Loiter Mode (Copter)

Executive Summary

Loiter Mode is the default GPS-assisted flight mode for ArduCopter. It automatically holds position, altitude, and heading when the sticks are centered. When the pilot moves the sticks, the vehicle accelerates up to a maximum speed. It is characterized by its "smooth" and "heavy" feel, as it resists rapid changes in direction to provide stable video.

Theory & Concepts

1. Velocity vs. Position Control

In robotics, there are two ways to hold position:

• Position Lock: "Stay at this GPS coordinate." (Stiff but can jitter).
• Velocity Lock: "Maintain 0 m/s speed." (Smooth but can drift).
• The Loiter Hybrid: ArduPilot Loiter is primarily a Velocity Controller. When you release the sticks, it calculates the braking velocity. Once stopped, it captures the current position as a "soft anchor" to prevent drift.

2. The Drift Feel

Loiter is designed for Cinematic Stabilty.

• The Math: By using an Acceleration-based input model, the drone feels heavy. It doesn't snap to a stop; it decelerates at a natural, smooth rate. This prevents the "Camera Shake" that occurs when a high-torque drone stops instantly.

Hardware Dependency Matrix

Loiter is critically dependent on a high-quality position estimate.

Sensor	Requirement	Code Implementation Notes
GPS	CRITICAL	Requires a 3D Lock and low HDOP. EKF variance issues will trigger a failsafe.
Compass	CRITICAL	Heading is required to translate "Forward" stick into "North/East" velocity. Compass interference causes "Toilet Bowling".
Barometer	REQUIRED	Used for altitude hold (Z-axis).

Control Architecture (Engineer's View)

Loiter uses the AC_Loiter library, which implements an Acceleration Controller.

1. Stick to Acceleration:
   • Pilot Stick Deflection is mapped to a target Acceleration (Lean Angle).
   • This differs from PosHold, which maps sticks to Velocity (Speed).
   • Result: The drone feels like it has "momentum." You push the stick, it leans and speeds up. You release, it leans back and slows down.
2. Velocity Integrator:
   • The requested acceleration is integrated into a target velocity.
   • This target velocity is fed into the Position Controller (AC_PosControl).
3. Braking Logic:
   • When sticks are centered, the controller enters a braking phase.
   • It calculates the distance required to stop based on LOIT_BRK_ACCEL and LOIT_BRK_JERK (smoothness).
   • It effectively generates a virtual waypoint at the stopping point and flies to it.

Pilot Interaction

• Moving: Push sticks to accelerate. The vehicle will speed up until it reaches LOIT_SPEED.
• Stopping: Release sticks. The vehicle will "coast" to a stop. The distance it coasts depends on how fast you were going and the Braking parameters.
• Repositioning: You can "nudge" the position at any time.

Failsafe Logic

• GPS Loss: If the EKF loses confidence in the position (GPS Glitch/Loss), the mode fails.
  • Reaction: It usually triggers an EKF Failsafe event, switching to Land or AltHold (which doesn't require GPS).
  • Danger: If it switches to AltHold, the wind will immediately blow it away. Be ready to take over.

Key Parameters

Parameter	Default	Description
LOIT_SPEED	1250	(cm/s) Max horizontal speed.
LOIT_ACC_MAX	500	(cm/s/s) Max acceleration. Higher = punchier.
LOIT_BRK_ACCEL	250	(cm/s/s) Deceleration rate when stopping. Higher = stops faster but jerks more.
LOIT_BRK_JERK	500	(cm/s/s/s) "Jerk" limit. How fast the braking acceleration ramps up. Lower = smoother start to braking.

Source Code Reference

• Mode Logic: ardupilot/ArduCopter/mode_loiter.cpp
• Controller Logic: AC_Loiter::update()

Practical Guide: Making Loiter Feel Like PosHold

Many FPV pilots hate Loiter because it feels "slushy" compared to the direct response of PosHold or Stabilize. You can tune Loiter to be much crisper.

The Problem: "The Drift"

By default, when you let go of the sticks, Loiter gently decelerates over 2-3 meters. This is great for filming, but bad for precision flying.

The Fix: Aggressive Braking

1. Increase LOIT_BRK_ACCEL:
   • Default: 250 cm/s/s (2.5 m/s).
   • Try: 500 - 800. This makes the drone brake twice as hard.
2. Increase LOIT_BRK_JERK:
   • Default: 500 cm/s/s/s.
   • Try: 1000 - 1500. This allows the braking force to ramp up instantly, rather than easing in.
3. Increase LOIT_ACC_MAX:
   • Default: 500.
   • Try: 700 - 900. This makes the drone accelerate faster when you push the stick.

Result

The drone will now stop almost instantly when you release the sticks, similar to a DJI drone in "Sport" mode, while still maintaining the GPS position lock safety.