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---------

Signed-off-by: Ramon Roche <mrpollo@gmail.com>
Co-authored-by: Ramon Roche <mrpollo@gmail.com>
This commit is contained in:
Hamish Willee
2025-03-13 16:08:27 +11:00
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co-authored by Ramon Roche
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# Flight Modes
Flight modes provide autopilot support to make it easier to manually fly the vehicle, to automate common tasks such as takeoff and landing, to execute autonomous missions, or to defer flight control to an external system.
For information about flight modes available to specific frames see the following topics:
- [Flight Modes (Multicopter)](../flight_modes_mc/index.md)
- [Flight Modes (Fixed-Wing)](../flight_modes_fw/index.md)
- [Flight Modes (VTOL)](../flight_modes_vtol/index.md)
- [Drive Modes (Differential Rover)](../flight_modes_rover/differential.md)
- [Drive Modes (Ackermann Rover)](../flight_modes_rover/ackermann.md)
::: info
The mode sub-topics in this section contain information that is common to all vehicles, but may not be relevant to the normal/default setup.
Generally you should see the vehicle-specific topics first, and navigate to these generalized topics when it becomes relevant.
:::
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# Offboard Mode (Generic/All Frames)
<img src="../../assets/site/position_fixed.svg" title="Position fix required (e.g. GPS)" width="30px" />
The vehicle obeys position, velocity, acceleration, attitude, attitude rates or thrust/torque setpoints provided by some source that is external to the flight stack, such as a companion computer.
The setpoints may be provided using MAVLink (or a MAVLink API such as [MAVSDK](https://mavsdk.mavlink.io/)) or by [ROS 2](../ros2/index.md).
PX4 requires that the external controller provides a continuous 2Hz "proof of life" signal, by streaming any of the supported MAVLink setpoint messages or the ROS 2 [OffboardControlMode](../msg_docs/OffboardControlMode.md) message.
PX4 enables offboard control only after receiving the signal for more than a second, and will regain control if the signal stops.
::: info
- This mode requires position or pose/attitude information - e.g. GPS, optical flow, visual-inertial odometry, mocap, etc.
- RC control is disabled except to change modes (you can also fly without any manual controller at all by setting the parameter [COM_RC_IN_MODE](../advanced_config/parameter_reference.md#COM_RC_IN_MODE) to 4: Stick input disabled).
- The vehicle must be already be receiving a stream of MAVLink setpoint messages or ROS 2 [OffboardControlMode](../msg_docs/OffboardControlMode.md) messages before arming in offboard mode or switching to offboard mode when flying.
- The vehicle will exit offboard mode if MAVLink setpoint messages or `OffboardControlMode` are not received at a rate of > 2Hz.
- Not all coordinate frames and field values allowed by MAVLink are supported for all setpoint messages and vehicles.
Read the sections below _carefully_ to ensure only supported values are used.
:::
## Description
Offboard mode is used for controlling vehicle movement and attitude, by setting position, velocity, acceleration, attitude, attitude rates or thrust/torque setpoints.
PX4 must receive a stream of MAVLink setpoint messages or the ROS 2 [OffboardControlMode](../msg_docs/OffboardControlMode.md) at 2 Hz as proof that the external controller is healthy.
The stream must be sent for at least a second before PX4 will arm in offboard mode, or switch to offboard mode when flying.
If the rate falls below 2Hz while under external control PX4 will switch out of offboard mode after a timeout ([COM_OF_LOSS_T](#COM_OF_LOSS_T)), and attempt to land or perform some other failsafe action.
The action depends on whether or not RC control is available, and is defined in the parameter [COM_OBL_RC_ACT](#COM_OBL_RC_ACT).
When using MAVLink the setpoint messages convey both the signal to indicate that the external source is "alive", and the setpoint value itself.
In order to hold position in this case the vehicle must receive a stream of setpoints for the current position.
When using ROS 2 the proof that the external source is alive is provided by a stream of [OffboardControlMode](../msg_docs/OffboardControlMode.md) messages, while the actual setpoint is provided by publishing to one of the setpoint uORB topics, such as [TrajectorySetpoint](../msg_docs/TrajectorySetpoint.md).
In order to hold position in this case the vehicle must receive a stream of `OffboardControlMode` but would only need the `TrajectorySetpoint` once.
Note that offboard mode only supports a very limited set of MAVLink commands and messages.
Operations, like taking off, landing, return to launch, may be best handled using the appropriate modes.
Operations like uploading, downloading missions can be performed in any mode.
## ROS 2 Messages
The following ROS 2 messages and their particular fields and field values are allowed for the specified frames.
In addition to providing heartbeat functionality, `OffboardControlMode` has two other main purposes:
1. Controls the level of the [PX4 control architecture](../flight_stack/controller_diagrams.md) at which offboard setpoints must be injected, and disables the bypassed controllers.
1. Determines which valid estimates (position or velocity) are required, and also which setpoint messages should be used.
The `OffboardControlMode` message is defined as shown.
```sh
# Off-board control mode
uint64 timestamp # time since system start (microseconds)
bool position
bool velocity
bool acceleration
bool attitude
bool body_rate
bool thrust_and_torque
bool direct_actuator
```
The fields are ordered in terms of priority such that `position` takes precedence over `velocity` and later fields, `velocity` takes precedence over `acceleration`, and so on.
The first field that has a non-zero value (from top to bottom) defines what valid estimate is required in order to use offboard mode, and the setpoint message(s) that can be used.
For example, if the `acceleration` field is the first non-zero value, then PX4 requires a valid `velocity estimate`, and the setpoint must be specified using the `TrajectorySetpoint` message.
| desired control quantity | position field | velocity field | acceleration field | attitude field | body_rate field | thrust_and_torque field | direct_actuator field | required estimate | required message |
| ------------------------ | -------------- | -------------- | ------------------ | -------------- | --------------- | ----------------------- | --------------------- | ----------------- | ------------------------------------------------------------------------------------------------------------------------------- |
| position (NED) | ✓ | - | - | - | - | - | - | position | [TrajectorySetpoint](../msg_docs/TrajectorySetpoint.md) |
| velocity (NED) | ✗ | ✓ | - | - | - | - | - | velocity | [TrajectorySetpoint](../msg_docs/TrajectorySetpoint.md) |
| acceleration (NED) | ✗ | ✗ | ✓ | - | - | - | - | velocity | [TrajectorySetpoint](../msg_docs/TrajectorySetpoint.md) |
| attitude (FRD) | ✗ | ✗ | ✗ | ✓ | - | - | - | none | [VehicleAttitudeSetpoint](../msg_docs/VehicleAttitudeSetpoint.md) |
| body_rate (FRD) | ✗ | ✗ | ✗ | ✗ | ✓ | - | - | none | [VehicleRatesSetpoint](../msg_docs/VehicleRatesSetpoint.md) |
| thrust and torque (FRD) | ✗ | ✗ | ✗ | ✗ | ✗ | ✓ | - | none | [VehicleThrustSetpoint](../msg_docs/VehicleThrustSetpoint.md) and [VehicleTorqueSetpoint](../msg_docs/VehicleTorqueSetpoint.md) |
| direct motors and servos | ✗ | ✗ | ✗ | ✗ | ✗ | ✗ | ✓ | none | [ActuatorMotors](../msg_docs/ActuatorMotors.md) and [ActuatorServos](../msg_docs/ActuatorServos.md) |
where ✓ means that the bit is set, ✘ means that the bit is not set and `-` means that the bit is value is irrelevant.
::: info
Before using offboard mode with ROS 2, please spend a few minutes understanding the different [frame conventions](../ros2/user_guide.md#ros-2-px4-frame-conventions) that PX4 and ROS 2 use.
:::
### Copter
- [px4_msgs::msg::TrajectorySetpoint](https://github.com/PX4/PX4-Autopilot/blob/main/msg/TrajectorySetpoint.msg)
- The following input combinations are supported:
- Position setpoint (`position` different from `NaN`). Non-`NaN` values of velocity and acceleration are used as feedforward terms for the inner loop controllers.
- Velocity setpoint (`velocity` different from `NaN` and `position` set to `NaN`). Non-`NaN` values acceleration are used as feedforward terms for the inner loop controllers.
- Acceleration setpoint (`acceleration` different from `NaN` and `position` and `velocity` set to `NaN`)
- All values are interpreted in NED (Nord, East, Down) coordinate system and the units are \[m\], \[m/s\] and \[m/s^2\] for position, velocity and acceleration, respectively.
- [px4_msgs::msg::VehicleAttitudeSetpoint](https://github.com/PX4/PX4-Autopilot/blob/main/msg/VehicleAttitudeSetpoint.msg)
- The following input combination is supported:
- quaternion `q_d` + thrust setpoint `thrust_body`.
Non-`NaN` values of `yaw_sp_move_rate` are used as feedforward terms expressed in Earth frame and in \[rad/s\].
- The quaternion represents the rotation between the drone body FRD (front, right, down) frame and the NED frame. The thrust is in the drone body FRD frame and expressed in normalized \[-1, 1\] values.
- [px4_msgs::msg::VehicleRatesSetpoint](https://github.com/PX4/PX4-Autopilot/blob/main/msg/VehicleRatesSetpoint.msg)
- The following input combination is supported:
- `roll`, `pitch`, `yaw` and `thrust_body`.
- All the values are in the drone body FRD frame. The rates are in \[rad/s\] while thrust_body is normalized in \[-1, 1\].
### Generic Vehicle
The following offboard control modes bypass all internal PX4 control loops and should be used with great care.
- [px4_msgs::msg::VehicleThrustSetpoint](https://github.com/PX4/PX4-Autopilot/blob/main/msg/VehicleThrustSetpoint.msg) + [px4_msgs::msg::VehicleTorqueSetpoint](https://github.com/PX4/PX4-Autopilot/blob/main/msg/VehicleTorqueSetpoint.msg)
- The following input combination is supported:
- `xyz` for thrust and `xyz` for torque.
- All the values are in the drone body FRD frame and normalized in \[-1, 1\].
- [px4_msgs::msg::ActuatorMotors](https://github.com/PX4/PX4-Autopilot/blob/main/msg/ActuatorMotors.msg) + [px4_msgs::msg::ActuatorServos](https://github.com/PX4/PX4-Autopilot/blob/main/msg/ActuatorServos.msg)
- You directly control the motor outputs and/or servo outputs.
- Currently works at lower level than then `control_allocator` module. Do not publish these messages when not in offboard mode.
- All the values normalized in \[-1, 1\]. For outputs that do not support negative values, negative entries map to `NaN`.
- `NaN` maps to disarmed.
## MAVLink Messages
The following MAVLink messages and their particular fields and field values are allowed for the specified vehicle frames.
### Copter/VTOL
- [SET_POSITION_TARGET_LOCAL_NED](https://mavlink.io/en/messages/common.html#SET_POSITION_TARGET_LOCAL_NED)
- The following input combinations are supported: <!-- https://github.com/PX4/PX4-Autopilot/blob/main/src/lib/FlightTasks/tasks/Offboard/FlightTaskOffboard.cpp#L166-L170 -->
- Position setpoint (only `x`, `y`, `z`)
- Velocity setpoint (only `vx`, `vy`, `vz`)
- Acceleration setpoint (only `afx`, `afy`, `afz`)
- Position setpoint **and** velocity setpoint (the velocity setpoint is used as feedforward; it is added to the output of the position controller and the result is used as the input to the velocity controller).
- Position setpoint **and** velocity setpoint **and** acceleration (the velocity and the acceleration setpoints are used as feedforwards; the velocity setpoint is added to the output of the position controller and the result is used as the input to the velocity controller; the acceleration setpoint is added to the output of the velocity controller and the result used to compute the thrust vector).
- PX4 supports the following `coordinate_frame` values (only): [MAV_FRAME_LOCAL_NED](https://mavlink.io/en/messages/common.html#MAV_FRAME_LOCAL_NED) and [MAV_FRAME_BODY_NED](https://mavlink.io/en/messages/common.html#MAV_FRAME_BODY_NED).
- [SET_POSITION_TARGET_GLOBAL_INT](https://mavlink.io/en/messages/common.html#SET_POSITION_TARGET_GLOBAL_INT)
- The following input combinations are supported: <!-- https://github.com/PX4/PX4-Autopilot/blob/main/src/lib/FlightTasks/tasks/Offboard/FlightTaskOffboard.cpp#L166-L170 -->
- Position setpoint (only `lat_int`, `lon_int`, `alt`)
- Velocity setpoint (only `vx`, `vy`, `vz`)
- _Thrust_ setpoint (only `afx`, `afy`, `afz`)
::: info
Acceleration setpoint values are mapped to create a normalized thrust setpoint (i.e. acceleration setpoints are not "properly" supported).
:::
- Position setpoint **and** velocity setpoint (the velocity setpoint is used as feedforward; it is added to the output of the position controller and the result is used as the input to the velocity controller).
- PX4 supports the following `coordinate_frame` values (only): [MAV_FRAME_GLOBAL](https://mavlink.io/en/messages/common.html#MAV_FRAME_GLOBAL).
- [SET_ATTITUDE_TARGET](https://mavlink.io/en/messages/common.html#SET_ATTITUDE_TARGET)
- The following input combinations are supported:
- Attitude/orientation (`SET_ATTITUDE_TARGET.q`) with thrust setpoint (`SET_ATTITUDE_TARGET.thrust`).
- Body rate (`SET_ATTITUDE_TARGET` `.body_roll_rate` ,`.body_pitch_rate`, `.body_yaw_rate`) with thrust setpoint (`SET_ATTITUDE_TARGET.thrust`).
### Fixed-wing
- [SET_POSITION_TARGET_LOCAL_NED](https://mavlink.io/en/messages/common.html#SET_POSITION_TARGET_LOCAL_NED)
- The following input combinations are supported (via `type_mask`): <!-- https://github.com/PX4/PX4-Autopilot/blob/main/src/lib/FlightTasks/tasks/Offboard/FlightTaskOffboard.cpp#L166-L170 -->
- Position setpoint (`x`, `y`, `z` only; velocity and acceleration setpoints are ignored).
- Specify the _type_ of the setpoint in `type_mask` (if these bits are not set the vehicle will fly in a flower-like pattern):
::: info
Some of the _setpoint type_ values below are not part of the MAVLink standard for the `type_mask` field.
:::
The values are:
- 292: Gliding setpoint.
This configures TECS to prioritize airspeed over altitude in order to make the vehicle glide when there is no thrust (i.e. pitch is controlled to regulate airspeed).
It is equivalent to setting `type_mask` as `POSITION_TARGET_TYPEMASK_Z_IGNORE`, `POSITION_TARGET_TYPEMASK_VZ_IGNORE`, `POSITION_TARGET_TYPEMASK_AZ_IGNORE`.
- 4096: Takeoff setpoint.
- 8192: Land setpoint.
- 12288: Loiter setpoint (fly a circle centred on setpoint).
- 16384: Idle setpoint (zero throttle, zero roll / pitch).
- PX4 supports the coordinate frames (`coordinate_frame` field): [MAV_FRAME_LOCAL_NED](https://mavlink.io/en/messages/common.html#MAV_FRAME_LOCAL_NED) and [MAV_FRAME_BODY_NED](https://mavlink.io/en/messages/common.html#MAV_FRAME_BODY_NED).
- [SET_POSITION_TARGET_GLOBAL_INT](https://mavlink.io/en/messages/common.html#SET_POSITION_TARGET_GLOBAL_INT)
- The following input combinations are supported (via `type_mask`): <!-- https://github.com/PX4/PX4-Autopilot/blob/main/src/lib/FlightTasks/tasks/Offboard/FlightTaskOffboard.cpp#L166-L170 -->
- Position setpoint (only `lat_int`, `lon_int`, `alt`)
- Specify the _type_ of the setpoint in `type_mask` (if these bits are not set the vehicle will fly in a flower-like pattern):
::: info
The _setpoint type_ values below are not part of the MAVLink standard for the `type_mask` field.
:::
The values are:
- 4096: Takeoff setpoint.
- 8192: Land setpoint.
- 12288: Loiter setpoint (fly a circle centred on setpoint).
- 16384: Idle setpoint (zero throttle, zero roll / pitch).
- PX4 supports the following `coordinate_frame` values (only): [MAV_FRAME_GLOBAL](https://mavlink.io/en/messages/common.html#MAV_FRAME_GLOBAL).
- [SET_ATTITUDE_TARGET](https://mavlink.io/en/messages/common.html#SET_ATTITUDE_TARGET)
- The following input combinations are supported:
- Attitude/orientation (`SET_ATTITUDE_TARGET.q`) with thrust setpoint (`SET_ATTITUDE_TARGET.thrust`).
- Body rate (`SET_ATTITUDE_TARGET` `.body_roll_rate` ,`.body_pitch_rate`, `.body_yaw_rate`) with thrust setpoint (`SET_ATTITUDE_TARGET.thrust`).
### Rover
- [SET_POSITION_TARGET_LOCAL_NED](https://mavlink.io/en/messages/common.html#SET_POSITION_TARGET_LOCAL_NED)
- The following input combinations are supported (in `type_mask`): <!-- https://github.com/PX4/PX4-Autopilot/blob/main/src/lib/FlightTasks/tasks/Offboard/FlightTaskOffboard.cpp#L166-L170 -->
- Position setpoint (only `x`, `y`, `z`)
- Specify the _type_ of the setpoint in `type_mask`:
::: info
The _setpoint type_ values below are not part of the MAVLink standard for the `type_mask` field.
::
The values are:
- 12288: Loiter setpoint (vehicle stops when close enough to setpoint).
- Velocity setpoint (only `vx`, `vy`, `vz`)
- PX4 supports the coordinate frames (`coordinate_frame` field): [MAV_FRAME_LOCAL_NED](https://mavlink.io/en/messages/common.html#MAV_FRAME_LOCAL_NED) and [MAV_FRAME_BODY_NED](https://mavlink.io/en/messages/common.html#MAV_FRAME_BODY_NED).
- [SET_POSITION_TARGET_GLOBAL_INT](https://mavlink.io/en/messages/common.html#SET_POSITION_TARGET_GLOBAL_INT)
- The following input combinations are supported (in `type_mask`): <!-- https://github.com/PX4/PX4-Autopilot/blob/main/src/lib/FlightTasks/tasks/Offboard/FlightTaskOffboard.cpp#L166-L170 -->
- Position setpoint (only `lat_int`, `lon_int`, `alt`)
- Specify the _type_ of the setpoint in `type_mask` (not part of the MAVLink standard).
The values are:
- Following bits not set then normal behaviour.
- 12288: Loiter setpoint (vehicle stops when close enough to setpoint).
- PX4 supports the coordinate frames (`coordinate_frame` field): [MAV_FRAME_GLOBAL](https://mavlink.io/en/messages/common.html#MAV_FRAME_GLOBAL).
- [SET_ATTITUDE_TARGET](https://mavlink.io/en/messages/common.html#SET_ATTITUDE_TARGET)
- The following input combinations are supported:
- Attitude/orientation (`SET_ATTITUDE_TARGET.q`) with thrust setpoint (`SET_ATTITUDE_TARGET.thrust`).
::: info
Only the yaw setting is actually used/extracted.
:::
## Offboard Parameters
_Offboard mode_ is affected by the following parameters:
| Parameter | Description |
| -------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
| <a id="COM_OF_LOSS_T"></a>[COM_OF_LOSS_T](../advanced_config/parameter_reference.md#COM_OF_LOSS_T) | Time-out (in seconds) to wait when offboard connection is lost before triggering offboard lost failsafe (`COM_OBL_RC_ACT`) |
| <a id="COM_OBL_RC_ACT"></a>[COM_OBL_RC_ACT](../advanced_config/parameter_reference.md#COM_OBL_RC_ACT) | Flight mode to switch to if offboard control is lost (Values are - `0`: _Position_, `1`: _Altitude_, `2`: _Manual_, `3`: *Return, `4`: *Land\*). |
| <a id="COM_RC_OVERRIDE"></a>[COM_RC_OVERRIDE](../advanced_config/parameter_reference.md#COM_RC_OVERRIDE) | Controls whether stick movement on a multicopter (or VTOL in MC mode) causes a mode change to [Position mode](../flight_modes_mc/position.md). This is not enabled for offboard mode by default. |
| <a id="COM_RC_STICK_OV"></a>[COM_RC_STICK_OV](../advanced_config/parameter_reference.md#COM_RC_STICK_OV) | The amount of stick movement that causes a transition to [Position mode](../flight_modes_mc/position.md) (if [COM_RC_OVERRIDE](#COM_RC_OVERRIDE) is enabled). |
| <a id="COM_RCL_EXCEPT"></a>[COM_RCL_EXCEPT](../advanced_config/parameter_reference.md#COM_RCL_EXCEPT) | Specify modes in which RC loss is ignored and the failsafe action not triggered. Set bit `2` to ignore RC loss in Offboard mode. |
## Developer Resources
Typically developers do not directly work at the MAVLink layer, but instead use a robotics API like [MAVSDK](https://mavsdk.mavlink.io/) or [ROS](http://www.ros.org/) (these provide a developer friendly API, and take care of managing and maintaining connections, sending messages and monitoring responses - the minutiae of working with _Offboard mode_ and MAVLink).
The following resources may be useful for a developer audience:
- [Offboard Control from Linux](../ros/offboard_control.md)
- [ROS/MAVROS Offboard Example (C++)](../ros/mavros_offboard_cpp.md)
- [ROS/MAVROS Offboard Example (Python)](../ros/mavros_offboard_python.md)
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# Return Mode (Generic Vehicle)
<img src="../../assets/site/position_fixed.svg" title="Position fix required (e.g. GPS)" width="30px" />
The _Return_ flight mode is used to _fly a vehicle to safety_ on an unobstructed path to a safe destination, where it should land.
The following topics should be read first if you're using these vehicle types:
- [Multicopter](../flight_modes_mc/return.md)
- [Fixed-wing (Plane)](../flight_modes_fw/return.md)
- [VTOL](../flight_modes_vtol/return.md)
::: info
- Mode is automatic - no user intervention is _required_ to control the vehicle.
- Mode requires a global 3d position estimate (from GPS or inferred from a [local position](../ros/external_position_estimation.md#enabling-auto-modes-with-a-local-position)).
- Flying vehicles can't switch to this mode without global position.
- Flying vehicles will failsafe if they lose the position estimate.
- Mode requires home position is set.
- Mode prevents arming (vehicle must be armed when switching to this mode).
- RC control switches can be used to change flight modes on any vehicle.
- RC stick movement in a multicopter (or VTOL in multicopter mode) will [by default](#COM_RC_OVERRIDE) change the vehicle to [Position mode](../flight_modes_mc/position.md) unless handling a critical battery failsafe.
- A VTOL will return as MC or FW based on its mode at the point the return mode was triggered.
In MC mode it will respect multicopter parameters, such as the landing "cone".
In FW mode it will respect fixed-wing parameters (ignore the cone), but unless using a mission landing, will transition to MC mode and land at the destination after loitering at the descent altitude.
<!-- https://github.com/PX4/PX4-Autopilot/blob/main/src/modules/commander/ModeUtil/mode_requirements.cpp -->
:::
## Overview
PX4 provides several mechanisms for choosing a safe return path, destination and landing, including using home location, rally ("safe") points, mission paths, and landing sequences defined in a mission.
All vehicles _nominally_ support all of these mechanisms, but not all of them make as much sense for particular vehicles.
For example, a multicopter can land virtually anywhere, so it doesn't make sense for it to use a landing sequence except in rare cases.
Similarly, a fixed-wing vehicle needs to fly a safe landing path: it can use the home location as a return point, but it won't try and land on it by default.
This topic covers all the possible return types that any vehicle _might_ be configured to use — the vehicle-specific return mode topics cover the default/recommended return type and configuration for each vehicle.
The following sections explain how to configure the [return type](#return_types), [minimum return altitude](#minimum-return-altitude) and [landing/arrival behaviour](#loiter-landing-at-destination).
<a id="return_types"></a>
## Return Types (RTL_TYPE)
PX4 provides four alternative approaches for finding an unobstructed path to a safe destination and/or landing, which are set using the [RTL_TYPE](#RTL_TYPE) parameter.
At high level these are:
- [Home/rally point return](#home_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](#mission-landing-rally-point-return-type-rtl-type-1) (`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](#mission-path-return-type-rtl-type-2) (`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](#closest-safe-destination-return-type-rtl-type-3) (`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.
<a id="home_return"></a>
### Home/Rally Point Return Type (RTL_TYPE=0)
This is the default return type for a [multicopter](../flight_modes_mc/return.md) (see topic for more information).
In this return type the vehicle:
- Ascends to a safe [minimum return altitude](#minimum-return-altitude) (above any expected obstacles).
- Flies via direct path to the home position or a rally point (whichever is closest)
- On [arrival](#loiter-landing-at-destination) descends to "descent altitude" and waits for a configurable time.
This time may be used to deploy landing gear.
- Lands or waits (this depends on landing parameters),
By default an MC or VTOL in MC mode will land and a fixed-wing vehicle circles at the descent altitude.
A VTOL in FW mode aligns its heading to the destination point, transitions to MC mode, and then lands.
::: info
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)
This is the default return type for a [fixed-wing](../flight_modes_fw/return.md) or [VTOL](../flight_modes_vtol/return.md) vehicle (see topics for more information).
In this return type the vehicle:
- Ascends to a safe [minimum return altitude](#minimum-return-altitude) (above any expected obstacles) if needed.
The vehicle maintains its initial altitude if that is higher than the minimum return altitude.
- Flies via direct constant-altitude path to a rally point or the start of a [mission landing pattern](#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](#loiter-landing-at-destination) at descent altitude (depending on landing parameters).
By default an MC or VTOL in MC mode will land, and a fixed-wing vehicle circles at the descent altitude.
A VTOL in FW mode aligns its heading to the destination point, transitions to MC mode, and then lands.
::: info
Fixed wing vehicles commonly also set [MIS_TKO_LAND_REQ](#MIS_TKO_LAND_REQ) to _require_ a mission 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](#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.
::: info
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 [minimum return altitude](#minimum-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 [minimum return altitude](#minimum-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 [minimum return altitude](#minimum-return-altitude) and flies to the home position (where it [lands or waits](#loiter-landing-at-destination)).
- **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 [minimum return altitude](#minimum-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](#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](#RTL_DESCEND_ALT)) and then [lands or waits](#loiter-landing-at-destination).
By default an MC or VTOL in MC mode will land, and a fixed-wing vehicle circles at the descent altitude.
A VTOL in FW mode aligns its heading to the destination point, transitions to MC mode, and then lands.
## Minimum Return Altitude
For most [return types](#return_types) a vehicle will ascend to a _minimum safe altitude_ before returning (unless already above that altitude), in order to avoid any obstacles between it and the destination.
::: info
The exception is when executing a [mission path return](#mission-path-return-type-rtl-type-2) from _within a mission_.
In this case the vehicle follows mission waypoints, which we assume are planned to avoid any obstacles.
:::
The return altitude for a fixed-wing vehicle or a VTOL in fixed-wing mode is configured using the parameter [RTL_RETURN_ALT](#RTL_RETURN_ALT) (does not use the code described in the next paragraph).
The return altitude for a multicopter or a VTOL vehicles in MC mode is configured using the parameters [RTL_RETURN_ALT](#RTL_RETURN_ALT) and [RTL_CONE_ANG](#RTL_CONE_ANG), which define a half cone centered around the destination (home location or safety point).
![Return mode cone](../../assets/flying/rtl_cone.jpg)
<!-- Original draw.io diagram can be found here: https://drive.google.com/file/d/1W72XeZYSOkRlBSbPXCCiam9NMAyAWSg-/view?usp=sharing -->
If the vehicle is:
- Above [RTL_RETURN_ALT](#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](#RTL_DESCEND_ALT) (whichever is higher).
- Outside the cone (3) it will first climb until it reaches [RTL_RETURN_ALT](#RTL_RETURN_ALT).
- Inside the cone:
- Above [RTL_DESCEND_ALT](#RTL_DESCEND_ALT) (4) it will return at its current altitude.
- Below [RTL_DESCEND_ALT](#RTL_DESCEND_ALT) (5) it will first ascend to `RTL_DESCEND_ALT`.
Note:
- If [RTL_CONE_ANG](#RTL_CONE_ANG) is 0 degrees there is no "cone":
- the vehicle returns at `RTL_RETURN_ALT` (or above).
- If [RTL_CONE_ANG](#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](#RTL_DESCEND_ALT) for the return.
## Loiter/Landing at Destination
Unless executing a [mission landing pattern](#mission-landing-pattern) as part of the return mode, the vehicle will arrive at its destination, and rapidly descend to the [RTL_DESCEND_ALT](#RTL_DESCEND_ALT) altitude, where it will loiter for [RTL_LAND_DELAY](#RTL_LAND_DELAY) before landing.
If `RTL_LAND_DELAY=-1` it will loiter indefinitely.
The default landing configuration is vehicle dependent:
- Multicopters are configured to hover for a short while, deploying landing gear if needed, and then land.
- Fixed-wing vehicles use a return mode with a [mission landing pattern](#mission-landing-pattern), as this enables automated landing.
If not using a mission landing, the default configuration is to loiter indefinitely, so the user can take over and manually land.
- VTOLs in MC mode fly and land exactly as a multicopter.
- VTOLS in FW mode head towards the landing point, transition to MC mode, and then land on the destination.
## Mission Landing Pattern
A mission landing pattern is a landing pattern defined as part of a mission plan.
This consists of a [MAV_CMD_DO_LAND_START](https://mavlink.io/en/messages/common.html#MAV_CMD_DO_LAND_START), one or more position waypoints, and a [MAV_CMD_NAV_LAND](https://mavlink.io/en/messages/common.html#MAV_CMD_NAV_LAND) (or [MAV_CMD_NAV_VTOL_LAND](https://mavlink.io/en/messages/common.html#MAV_CMD_NAV_VTOL_LAND) for a VTOL Vehicle).
Landing patterns defined in missions are the safest way to automatically land a _fixed-wing_ vehicle on PX4.
For this reason fixed-wing vehicles are configured to use [Mission landing/really point return](#mission-landing-rally-point-return-type-rtl-type-1) by default.
## Parameters
The RTL parameters are listed in [Parameter Reference > Return Mode](../advanced_config/parameter_reference.md#return-mode) (and summarised below).
| Parameter | Description |
| ----------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| <a id="RTL_TYPE"></a>[RTL_TYPE](../advanced_config/parameter_reference.md#RTL_TYPE) | Return mechanism (path and destination).<br>`0`: Return to a rally point or home (whichever is closest) via direct path.<br>`1`: Return to a rally point or the mission landing pattern start point (whichever is closest), via direct path. If neither mission landing or rally points are defined return home via a direct path. If the destination is a mission landing pattern, follow the pattern to land.<br>`2`: Use mission path fast-forward to landing if a landing pattern is defined, otherwise fast-reverse to home. Ignore rally points. Fly direct to home if no mission plan is defined.<br>`3`: Return via direct path to closest destination: home, start of mission landing pattern or safe point. If the destination is a mission landing pattern, follow the pattern to land. |
| <a id="RTL_RETURN_ALT"></a>[RTL_RETURN_ALT](../advanced_config/parameter_reference.md#RTL_RETURN_ALT) | Return altitude in meters (default: 60m) when [RTL_CONE_ANG](../advanced_config/parameter_reference.md#RTL_CONE_ANG) is 0. If already above this value the vehicle will return at its current altitude. |
| <a id="RTL_DESCEND_ALT"></a>[RTL_DESCEND_ALT](../advanced_config/parameter_reference.md#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) |
| <a id="RTL_LAND_DELAY"></a>[RTL_LAND_DELAY](../advanced_config/parameter_reference.md#RTL_LAND_DELAY) | Time to wait 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). |
| <a id="RTL_MIN_DIST"></a>[RTL_MIN_DIST](../advanced_config/parameter_reference.md#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. |
| <a id="RTL_CONE_ANG"></a>[RTL_CONE_ANG](../advanced_config/parameter_reference.md#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). |
| <a id="COM_RC_OVERRIDE"></a>[COM_RC_OVERRIDE](../advanced_config/parameter_reference.md#COM_RC_OVERRIDE) | Controls whether stick movement on a multicopter (or VTOL in MC mode) causes a mode change to [Position mode](../flight_modes_mc/position.md) (except when vehicle is handling a critical battery failsafe). This can be separately enabled for auto modes and for offboard mode, and is enabled in auto modes by default. |
| <a id="COM_RC_STICK_OV"></a>[COM_RC_STICK_OV](../advanced_config/parameter_reference.md#COM_RC_STICK_OV) | The amount of stick movement that causes a transition to [Position mode](../flight_modes_mc/position.md) (if [COM_RC_OVERRIDE](#COM_RC_OVERRIDE) is enabled). |
| <a id="RTL_LOITER_RAD"></a>[RTL_LOITER_RAD](../advanced_config/parameter_reference.md#RTL_LOITER_RAD) | [Fixed-wing Only] The radius of the loiter circle (at [RTL_LAND_DELAY](#RTL_LAND_DELAY)). |
| <a id="MIS_TKO_LAND_REQ"></a>[MIS_TKO_LAND_REQ](../advanced_config/parameter_reference.md#MIS_TKO_LAND_REQ) | Specify whether a mission landing or takeoff pattern is _required_. Generally fixed-wing vehicles set this to require a landing pattern but VTOL do not. |
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