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, safety (rally) 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 location return (RTL_TYPE=0): Ascend to safe altitude and return via a direct path to the home location.
  • Mission landing return (RTL_TYPE=1): Ascend to a safe altitude, fly direct to start of mission landing (if defined) and land.
  • Mission path return (RTL_TYPE=2): Use mission path and continue to mission landing (if defined). If no mission landing defined, reverse mission to home.
  • Closest safe destination return (RTL_TYPE=3): Ascend to a safe altitude and return via direct path to closest destination: home, mission landing pattern or safety point (also called a "rally point").

For the mission based return types, the vehicle will fall back to a home location return if no mission/mission landing is defined.

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

Home Location Return Type (RTL_TYPE=0)

In this return type the vehicle:

  • Ascends to a safe return altitude (above any expected obstacles).
  • Flies to the home position via direct path
  • Land or waits at descent altitude

This return type is the fallback for the other types if no mission or mission landing is defined.

This return type is commonly referred to as Return to Launch (RTL) and Return to Home (RTH).

Mission Landing Return Type (RTL_TYPE=1)

This return type uses a mission landing pattern (if defined) to set the return destination and landing behaviour.

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.

If a mission landing has been defined, the vehicle will:

  • Ascend to a safe return altitude above any expected obstacles.
  • Fly a direct path to the start of a mission landing sequence
  • Descend/ascend to the specified waypoint altitude
  • Execute the landing sequence.

If no mission landing is defined, PX4 will revert to a Home Location Return.

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.

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: Same as Mission Landing Return.
    • 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 RTL was triggered in mission mode (above)
  • Manual modes: Same as Home Location Return.

If no mission is defined PX4 will revert to a Home Location Return.

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 safe 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 safety 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).

Return mode cone

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:

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).

Parameter Description
RTL_TYPE Return mechanism (path and destination). 0: Return to safe point via direct path, 1: Use mission landing if defined (otherwise type 0), 2: Use mission path either forward to landing (if landing defined) or reversed to home (or type 0 if no mission).
RTL_RETURN_ALT Return altitude in meters (default: 60m) when RTL_CONE_ANG is 0. If already above this value the vehicle will return at its current altitude.
RTL_DESCEND_ALT Minimum return altitude and altitude at which the vehicle will slow or stop its initial descent from a higher return altitude (default: 30m)
RTL_LAND_DELAY Time to hover at RTL_DESCEND_ALT before landing (default: 0.5s) -by default this period is short so that the vehicle will simply slow and then land immediately. If set to -1 the system will loiter at RTL_DESCEND_ALT rather than landing. The delay is provided to allow you to configure time for landing gear to be deployed (triggered automatically).
RTL_MIN_DIST Minimum horizontal distance from home position to trigger ascent to the return altitude specified by the "cone". If the vehicle is horizontally closer than this distance to home, it will return at its current altitude or RTL_DESCEND_ALT (whichever is higher) instead of first ascending to RTL_RETURN_ALT).
RTL_CONE_ANG Half-angle of the cone that defines the vehicle RTL return altitude. Values (in degrees): 0, 25, 45, 65, 80, 90. Note that 0 is "no cone" (always return at RTL_RETURN_ALT or higher), while 90 indicates that the vehicle must return at the current altitude or RTL_DESCEND_ALT (whichever is higher).
COM_RC_OVERRIDE If enabled stick movement gives control back to the pilot in Position mode (except when vehicle is handling a critical battery failsafe). Enabled by default.
NAV_LOITER_RAD [Fixed Wing Only] The radius of the loiter circle (at RTL_LAND_DELAY.
© PX4 Dev Team. License: CC BY 4.0            Updated: 2019-11-21 19:33:57

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