ArduPilot Landing Quick Reference

ArduPilot Landing Quick Reference

TL;DR - Key Findings

✅ Rangefinder directly overrides barometer when in range
✅ Prevents powered crash into ground (Scenario 2)
⚠️ Cannot prevent early flare causing long landing (Scenario 1)
✅ 6m rangefinder is sufficient for safety

How It Works (Simple)

When rangefinder in range (≤6m):
  landing_height = rangefinder_altitude (NOT barometer!)
  
When rangefinder NOT in range (>6m):
  landing_height = barometer_altitude (can be wrong!)

The Two Scenarios

Scenario Baro Error Result Danger Level
Baro reads LOW (-20m) Thinks underground ⚠️ Flares early → LONG landing Low (needs long runway)
Baro reads HIGH (+20m) Thinks too high ❌ Continues powered descent HIGH (crash risk)

Rangefinder Protection

Landing Sequence (Simple)

  1. Approach → Descend on glide slope (use baro)
  2. Flare Trigger → At LAND_FLARE_ALT or rangefinder < flare alt
  3. Flare → Cut throttle, nose up, shallow descent
  4. Touchdown → Land at TECS_LAND_SINK rate
  5. Rollout → Maintain heading with rudder
  6. Disarm → Auto-disarm after 20 seconds

Essential Parameters

# Enable rangefinder for landing
RNGFND_LANDING = 1        # or 3 for both up/down

# Flare triggers (conservative for 6m sensor)
LAND_FLARE_ALT = 10       # meters (must be < sensor range!)
LAND_FLARE_SEC = 2.0      # seconds

# Landing behavior
TECS_LAND_SINK = 0.25     # final descent rate (m/s)
TECS_LAND_THR = 30        # approach throttle (%)
LAND_SLOPE_RCALC = 2.0    # allow slope adjustment

# Safety
LAND_DISARMDELAY = 20     # auto-disarm delay (seconds)

Mission Planning Rules

Waypoint Spacing

Pre-approach WP:  400-600m from touchdown, 40-60m altitude
Final approach WP: 200-300m from touchdown, 20-30m altitude
NAV_LAND:         0m altitude (touchdown point)

Example Mission

WP1: Pre-approach  (500m, 50m alt)
WP2: Final approach (250m, 25m alt)  ← Last WP before landing
WP3: NAV_LAND      (0m, 0m alt)      ← Touchdown point

Troubleshooting

Problem Likely Cause Solution
Lands 200m long Baro reads LOW / Early flare Long runway or pre-calibrate baro
Hard landing / crash Baro reads HIGH / Late flare Rangefinder should prevent! Check RNGFND_LANDING
Overshoots waypoints Too fast / tight spacing Space waypoints wider
Flares too late LAND_FLARE_ALT too low Increase to 10-15m
Flares too early LAND_FLARE_ALT too high Decrease to 5-8m (but riskier!)

Advanced: Dry Run Calibration (LUA Script)

For long missions (200km+) where baro drift exceeds rangefinder range:

1. Descend on approach until rangefinder triggers
2. Note rangefinder altitude (e.g., 6m)
3. Pull up to safe altitude
4. Recalibrate barometer: baro_offset = -20m (or whatever error detected)
5. Execute actual landing with corrected baro

This ensures barometer matches reality within rangefinder range.

Safety Notes

✅ DO: - Use shallow glide slopes (3-6°) - Set LAND_FLARE_ALT within rangefinder range (< 6m sensor → use 3-5m) - Plan for long runways (300-500m minimum) - Test landing at home field first - Enable RNGFND_LANDING

❌ DON’T: - Land without rangefinder on long missions - Use steep glide slopes (>10°) - Set LAND_FLARE_ALT > rangefinder range - Arm in hot vehicle after cold transport (baro drift!) - Skip testing with telemetry monitoring

Code Reference

Key files in ArduPilot source: - ArduPlane/altitude.cpp - Rangefinder correction logic - libraries/AP_Landing/AP_Landing_Slope.cpp - Flare trigger logic - ArduPlane/commands_logic.cpp - Landing verification

Formula: landing_height = baro_altitude - (baro_altitude - rangefinder_altitude)

Summary

Your 6m rangefinder provides excellent crash prevention for the dangerous scenario (baro reads high). The limitation is it cannot prevent early flare when baro reads low, resulting in long landings. Solution: use long runways or pre-landing calibration scripts.

Bottom line: The rangefinder is asymmetric protection - great at preventing crashes, limited at preventing overshoots.