RTL Mode (Copter)

Executive Summary

Return to Launch (RTL) is the primary failsafe mode. When engaged, it navigates the vehicle back to its home position (or the nearest Rally Point) at a safe altitude and automatically lands. It uses a predefined state machine to ensure obstacles are cleared before horizontal movement begins.

Theory & Concepts

1. Barrier Clearance

The #1 cause of RTL failure is the drone hitting an obstacle on the way home.

• The Logic: ArduPilot always Climbs First.
• Safety: It compares your current altitude to RTL_ALT. If you are lower, it climbs straight up before starting the horizontal trip. This ensures it clears trees, buildings, or fences.

2. The Cone of Silence

Why does the drone sometimes not climb when you trigger RTL?

• The Problem: If you are 2 meters from your landing spot, you don't want the drone to climb to 30m just to fly 2m sideways.
• The Geometry: ArduPilot builds a "Virtual Cone" above the landing point. If you are inside this cone, the RTL altitude is limited to your current height, forcing an immediate descent and land.

Hardware Dependency Matrix

RTL is an autonomous navigation mode dependent on accurate positioning.

Sensor	Requirement	Code Implementation Notes
GPS	CRITICAL	Requires a valid 3D position lock. If GPS is lost during flight, RTL cannot be engaged (the vehicle may land in place or drift).
Compass	CRITICAL	Required for heading control during the return path.
Barometer	REQUIRED	Primary source for altitude relative to Home.
Terrain Data	OPTIONAL	Required if RTL_ALT_TYPE is set to "Terrain" to climb over hills.

Control Architecture (Engineer's View)

RTL operates as a rigid State Machine defined in ModeRTL::run().

1. Stage 1: Initial Climb (INITIAL_CLIMB)
   • The vehicle stops horizontal movement.
   • It climbs to RTL_ALT.
   • Optimization: If already above RTL_ALT, it maintains current altitude.
   • Cone Logic: If close to home, RTL_CONE_SLOPE limits the climb to prevent shooting up just to come right back down.
2. Stage 2: Return Home (RETURN_HOME)
   • The vehicle flies a straight line to the Home coordinates.
   • Speed is defined by WPNAV_SPEED.
   • Nose orientation is controlled by WP_YAW_BEHAVIOR (usually points to home).
3. Stage 3: Loiter at Home (LOITER_AT_HOME)
   • Once over home, it pauses for RTL_LOIT_TIME (ms). This gives the pilot a chance to visually acquire the drone before landing.
4. Stage 4: Final Descent / Landing (FINAL_DESCENT / LAND)
   • It descends to RTL_ALT_FINAL.
   • If RTL_ALT_FINAL is 0 (default), it proceeds to land and disarm.

Terrain Following

RTL can be configured to follow terrain using RTL_ALT_TYPE.

• 0 (Relative): Altitude is relative to Home. (Simple geometry).
• 1 (Terrain): Altitude is relative to the ground beneath the drone.
  • Requirement: Needs a Rangefinder OR a valid Terrain Database on the flight controller.
  • Calculation: compute_return_target() recalculates the target altitude dynamically.

Failsafe Logic

• GPS Glitch: If GPS variance becomes too high during the return leg, the vehicle may switch to Land mode immediately to prevent flyaways.
• Rally Points: If Rally Points are configured, RTL will choose the closest Rally Point instead of Home.

Key Parameters

Parameter	Default	Description
RTL_ALT	1500	(cm) Return altitude (15m).
RTL_ALT_FINAL	0	(cm) Altitude to hover at after returning. 0 = Land immediately.
RTL_LOIT_TIME	5000	(ms) Time to hover over home before landing.
RTL_CONE_SLOPE	0.5	Defines the slope of the "virtual cone" above home. High values force a steeper climb even when close to home.
RTL_SPEED	0	(cm/s) Speed for the return leg. 0 = Use WPNAV_SPEED.

Source Code Reference

• Mode Logic: ardupilot/ArduCopter/mode_rtl.cpp
• Target Calculation: ModeRTL::compute_return_target()

Practical Guide: Configuring RTL Safety

RTL is your insurance policy. Don't trust the defaults.

Step 1: The Return Altitude

Set RTL_ALT to be higher than the tallest obstacle in your flight area (trees/buildings) plus a 10m buffer.

• Example: If trees are 20m high, set RTL_ALT = 3000 (30m).
• Warning: Do not set this higher than 120m (400ft) to avoid violating airspace rules.

Step 2: The Cone of Silence

If you fly in tight spaces (like a backyard), the default "Cone" might force the drone to climb into a tree canopy when you just wanted it to land.

• The Fix: Increase RTL_CONE_SLOPE.
• Logic: A steeper slope means the drone enters the cone sooner.
• Value: Try 3.0. This effectively disables the cone for short distances, forcing the drone to climb to RTL_ALT almost immediately, which is safer if you have obstacles nearby but clear sky above.

Step 3: The "Wait"

Always set RTL_LOIT_TIME to at least 5000 (5 seconds).

• Why? This gives you time to look up, spot the drone overhead, and verify it is descending on the correct spot. If it's drifting (bad GPS), you have 5 seconds to switch to Stabilize and save it.