返航模式

The Return flight mode is used to fly a vehicle to safety on an unobstructed path to a safe destination, where it should land.

PX4 提供了几种机制来选择安全的返航路径，返航目的地和着陆，包括使用其实位置，集结（“安全”）点，任务路径和任务着陆顺序。

• Multicopter
• Fixed-wing (Plane)
• 垂直起降

:::note

• 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).
  • 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).
• 遥控开关可以在任何无人机上更改飞行模式。
• RC stick movement in a multicopter (or VTOL in multicopter mode) will by default change the vehicle to Position mode 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.

:::

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, minimum return altitude and landing/arrival behaviour.

返航类型（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 parameter.

At high level these are:

• 返航到起始位置/集结点（RTL_TYPE=0）: 上升到安全高度并通过直接路径返回到最近的集结点或起始地点。
• 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. 如果未定义任务着陆点或集结点，请通过直接路径返回起始位置。
• 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. 如果目的地是飞行任务着陆模式，则按照该模式降落。

以下各节提供了每种类型的更详细说明。

起始位置/集结点返航类型（RTL_TYPE=0）

This is the default return type for a multicopter (see topic for more information).

无人机在该返航类型中：

• 爬升到一个安全的返航高度（高于任何可预期的障碍物）。
• 通过直接路径飞往起始位置或集结点（以最近者为准）。
• On arrival 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.

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

任务着陆/集结点返航类型 (RTL_TYPE=1)

This is the default return type for a fixed-wing or VTOL vehicle (see topics for more information).

无人机在该返航类型中：

• Ascends to a safe 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 (whichever is closest). 如果未定义任务降落或集结点，无人机通过直接路径返回到起始位置。
• 如果目的地是飞行任务着陆模式，则按照该模式降落。
• If the destination is a rally point or home it will land or wait 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.

Fixed wing vehicles commonly also set MIS_TKO_LAND_REQ to require a mission landing pattern.

任务路径返航类型（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. 集结点，如果有的话，将被忽略。

:::note 该行为相当复杂，因为它取决于飞行模式以及是否定义了任务和任务着陆。 :::

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.
• 任务模式以外的自动模式：
  • Ascend to a safe minimum return altitude above any expected obstacles.
  • 直接飞到最近的航点（对固定翼而言，不是着陆航点），然后降落到航点高度。
  • 从该航点以快速模式继续执行任务。
• 手动模式:
  • Ascend to a safe minimum return altitude above any expected obstacles.
  • 直接飞到降落序列位置并下降到航点高度。
  • 使用任务降落模式着陆。

Mission without landing pattern defined:

• 任务模式:
  • 从上一个航点开始以“快退”（反向）飞行的任务
    • 跳，延迟和其他任何非定位命令都会被忽略，悬停和其他位置航点将转换为简单航点。
    • VTOL 无人机在反向飞行任务之前切换到固定翼模式（如果需要）。
  • On reaching waypoint 1, the vehicle ascends to the minimum return altitude and flies to the home position (where it lands or waits).
• 任务模式以外的自动模式：
  • 直接飞到最近的航点（对固定翼而言，不是着陆航点），然后降落到航点高度。
  • 反向执行任务，就像在任务模式中触发返航模式一样（上图）。
• Manual modes: Fly directly to home location and land.

如果未定义任务，PX4 将直接飞回起始位置并着陆（集结点将被忽略）。

如果任务在返航模式期间发生更改，则将按照与上述相同的规则根据新任务重新评估行为（例如，如果新任务没有降落顺序并且你在一个任务中，则任务将被逆转）。

最近的安全目的地返回类型（RTL_TYPE=3）

无人机在该返航类型中：

• Ascends to a safe minimum return altitude (above any expected obstacles).
• 飞到最近目的地的直接路径：起始位置，任务着陆模式或集结点。
• 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 lands or waits. 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.

悬停/降落在目的地

For most 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.

The exception is when executing a mission path return 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 (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 and RTL_CONE_ANG, which define a half cone centered around the destination (home location or safety point).

如果无人机是：

• 高于RTL_RETURN_ALT (1)，无人机将在当前高度返航。
• 在圆锥下方，它将返回与圆锥（2）或 RTL_DESCEND_ALT （以较高者为准）相交的位置。
• 在圆锥（3）之外，它将首先爬升，直到达到 RTL_RETURN_ALT 。
• 在圆锥内：
  • 高于RTL_DESCEND_ALT (4)，无人机将在当前高度返航。
  • 低于 RTL_DESCEND_ALT (5)，它将会先爬升到 RTL_DESCEND_ALT的高度。

注意：

• 如果 RTL_CONE_ANG 为 0 度，则没有 "圆锥"：
  • 无人机在RTL_RETURN_ALT的高度或者（或以上）返航。
• 如果 RTL_CONE_ANG 是90度，无人机将更大程度在 RTL_DESCEND_ALT 的高度和当前高度返航。
• 无人机总是会爬升到至少RTL_DESCEND_ALT的高度返航。

无人机默认行为

Unless executing a mission landing pattern as part of the return mode, the vehicle will arrive at its destination, and rapidly descend to the RTL_DESCEND_ALT altitude, where it will loiter for RTL_LAND_DELAY before landing. If RTL_LAND_DELAY=-1 it will loiter indefinitely.

The default landing configuration is vehicle dependent:

• 如果定义了任务降落，直接飞往任务降落起点，然后着陆。
• Fixed-wing vehicles use a return mode with a 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, one or more position waypoints, and a MAV_CMD_NAV_LAND (or 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 by default.

参数

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

参数	描述
RTL_TYPE	返航机制（路径和目的地）。 0：通过直接路径返回到集结点或起始点（以最近者为准）。 1：返回到集结点或任务着陆模式的起点（以最近者为准），通过直接路径。 如果未定义任务着陆点或集结点，通过直接路径返回起始位置。 如果目的地是任务着陆模式，则按照该模式着陆。 2：如果定义了着陆模式，则使用任务路径快速着陆，否则快速返回起始位置。 忽略集结点。 如果没有定义任务规划，直接飞往起始位置。 3：通过直接路径返航到最近目的地：起始位置，任务降落起始点或者安全点。 如果目的地是飞行任务着陆模式，则按照该模式降落。
RTL_RETURN_ALT	当RTL_CONE_ANG为 0 时，返航高度以米为单位（默认：60 米）。 如果已经超过这个值, 飞机将返回当前的高度。
RTL_DESCEND_ALT	最小返航高度和无人机从较高的返航高度到减速或者停止的初始下降高度（默认： 30米）。
RTL_LAND_DELAY	着陆前在RTL_DESCEND_ALT高度悬停时间（默认： 0.5秒）- 默认情况下这个段时间很短，因此无人机智慧减速然后立即着陆。 如果设置为-1，系统将在 RTL_DESCEND_ALT 高度悬停而不是降落。 延迟能够使你为起落架的展开部署配置时间（自动触发）。
RTL_MIN_DIST	能够触发无人机上升到返航高度，距离起始位置的最小水平距离由那个"锥形"指定。 如果无人机在水平方向比这个相对于起始位置的距离更近，它将在当前高度或者在RTL_DESCEND_ALT高度（以较高者为准）返航，而不是先上升到RTL_RETURN_ALT）。
RTL_CONE_ANG	圆锥半角决定无人机的 RTL 返航高度。 数值(度数)：0、25、45、65、80、90。 请注意，0 为“无圆锥”（始终返回RTL_RETURN_ALT或更高），而 90 则表示无人机必须在当前高度或在RTL_DESCEND_ALT高度（以较高者为准）返航。
COM_RC_OVERRIDE	Controls whether stick movement on a multicopter (or VTOL in MC mode) causes a mode change to Position mode (except when vehicle is handling a critical battery failsafe). 可以分别为自动模式和 offboard 模式启用此功能，默认情况下在自动模式下启用此功能。
COM_RC_STICK_OV	The amount of stick movement that causes a transition to Position mode (if COM_RC_OVERRIDE is enabled).
NAV_LOITER_RAD	[Fixed-wing Only] The radius of the loiter circle (at RTL_LAND_DELAY).
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.