Return Mode

The Return flight mode is used to fly a vehicle to safety on an unobstructed path to a safe destination, where it may either wait (hover or circle) or land.

PX4 provides several mechanisms for choosing a safe return path, destination and landing, including using home location, rally ("safe") points, mission paths, and mission landing sequences.

The following sections explain how to configure the return type, return altitude and landing/arrival behaviour. At the end there are sections explaining the default (preconfigured) behaviour for each vehicle type.

• This mode requires GPS.
• This mode is automatic - no user intervention is required to control the vehicle.
• RC control switches can be used to change flight modes on any vehicle. The effect of RC stick movement depends on the vehicle type.

Return Types (RTL_TYPE)

PX4 provides four alternative approaches (RTL_TYPE) for finding an unobstructed path to a safe destination and/or landing:

• Home/rally point return (RTL_TYPE=0): Ascend to safe altitude and return via a direct path to the closest rally point or home location.
• Mission landing/rally point return (RTL_TYPE=1): Ascend to a safe altitude, fly direct to the closest destination other than home: rally point or start of mission landing. If no mission landing or rally points are defined, return home via direct path.
• Mission path return (RTL_TYPE=2): Use mission path and fast-continue to mission landing (if defined). If no mission landing defined, fast-reverse mission to home. If no mission defined, return direct to home (rally points are ignored).
• Closest safe destination return (RTL_TYPE=3): Ascend to a safe altitude and return via direct path to closest destination: home, start of mission landing pattern, or rally point. If the destination is a mission landing pattern, follow the pattern to land.

More detailed explanations for each of the types are provided in the following sections.

Home/Rally Point Return Type (RTL_TYPE=0)

In this return type the vehicle:

• Ascends to a safe return altitude (above any expected obstacles).
• Flies via direct path to the home position or a rally point (whichever is closest)
• Land or waits at descent altitude (depending on landing parameters).

If no rally points are defined, this is the same as a Return to Launch (RTL)/Return to Home (RTH).

Mission Landing/Rally Point Return Type (RTL_TYPE=1)

In this return type the vehicle:

• Ascends to a safe return altitude (above any expected obstacles).
• Flies via direct path to a rally point or the start of a mission landing pattern (whichever is closest). If no mission landing or rally points are defined the vehicle instead returns home via a direct path.
• If the destination is a mission landing pattern it will follow the pattern to land.
• If the destination is a rally point or home it will land or wait at descent altitude (depending on landing parameters).

A mission landing pattern consists of a MAV_CMD_DO_LAND_START, one or more position waypoints, and a MAV_CMD_NAV_LAND.

When this type is set PX4 will reject any mission without a valid landing pattern.

Mission Path Return Type (RTL_TYPE=2)

This return type uses the mission (if defined) to provide a safe return path, and the mission landing pattern (if defined) to provide landing behaviour. If there is a mission but no mission landing pattern, the mission is flown in reverse. Rally points, if any, are ignored.

The behaviour is fairly complex because it depends on the flight mode, and whether a mission and mission landing are defined.

Mission with landing pattern:

• Mission mode: Mission is continued in "fast-forward mode" (jumps, delay and any other non-position commands ignored, loiter and other position waypoints converted to simple waypoints) and then lands.
• Auto mode other than mission mode:
  • Ascend to a safe return altitude above any expected obstacles.
  • Fly directly to closest waypoint (for FW not a landing WP) and descend to waypoint altitude.
  • Continue mission in fast forward mode from that waypoint.
• Manual modes:
  • Ascend to a safe return altitude above any expected obstacles.
  • Fly directly to landing sequence position and descend to waypoint altitude
  • Land using mission landing pattern

Mission without landing pattern defined:

• Mission mode:
  • Mission flown "fast-backward" (in reverse) starting from the previous waypoint
    • Jumps, delay and any other non-position commands ignored, loiter and other position waypoints converted to simple waypoints.
    • VTOL vehicles transition to FW mode (if needed) before flying the mission in reverse.
  • On reaching waypoint 1, the vehicle ascends to the return altitude and flies to the home position (where it lands or waits).
• Auto mode other than mission mode:
  • Fly directly to closest waypoint (for FW not a landing WP) and descend to waypoint altitude.
  • Continue the mission in reverse, exactly as though Return mode was triggered in mission mode (above)
• Manual modes: Fly directly to home location and land.

If no mission is defined PX4 will fly directly to home location and land (rally points are ignored).

If the mission changes during return mode, then the behaviour is re-evaluated based on the new mission following the same rules as above (e.g. if the new mission has no landing sequence and you're in a mission, the mission is reversed).

Closest Safe Destination Return Type (RTL_TYPE=3)

In this return type the vehicle:

• Ascends to a safe return altitude (above any expected obstacles).
• Flies a direct path to the closest destination of: home location, mission landing pattern or rally point.
• If the destination is a mission landing pattern the vehicle will follow the pattern to land
• If the destination is a home location or rally point, the vehicle will descend to the descent altitude (RTL_DESCEND_ALT) and then Land or waits.

Return Altitude

A vehicle will usually first ascend to a safe altitude before returning (in order to avoid any obstacles between it and the destination).

The exception is when executing a mission path return from within a mission. In this case the vehicle follows mission waypoints, which we assume avoid any obstacles).

The return altitude can be configured using the parameters RTL_RETURN_ALT and RTL_CONE_ANG (which define a half cone centered around the destination - home location or safety point).

If the vehicle is:

• Above RTL_RETURN_ALT (1) it will return at its current altitude.
• Below the cone it will return where it intersects the cone (2) or RTL_DESCEND_ALT (whichever is higher).
• Outside the cone (3) it will first climb until it reaches RTL_RETURN_ALT.
• Inside the cone:
  • Above RTL_DESCEND_ALT (4) it will return at its current altitude.
  • Below RTL_DESCEND_ALT (5) it will first ascend to RTL_DESCEND_ALT.

Note:

• If RTL_CONE_ANG is 0 degrees there is no "cone":
  • the vehicle returns at RTL_RETURN_ALT (or above).
• If RTL_CONE_ANG is 90 degrees the vehicle will return at the greater of RTL_DESCEND_ALT and the current altitude.
• The vehicle will always ascend at least RTL_DESCEND_ALT for the return.

RC stick movement will by default change the vehicle to Position mode unless handling a critical battery failsafe.

Hover/Landing at Destination

Unless executing a mission landing (e.g. if executing a home location return or closest safe destination return), the vehicle will arrive at its destination, and rapidly descend to the RTL_DESCEND_ALT altitude.

The vehicle will the loiter for a specified time (RTL_LAND_DELAY) and then land. If RTL_LAND_DELAY=-1 it will loiter indefinitely.

Vehicle Default Behaviour

The mode is implemented in almost exactly the same way in all vehicle types (the exception being that fixed wing vehicles will circle rather than hover when waiting), and are hence tuned using the same parameters.

However the default configuration is tailored to suit the vehicle type, as described below.

Multi-Copter (MC)

Multicopters use a home location return by default (and the following configuration):

• Ascend to RTL_RETURN_ALT (RTL_CONE_ANG=0 - cone not used).
• Fly to the home position in a straight line and constant altitude (if already above the return altitude it will return at its current altitude).
• Rapidly descend to the RTL_DESCEND_ALT altitude.
• Land more or less immediately (small RTL_LAND_DELAY).

Fixed Wing (FW)

Fixed-wing aircraft use a mission landing return type by default:

• If a mission landing is defined, fly direct to the mission landing start point and then land.
• Otherwise fly directly to the home position and circle above it at radius NAV_LOITER_RAD.

If not following a mission landing, and RTL_LAND_DELAY is set to -1, the vehicle will land as described in the topic: Landing (Fixed Wing). RC stick movement is ignored.

VTOL

VTOL aircraft use a mission landing return type by default:

• If a mission landing is defined, fly direct to the mission landing start point and then land.
• Otherwise fly directly to the home position, transition to multicopter mode, and land as a multicopter.

  If not in a mission landing, a VTOL in FW mode will always transition back to MC just before landing (ignoring NAV_FORCE_VT).

Parameters

The RTL parameters are listed in Parameter Reference > Return Mode (and summarised below).