The DO_LAND_START mission item can be used for two purposes:
To provide an automatic re-attempt for an aborted NAV_LAND autolanding in a mission. (See How to Abort an Autolanding)
To provide an automatic landing for an RTL.
To trigger an automatic landing as part of an RTL (return to launch) you need to do two things:
add a DO_LAND_START mission item to your mission, just before the start of your landing sequence which sets up the approach waypoints and ends with a NAV_LAND item.
set the RTL_AUTOLAND parameter to 1 or 2
The way it works is that when the plane enters an RTL it checks to see
if the parameter RTL_AUTOLAND is set to 1 or 2. If it is 1 or 2, then the
current mission is searched for a mission item of type
If one is found then the plane will automatically enter AUTO mode and
land, starting at the part of the mission just after the
the “landing sequence” whose start is the
DO_LAND_START marker, does not actually
have to include a landing! it can be any sequence of valid mission commands. For example, flying to a location, and changing to an indefinite VTOL loiter (applies only to a QuadPlane, obviously).
The exact behaviour depends on the RTL_AUTOLAND value:
If RTL_AUTOLAND = 1, then the plane will first RTL as normal, then when it starts circling the return point (home or a rally point) it will then switch to the AUTO mission after the
If RTL_AUTOLAND = 2, then the plane will bypass the RTL completely and go straight to the landing sequence after the
If you are already in a landing sequence (NAV_LAND or DO_START_LAND sequence), then it will continue.
You can optionally include more than one
DO_LAND_START mission item
in your mission. If that is done then the latitude/longitude/altitude of the
DO_LAND_START mission items is used to choose which landing sequence
to use. The
DO_LAND_START closest to the current location in all three dimensions is used.
This can be useful if you have multiple landing sequences for different
wind conditions or different areas.
if you execute an RTL due to a battery failsafe and the MIS_OPTIONS bit 1 (Use distance to land) is set, then if continuing the mission results in a closer landing point, then the mission will proceed.
ArduPilot checks to see if there is a
DO_LAND_START mission items before arming, and RTL_AUTOLAND is set to 0. If so, a pre-arm failure condition will prevent arming. If it is desired to use a
DO_LAND_START only for aborted autolandings and not as an RTL action override, then set RTL_AUTOLAND = 3, to allow arming. Its use for an aborted autolanding is explained in How to Abort an Autolanding.