_CMD_NAV_LOITER_TO_ALT at 50m AGL which is required for final approach, but the plane glides to it in a straight descending line, potentially crashing into terrain.

I would like the vehicle to complete the RTL route on altitude set in RTL_ALTITUDE parameter, also when landing in AUTO mode, and only descend when close to landing sequence.

Workaround
By adding a MAV_CMD_NAV_WAYPOINT after MAV_CMD_DO_LAND_START and before MAV_CMD_NAV_LOITER_TO_ALT, I can adjust the return altitude on this mission item, and have the plane climb to it immediately by setting bit 14 of FLIGHT_OPTIONS. I am including the Lua function below.

The problem is, that this script needs to reliably detect when RTL mode is triggered to update the waypoint. When using RTL_AUTOLAND the vehicle switches to AUTO immediately, and I might miss it between the calls of the script. When changing the mode from GCS, I can listen to MAV_CMD_DO_SET_MODE, but can't detect RTL triggered by another script or RC transmitter. Finally, I can't simply look at the index of current waypoint, because I don't want to climb to RTL altitude when mission proceeds to landing sequence as planned.

Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[x ] Plane
[ ] Rover
[ ] Submarine

Additional context
I'd be happy to contribute either a reliable way to detect RTL in Lua when using Automatic landing or some more involved behaviour for RTL, as preferred by the dev team.

I searched the issues for RTL_AUTOLAND and RTL_ALTITUDE but didn't find anything with both for my use case. There's a thread Altitude behaviour on RTL using DO_LAND_START with RTL_AUTOLAND=2 which confirms that adding a waypoint is the way to do it.

-- Update first waypoint of landing sequence to recreate RTL_ALTITUDE behavior in AUTO
-- 
-- This must be called every time the vehicle enters RTL. Because the altitude target is specified
-- as a regular waypoint with large acceptance radius, the vehicle might not reach it if it's already close.
-- Note that if RTL is switched into AUTO and vehicle continues the mission it will fly to RTL altitude,
-- this can be reset to original value by reuploading the mission plan from GCS.
local function update_rtl_altitude()
    local doland_idx = find_do_land_start_index()
    if not doland_idx then
        return
    end

    local rtl_idx = doland_idx + 1
    local rtl_item = mission:get_item(rtl_idx)
    if (not rtl_item) or rtl_item:command() ~= MAV_CMD_NAV_WAYPOINT then
        return
    end

    local rtl_altitude = RTL_ALTITUDE:get()
    if not rtl_altitude then
        return
    end

    if rtl_altitude == -1 then
        local current_rel_alt = -ahrs:get_relative_position_D_home()
        rtl_item:z(current_rel_alt)
    else
        rtl_item:z(rtl_altitude)
    end

    -- Never write altitude smaller than 50 m
    if rtl_item:z() < 50 then
        rtl_item:z(50)
    end
    -- Ensure the altitude is relative to home like the parameter
    rtl_item:frame(MAV_FRAME_GLOBAL_RELATIVE_ALT)

    mission:set_item(rtl_idx, rtl_item)
    gcs:send_text(MAV_SEVERITY.NOTICE, string.format("Doing RTL at %.0f m (REL)", rtl_item:z()))

    -- If already flying there, update the navigation target after mission was modified
    if mission:get_current_nav_index() == rtl_idx then
        mission:set_current_cmd(rtl_idx)
    end
end

Contribution on behalf of @FlyfocusUAV

[LupusTheCanine]

You can use waypint with Lat=lon=0 to immediately set altitude to RTL_ALT value after entering RTL.
followed by waypoint with RTL_ALT at target coordinates.
Unfortunately acceptance radius seems to be limited to 256m so it can't really be used to skip these waypoints when close to home.
If your mission is offset you could use DO_LAND_START placed further out toward the mission direction and one over the landing area to select when those waypoints will happen.

Another option is to use RTL_AUTOLAND=1 as the plane will follow RTL_ALTITUDE when returning to home. Rally points are unfortunately limited to returning at rally waypoint altitude :-(

[Maarrk]

You can use waypint with Lat=lon=0 to immediately set altitude to RTL_ALT value after entering RTL.
followed by waypoint with RTL_ALT at target coordinates.

I didn't know about lat=lon=0 behaviour, but getting to altitude is solved if I can detect RTL being entered

Another option is to use RTL_AUTOLAND=1 as the plane will follow RTL_ALTITUDE when returning to home.

This is an interesting idea which I will explore more, thanks!

[Maarrk]

It turns out that we're doing a lot of A to B flights, where we can't land where we release the plane, so RTL_AUTOLAND=1 is not an option for us.

For now, I gave up on detecting RTL and went with opposite approach. The plane always uses RTL_ALTITUDE when approaching landing, even during normal AUTO mission without using RTL at all.

---

A dream would be to have a new value for RTL_AUTOLAND, so the complete table is like this additions in italic:

Value	Meaning
0	Disable
1	Fly HOME at RTL_ALTITUDE then land via DO_LAND_START mission item
2	Go directly to landing sequence via DO_LAND_START mission item
3	OnlyForGoAround
4	Go directly to landing sequence via DO_RETURN_PATH_START mission item
5	Fly to closest DO_LAND_START mission item at RTL_ALTITUDE and continue the landing sequence

The value in 5 probably needs NAV_LOITER_TO_ALTITUDE so the glide path doesn't get affected when coming from different altitudes, but that can be done in stock QGC already. Since it's just mixing functionality from two already present behaviours, I don't expect it to be too complex to implement. We'll do our testing and report here or in a new PR if it's viable.