Optical Flow and Rangefinder

This guide explains how to use optical flow sensors and rangefinders (LIDAR/SONAR) with INAV and to enable terrain following and GPS-free position hold.

Overview

Rangefinders (LIDAR/SONAR) measure altitude above ground level (AGL), while optical flow sensors track horizontal movement by analyzing the ground below. When used together, they enable:

• Terrain following - Maintain constant height above ground using Surface mode
• GPS-free position hold - Hold position indoors or in GPS-denied areas (multirotor only)
• Improved landing detection - More accurate landing for multirotors and fixed wings

note

Optic flow and rangefinders are most commonly used with Surface mode, which modifies altitude-controlled flight modes to follow terrain contours instead of maintaining absolute altitude.

---

Supported Hardware

Rangefinder (LIDAR/SONAR) Sensors

Type	Model	Range	Connection	Notes
Laser (ToF)	VL53L0X	0-75cm	I2C	Most common, inexpensive
Laser (ToF)	VL53L1X	0-400cm	I2C	Extended range version
Laser (ToF)	TOF10120	0-200cm	I2C	Compact and lightweight
Laser (ToF)	TeraRanger Evo	30-600cm	I2C/UART	Professional grade, various models
Ultrasonic	SRF10	0-600cm	I2C	Legacy support
Ultrasonic	US42	0-645cm	I2C	Maxbotix sensor
Radar	NanoRadar NRA15/NRA24	0-200m	UART	Long range, experimental
Serial/MSP	MSP Protocol	Varies	UART/MSP	External sensors via companion computer
Serial	Benewake (TFmini, etc.)	Varies	UART	Popular LIDAR modules

Not Supported: HC-SR04 and US-100 ultrasonic sensors are NOT supported.

Optical Flow Sensors

Type	Model	Features	Connection	Notes
PixArt	PMW3901-based (CXOF)	Motion tracking chip	SPI/I2C	Common in flight controllers
MSP Protocol	External via companion	Custom solutions	UART/MSP	Requires external processor

---

Setup Guide

1. Hardware Installation

Rangefinder Mounting:

• Mount facing straight down from the aircraft
• Keep away from propeller wash turbulence
• Clear field of view to ground (no obstructions)
• Typical mounting: bottom center of frame

Optical Flow Mounting:

• Mount facing straight down
• Should be level with rangefinder
• Needs clear view of ground texture
• Works best 10cm-3m above ground

2. Configuration

Enable Rangefinder

In INAV Configurator → Configuration Tab:

1. Set rangefinder_hardware to your sensor type (e.g., VL53L0X)
2. Save and reboot

CLI Settings:

# Select your rangefinder hardware
set rangefinder_hardware = VL53L0X

# Optional: Enable median filtering for smoother readings
set rangefinder_median_filter = ON

# Maximum altitude for rangefinder use (in cm, default 200cm = 2m)
set inav_max_surface_altitude = 200

# Rangefinder weight in altitude estimation (default 3.5)
set inav_w_z_surface_p = 3.5

# Rangefinder weight in climb rate estimation (default 6.1)
set inav_w_z_surface_v = 6.1

# Save settings
save

Enable Optical Flow (Optional)

CLI Settings:

# Select your optic flow hardware
set opflow_hardware = CXOF

# Scale factor for your specific module (default 10.5)
set opflow_scale = 10.5

# Alignment (default CW0FLIP - depends on mounting)
set align_opflow = CW0FLIP

# Optic flow weight in horizontal position estimation
set inav_w_xy_flow_p = 1.0

# Optic flow weight in horizontal velocity estimation
set inav_w_xy_flow_v = 2.0

# Save settings
save

3. Calibration

Rangefinder Calibration

Rangefinders typically require no calibration. Verify operation:

1. Connect to INAV Configurator
2. Observe sensor readings in the Sensors tab
3. Move aircraft up/down to verify readings change appropriately

Optical Flow Calibration

In INAV Configurator → Calibration Tab:

1. Connect your aircraft to INAV Configurator
2. Go to the Calibration Tab
3. Place aircraft on a flat, stable surface
4. Click "Calibrate Optic Flow"
5. Aircraft must remain stationary for ~30 seconds during auto-calibration
6. Verification: Settings accgain_x, accgain_y, accgain_z should NOT be 4096

Verification (CLI): After calibration, verify these settings have changed from defaults:

• opflow_scale should be adjusted from the default 10.5
• Hardware-specific calibration values should be stored

If calibration fails or optic flow isn't working correctly, see the Optic Flow and Rangefinder Troubleshooting section.

---

Using Surface Mode

Surface mode enables terrain following - which is where the aircraft maintains a constant altitude above ground instead of absolute altitude. This is the primary use case for rangefinders.

Platform Support

• ✅ Multirotors: Full support with all altitude-holding modes
• ❌ Fixed Wing: Surface mode NOT available (barometric altitude only)

Compatible Flight Modes (Multirotors)

Surface mode can be combined with these altitude-controlled modes:

Mode	Without SURFACE	With SURFACE Enabled
ALTHOLD	Maintains absolute altitude (barometric)	Maintains height above ground (terrain following)
POSHOLD	Holds 3D position with absolute altitude	Holds 2D position with terrain-relative altitude
CRUISE	Velocity control with absolute altitude	Velocity control with terrain-relative altitude

Throttle Stick Behavior in Surface Mode

When Surface mode is active:

• Throttle stick controls altitude above ground (AGL)
• Stick range: 0 to nav_max_terrain_follow_alt (default 100cm)
• Example: With default settings, full throttle = 1 meter above ground
• Mid-throttle = ~50cm above ground

Important: Adjust nav_max_terrain_follow_alt based on your rangefinder's reliable range.

Enabling Surface Mode

1. Go to Modes tab in configurator
2. Add SURFACE mode
3. Assign to an AUX channel switch
4. Save

Recommended Setup:

# Set maximum terrain following altitude (in cm)
# Should be less than your rangefinder's max reliable range
set nav_max_terrain_follow_alt = 100

# Save
save

Surface Mode Configuration

Key Settings:

Setting	Default	Range	Description
nav_max_terrain_follow_alt	100cm	0-1000cm	Maximum altitude above ground in Surface mode
inav_max_surface_altitude	200cm	0-1000cm	Maximum rangefinder range to use for surface tracking
inav_w_z_surface_p	3.5	0-100	Weight of rangefinder in altitude estimation (higher = more responsive)
inav_w_z_surface_v	6.1	0-100	Weight of rangefinder in climb rate estimation

Tuning Tips:

• inav_w_z_surface_p too high: Oscillation, sensitive to rangefinder noise
• inav_w_z_surface_p too low: Slow response to terrain changes
• Start with defaults and adjust gradually

---

GPS Free Position Hold with Optical Flow

Prerequisites:

1. Rangefinder installed and working
2. Optical flow sensor installed and calibrated
3. Both sensors enabled in configuration

How It Works:

• Optical flow detects horizontal movement relative to ground
• Rangefinder provides altitude for flow calculations
• Together they estimate X/Y position and velocity
• POSHOLD mode uses this to maintain position without GPS

Limitations:

• Works best 10cm-3m above ground
• Requires textured ground surface (grass, carpet work well; smooth floors don't)
• Position estimate drifts over time (no absolute reference)
• GPS will override flow if available

Configuration:

# Enable optic flow for position estimation
set opflow_hardware = CXOF

# Flow weight settings (adjust for your setup)
set inav_w_xy_flow_p = 1.0   # Position weight
set inav_w_xy_flow_v = 2.0   # Velocity weight

# Enable rangefinder for altitude reference
set rangefinder_hardware = VL53L0X
set inav_max_surface_altitude = 300  # 3m max range

save

---

Troubleshooting

Rangefinder Issues

Problem: No rangefinder readings in configurator

• Check I2C/UART connection and wiring
• Verify correct rangefinder_hardware selection
• Ensure sensor has power (3.3V or 5V depending on model)
• Check for conflicting I2C devices (same address)

Problem: Erratic altitude readings

• Enable rangefinder_median_filter = ON
• Check for propeller wash affecting sensor
• Ensure sensor is mounted rigidly (no vibration)
• Verify clear line of sight to ground

Problem: Surface mode not working

• Verify rangefinder readings are valid in Sensors tab
• Check inav_max_surface_altitude is appropriate for your sensor
• Ensure aircraft is within rangefinder range
• Multirotor only - fixed wings don't support Surface mode

Optical Flow Issues

Problem: No optic flow data

• Verify SPI/I2C connection
• Check opflow_hardware setting matches your sensor
• Ensure sensor has clear view of ground
• Check alignment setting align_opflow

Problem: Position drift in GPS-free POSHOLD

• Calibrate optical flow sensor
• Verify rangefinder is working (flow needs altitude reference)
• Ensure ground has visible texture (patterns, grass, etc.)
• Flow works poorly over smooth/uniform surfaces
• Check opflow_scale calibration

Problem: "Toilet bowling" or circling

• This usually indicates compass calibration issues, not optic flow
• Recalibrate compass following Compass calibration
• Ensure compass is away from magnetic interference

---

Safety Warnings

⚠️ Important Safety Information:

1. Test altitude limits: Always verify your rangefinder's reliable range before flying
2. Failsafe behavior: If rangefinder signal is lost, aircraft will slowly descend
3. Terrain clearance: Set nav_max_terrain_follow_alt conservatively (under rangefinder max range)
4. Environmental factors:
   • Rangefinders may give false readings over water or reflective surfaces
   • Optic flow requires textured ground surface
   • Very low or very high altitude reduces accuracy
5. Fixed wing limitation: Surface mode is NOT available for fixed wings
6. Indoor flight: GPS-free position hold requires good lighting for optical flow

---

Advanced: Position Estimator Weights

INAV uses a sensor fusion algorithm that combines multiple sensors. You can adjust how much each sensor influences the position estimate.

Surface/Rangefinder Weights:

set inav_w_z_surface_p = 3.5    # Altitude position weight
set inav_w_z_surface_v = 6.1    # Altitude velocity weight

Optical Flow Weights:

set inav_w_xy_flow_p = 1.0      # Horizontal position weight
set inav_w_xy_flow_v = 2.0      # Horizontal velocity weight

General Principle:

• Higher weight = more influence from that sensor
• Lower weight = less influence, more from other sensors (GPS, barometer, etc.)
• Increase weights if sensor is accurate and reliable
• Decrease weights if sensor is noisy or unreliable

---

Examples and Use Cases

Example 1: Basic Terrain Following (Multirotor)

Goal: Fly over uneven terrain maintaining constant ground clearance

Setup:

1. Install VL53L0X rangefinder (I2C)
2. Configure: set rangefinder_hardware = VL53L0X
3. Set terrain follow limit: set nav_max_terrain_follow_alt = 100
4. Enable Surface mode on a switch

Flight:

1. Arm in ANGLE mode
2. Enable ALTHOLD mode
3. Toggle Surface mode ON
4. Throttle stick now controls height above ground (0-100cm)
5. Fly over terrain - aircraft follows ground contours

Example 2: Indoor GPS-Free Position Hold

Goal: Stable hover indoors without GPS

Setup:

1. Install VL53L0X rangefinder + CXOF optical flow sensor
2. Configure both sensors in CLI
3. Enable Surface mode + POSHOLD mode switches

Flight:

1. Find area with textured floor/ground
2. Arm and hover in ANGLE mode
3. Enable POSHOLD mode
4. Enable Surface mode
5. Release sticks - aircraft holds position using optic flow
6. Note: Position will drift slowly over time (no GPS reference)

Example 3: Automated Landing with Rangefinder

Goal: Smooth automated landing for multirotor

Setup:

1. Install rangefinder
2. Configure: set rangefinder_hardware = VL53L0X
3. Set landing detection: set nav_disarm_on_landing = ON

Behavior:

• RTH landing uses rangefinder for precise altitude control
• Landing detection activates when rangefinder shows ground contact
• Automatic disarm on touchdown

---

Related Documentation

• Navigation PID tuning MC - More Rangefinder and Optical flow related information
• Modes - All flight modes including SURFACE
• Navigation Modes - ALTHOLD, POSHOLD, CRUISE
• Settings - Complete CLI settings reference

---

Frequently Asked Questions

Q: Can I use Surface mode with fixed wings? A: No, Surface mode is only available for multirotors. Fixed wings use barometric / GPS altitude only.

Q: Do I need GPS if I have optic flow? A: No, but optic flow position estimates drift over time without an absolute reference. GPS provides that reference for long-term accuracy. You DO need a compass for yaw / heading measurement.

Q: What's the maximum rangefinder range I should use? A: Set inav_max_surface_altitude to your sensor's reliable range, not maximum range. For VL53L0X, use 50-60cm even though it can read up to 75cm under ideal conditions (white tile floor, indoors).

Q: Can I combine Surface mode with waypoint missions? A: Surface mode is designed for manual flight modes (ALTHOLD, POSHOLD, CRUISE). Waypoint missions use absolute altitude from barometer/GPS.

Q: My rangefinder readings jump around - how do I fix this? A: Enable median filtering: set rangefinder_median_filter = ON. Also check for vibration and ensure sensor has clear view to ground.

---

Technical Details

For developers and advanced users, see the technical implementation documentation:

• Rangefinder Hardware Support
• Source code: src/main/sensors/rangefinder.c and src/main/sensors/opflow.c
• Position estimator: src/main/navigation/navigation_pos_estimator_*.c

---