ArduPilot Plane Automatic Landing with Rangefinder

Understanding Barometer Drift Problem

During flight, the barometer altitude can drift due to: - Temperature changes inside the vehicle (battery heat, video transmitter heat, etc.) - Atmospheric pressure changes during flight - Temperature differences between takeoff location and flying site

Result: The barometer can report altitude errors of ±5 to ±20 meters (or more) on long flights, even though most barometers are temperature compensated.

How Rangefinder Corrects Altitude During Landing

The Key Formula

When the rangefinder is in range (typically ≤6-12m for cheap sensors), the autopilot calculates:

correction = baro_altitude - rangefinder_altitude
landing_height = baro_altitude - correction
               = baro_altitude - (baro_altitude - rangefinder_altitude)
               = rangefinder_altitude  ✓

Once rangefinder is in range, it directly overrides the barometer for landing altitude!

Code Evidence

From ArduPlane/altitude.cpp:

float Plane::get_landing_height(bool &rangefinder_active)
{
    // get basic height above target
    height = height_above_target();  // Barometer-based
    
    // possibly correct with rangefinder
    height -= rangefinder_correction();  // Subtract correction
    rangefinder_active = rangefinder_state.in_range;
    
    return height;  // = rangefinder altitude when in range!
}

Landing Sequence Order

1. Approach Phase

• Navigate to pre-approach waypoint
• Navigate to final approach waypoint (last WP before NAV_LAND)
• Descend on glide slope at TECS_LAND_THR throttle
• Use barometer for altitude (rangefinder not in range yet)

2. Flare Trigger (first condition met)

• Time-based: LAND_FLARE_SEC seconds before ground impact
• Altitude-based: LAND_FLARE_ALT meters above ground
• Rangefinder correction active if within sensor range

3. Flare Execution

• Throttle cuts to between THR_MIN and zero
• At TECS_FLARE_HGT: Transition sink rate to TECS_LAND_SINK
• Roll limited to LEVEL_ROLL_LIMIT (default 5°)
• Navigate along extended line through landing point

4. Final Phase

• Maintain TECS_LAND_SINK descent rate (default 0.25 m/s)
• Pitch controlled to minimum LAND_PITCH_DEG
• Rangefinder fully active (provides accurate ground distance)

5. Touchdown and Rollout

• Below GROUND_STEER_ALT: Rudder maintains heading
• After LAND_DISARMDELAY seconds (default 20s): Auto-disarm

Two Critical Scenarios with 6m Rangefinder

Scenario 1: Baro Drift -20m (reads LOW)

Situation: - Actual altitude at ground level: 0m - Barometer reports: -20m (thinks it’s 20m underground)

What Happens:

Result: - ❌ Flare starts 20m too high (at 30m instead of 10m) - ✅ Rangefinder prevents crash after 6m detection - ⚠️ Landing is LONG (overshoots touchdown point by ~100-200m) - ✅ Safe if runway is long enough - ❌ Cannot fix on first approach (baro error > rangefinder range)

Solution: Use the “dry run calibration” LUA script: 1. Descend until rangefinder triggers 2. Pull up immediately 3. Recalibrate baro to known rangefinder altitude 4. Execute actual landing with corrected baro

Scenario 2: Baro Drift +20m (reads HIGH)

Situation: - Actual altitude at ground level: 0m - Barometer reports: +20m (thinks it’s 20m high)

What Happens:

Result: - ⚠️ Plane continues powered descent past intended flare altitude - ✅ Rangefinder detects ground at 6m actual altitude - ✅ Flare triggers immediately based on rangefinder - ✅ Prevents powered crash into ground! - ✅ Landing point is approximately correct (maybe slightly short)

Safety Mechanism from Code:

// AP_Landing_Slope.cpp line 103
if ((on_approach_stage && below_flare_alt) ||
    (on_approach_stage && below_flare_sec) ||
    (!rangefinder_state_in_range && wp_proportion >= 1) ||  // Safety catch!
    probably_crashed)

Even if rangefinder fails, flare triggers when plane passes the landing point.

Key Insights

Rangefinder Limitations

With a 6m range rangefinder: - ❌ Cannot prevent early flare (when you’re too high to detect ground) - ✅ CAN prevent late/no flare (detects ground and forces correction)

Critical Parameters for Your Setup

RNGFND_LANDING = 1 or 3     # Enable rangefinder for landing
LAND_FLARE_ALT = 10         # Flare altitude (meters) - conservative value
LAND_FLARE_SEC = 2.0        # Flare time (seconds)
TECS_LAND_SINK = 0.25       # Final descent rate (m/s)
LAND_SLOPE_RCALC = 2.0      # Allow slope recalc for rangefinder bumps

Best Practices

1. Use shallow glide slopes (3-6 degrees) to minimize overshoot in Scenario 1
2. Space waypoints adequately to allow turns to complete before final approach
3. Set conservative LAND_FLARE_ALT (10-15m) to ensure rangefinder engagement
4. Ensure long runway to handle potential overshoot from early flare
5. Calibrate barometer just before landing if possible (arm at ground level)
6. Consider dry-run calibration script for long-range missions

Why Documentation Emphasizes Shallow Slopes

The documentation recommends shallow glide slopes and conservative flare altitudes because: - Minimizes overshoot distance in Scenario 1 (early flare) - Gives more time for rangefinder to engage and correct - Reduces approach speed and energy at touchdown - Makes landing more survivable if things go wrong

Conclusion

Your cheap 6m rangefinder is highly effective at preventing the dangerous Scenario 2 (powered descent into ground). For Scenario 1, you need either: - A long runway to handle overshoot, OR - A pre-landing baro calibration routine (custom LUA script)

The rangefinder’s limited range makes it asymmetric in protection: excellent crash prevention, limited overshoot prevention.