inav/docs/Rangefinder.md

Rangefinder

Rangefinders are devices used to measure altitude Above Ground Level (AGL), the distance to the ground directly beneath the aircraft.

Features

Rangefinders in INAV are used for:

• Landing detection for multirotors - precise touchdown detection
• Automated landing support for fixed wings - smooth descent control
• Terrain following (Surface mode) - maintain constant height above ground (multirotors only)
• Position estimation - when combined with optical flow sensors, enables GPS-free navigation

Hardware

INAV supports the following rangefinder types:

Time-of-Flight (ToF) Laser Sensors

• VL53L0X - STMicroelectronics laser rangefinder, 0-75cm range (I2C)
• VL53L1X - STMicroelectronics laser rangefinder, 0-400cm range (I2C)
• TOF10120 - Small & lightweight laser sensor, 0-200cm range (I2C)
• TERARANGER_EVO - TeraRanger Evo series, 30-600cm range depending on model (I2C/UART)
  • https://www.terabee.com/sensors-modules/lidar-tof-range-finders/#individual-distance-measurement-sensors

Ultrasonic Sensors

• SRF10 - Devantech SRF10, 0-600cm range (I2C)
• US42 - Maxbotix US-42, 0-645cm range (I2C)
• USD1_V0 - USD1 ultrasonic sensor variant (UART)

Radar Sensors

• NRA - NanoRadar NRA15/NRA24 millimeter-wave radar (UART)

NRA15/NRA24 sensors can use US-D1_V0 or NRA protocol depending on firmware configuration:

Radar	Protocol	Resolution	Name in configurator
NRA15	US-D1_V0	0-30m (±4cm)	USD1_V0
NRA15	NRA	0-30m (±4cm)	NRA
NRA15	NRA	0-100m (±10cm)	NRA
NRA24	US-D1_V0	0-50m (±4cm)	USD1_V0
NRA24	US-D1_V0	0-200m (±10cm)	USD1_V0
NRA24	NRA	0-50m (±4cm)	NRA
NRA24	NRA	0-200m (±10cm)	NRA

Serial/External Sensors

• MSP - External rangefinder via MSP protocol (UART/MSP)
• BENEWAKE - Benewake LIDAR modules (TFmini, etc.) via serial protocol (UART)

Development/Testing

• FAKE - Simulated rangefinder for testing without hardware

Configuration

Enable rangefinder in CLI:

set rangefinder_hardware = VL53L0X  # or your sensor type
save

Optional median filtering for smoother readings:

set rangefinder_median_filter = ON
save

Position Estimation Weights

Control how rangefinder data is fused into altitude estimates:

set inav_max_surface_altitude = 200      # Max rangefinder range to use (cm)
set inav_w_z_surface_p = 3.5             # Position weight
set inav_w_z_surface_v = 6.1             # Velocity weight

Surface Mode (Terrain Following)

Surface mode enables terrain following on multirotors only by maintaining constant altitude above ground instead of absolute altitude.

Activation: Enable SURFACE mode via RC switch (requires rangefinder configured)

Compatible with:

• ALTHOLD - Maintains height above ground
• POSHOLD - Holds position with terrain-relative altitude
• CRUISE - Velocity control with terrain following

Settings:

set nav_max_terrain_follow_alt = 100    # Max altitude in Surface mode (cm)

Important: Always set nav_max_terrain_follow_alt less than inav_max_surface_altitude to prevent attempting to use the sensor beyond its reliable range. For example, if your rangefinder is reliable up to 200cm, set inav_max_surface_altitude = 200 and nav_max_terrain_follow_alt = 100 (or lower).

Note: Surface mode is NOT available on fixed wing aircraft.

Connections

I2C Rangefinders

I2C sensors (VL53L0X, VL53L1X, TOF10120, SRF10, US42, TERARANGER_EVO) connect to the flight controller's I2C port and are auto-detected when configured.

Serial Rangefinders

UART-based sensors (MSP, BENEWAKE, NRA, USD1_V0) require:

1. Assign UART port in Ports tab
2. Configure rangefinder_hardware setting
3. Set appropriate baud rate

Optical Flow Integration

When a rangefinder is combined with an optical flow sensor:

• Rangefinder provides altitude (Z-axis)
• Optical flow provides horizontal movement (X/Y axes)
• Together they enable GPS-free position hold indoors

See wiki for optical flow setup: https://github.com/iNavFlight/inav/wiki/Optic-Flow-and-Rangefinder

Constraints

Not Supported:

• HC-SR04 ultrasonic sensors
• US-100 ultrasonic sensors

Limitations:

• Surface mode: Multirotors only
• Tilt angle: Rangefinder disabled if aircraft tilt exceeds sensor detection cone
• Range: Set inav_max_surface_altitude to sensor's reliable range, not maximum range

Troubleshooting

No rangefinder readings:

• Verify correct rangefinder_hardware selection
• Check wiring and power (3.3V or 5V depending on sensor)
• Ensure I2C address is unique (no conflicts)
• Check sensor has clear line of sight to ground

Erratic readings:

• Enable rangefinder_median_filter = ON
• Check for vibration - mount sensor rigidly
• Verify sensor is not affected by propeller wash
• Ensure ground surface has sufficient texture (not smooth/reflective)

Surface mode not working:

• Verify SURFACE mode is enabled via switch
• Check rangefinder is providing valid data
• Ensure altitude is below nav_max_terrain_follow_alt (operational limit)
• Ensure altitude is within sensor's reliable range (set by inav_max_surface_altitude)
• Confirm platform is multirotor (fixed wings not supported)

References

• INAV Wiki: Optic Flow and Rangefinder Setup
• Settings Reference