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Add documentation on LANDING_TARGET

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Hamish Willee
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en/SUMMARY.md
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* [Arm Authorization Protocol](protocol/arm_authorization.md)
* [Image Transmission Protocol](protocol/image_transmission.md)
* [File Transfer Protocol (FTP)](protocol/ftp.md)
* [Landing Target Protocol](protocol/landing_target.md)
* [Message Signing](guide/message_signing.md)
* [Serialization](guide/serialization.md)
* [Routing](guide/routing.md)
en/protocol/landing_target.md
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# Landing Target Protocol
The landing target protocol/message communicates the position of one or more targets from MAVLink positioning system(s) to an autopilot.
A multicopter or VTOL system can use the message to land with far greater positional accuracy than provided by conventional GPS
(GPS provides position within several meters while a landing-target system might reasonably provide centimetre-level precision landing).
Positioning system might typically consist of an onboard companion computer with a vision system that can detect a light beacon or target image.
Radio beacons and different types of visual markers and tags are also supported.
## Protocol Messages
The message used by this protocol is [LANDING_TARGET](../messages/common.md#LANDING_TARGET).
This is *broadcast* by the positioning system to indicate the position of a particular target at a particular time.
A typical broadcast rate would be (TBD).
The original MAVLink 1 message was designed with the assumption that the target is captured from a downward facing camera, and provides fields that are relative to the captured image.
MAVLink 2 extended the message to provide positional information in terms of standard frames ([MAV_FRAME](../messages/common.md#MAV_FRAME)), a quaternion and the type of landing targets ([LANDING_TARGET_TYPE](../messages/common.md#LANDING_TARGET_TYPE)).
This allows more flexibility for the types of target that can be supported, simplifies the code required by the autopilot, and allows the autopilot to control both landing position and orientation on (some) targets.
Different systems may support either (or presumably both) sets of fields.
These are discussed below.
## Target Relative to Captured Image {#camera_image_relative}
The [LANDING_TARGET](../messages/common.md#LANDING_TARGET) fields that are relative to a captured image are shown below:
Field Name | Type | Units | Values | Description
--- | --- | --- | --- | ---
angle_x | float | rad | | X-axis angular offset of the target from the center of the image
angle_y | float | rad | | Y-axis angular offset of the target from the center of the image
distance | float | m | | Distance to the target from the vehicle
size_x | float | rad | | Size of target along x-axis
size_y | float | rad | | Size of target along y-axis
The positional information can be interpreted as described below.
Imagine a ray pointing from the camera's principal point to the target.
The x angle (`angle_x`) is the angle spanned by that ray and the x-axis of the image plane. The same holds for the y angle (`angle_y`).
In other words, the x angle is a function of the x pixel coordinate of the target (denoted by *u*̅in the image below), the y angle is a function of the y pixel coordinate (denoted *v* in the image below).
Using the angle rather than *u̅/v̅* pixel coordinates has the advantage that the effect of the camera lens is already accounted for.
Otherwise the receiver of the message would need to know the camera field of view etc.
![Landing Target Fields](../../assets/protocols/landing_target_definitions.png)
The sizes in x and y direction are analogous (`size_x`/`size_y`).
They describe the angle between the smallest and biggest pixel in x/y direction respectively of the target as seen in the image.
> **Tip** ArduPilot supports messages with these fields only (at time of writing).
The MAVLink 2 extension fields are ignored.
## Target as Position/Quaternion (MAVLink 2 and later) {#positional}
The message fields that are used to providing target as a position/quaternion are shown below.
Their use is obvious.
Field Name | Type | Units | Values | Description
--- | --- | --- | --- | ---
frame | uint8_t | | [MAV_FRAME](../messages/common.md#MAV_FRAME) | Coordinate frame used for following fields.
x | float | m | | X Position of the landing target on `MAV_FRAME`
y | float | m | | Y Position of the landing target on `MAV_FRAME`
z | float | m | | Z Position of the landing target on `MAV_FRAME`
q | float[4] | | | Quaternion of landing target orientation (w, x, y, z order, zero-rotation is 1, 0, 0, 0)
type | uint8_t | | [LANDING_TARGET_TYPE](../messages/common.md#LANDING_TARGET_TYPE) | Type of landing target
position_valid | uint8_t | | | Boolean indicating known position (1) or default unknown position (0), for validation of positioning of the landing target
> **Tip** PX4 supports this form of positioning in [MAV_FRAME_LOCAL_NED](../messages/common.md#MAV_FRAME_LOCAL_NED) (only).
The original (MAVLink 1) fields are ignored.