simulation/docs/gps_denied_navigation.md

GPS-Denied Navigation

How the system navigates without GPS.

Principle

All navigation uses relative positioning from visual sensors. GPS is only used for geofencing (safety boundaries).

Function	GPS Used?
Position estimation	No - visual odometry
Waypoint navigation	No - local coordinates
Velocity control	No - optical flow
Geofencing	Yes - safety only

Position Estimation

Visual Odometry

1. Detect features in camera image (ORB, SIFT)
2. Match features between consecutive frames
3. Estimate camera motion from feature displacement
4. Accumulate motion into position estimate

Optical Flow

1. Capture ground images from downward camera
2. Measure pixel displacement between frames
3. Convert to velocity using altitude
4. Integrate for position

Sensor Fusion

Extended Kalman Filter combines:

• Visual odometry (position)
• Optical flow (velocity)
• IMU (acceleration, rotation)
• Barometer (altitude)

Output: Full 6-DOF pose estimate

ArduPilot Configuration

Key parameters for GPS-denied operation:

# EKF Source Configuration
EK3_SRC1_POSXY = 6    # External Nav for position
EK3_SRC1_VELXY = 6    # External Nav for velocity
EK3_SRC1_POSZ = 1     # Barometer for altitude

# Disable GPS for navigation
GPS_TYPE = 0          # No GPS (or keep for geofence)

# Enable external navigation
VISO_TYPE = 1         # Enable visual odometry input

# Arming checks
ARMING_CHECK = 0      # Disable pre-arm checks (for testing)

See config/ardupilot_gps_denied.parm for complete parameters.

Sending Position to ArduPilot

Visual odometry sends position via MAVLink:

# VISION_POSITION_ESTIMATE message
msg = mavutil.mavlink.MAVLink_vision_position_estimate_message(
    usec=timestamp_us,
    x=position_x,      # meters, NED frame
    y=position_y,
    z=position_z,
    roll=roll,         # radians
    pitch=pitch,
    yaw=yaw
)

Drift Mitigation

Visual odometry accumulates drift over time. Strategies:

1. Loop Closure: Recognize previously visited locations
2. Landmark Matching: Use known visual markers
3. Multi-Sensor Fusion: Weight sensors by confidence
4. Periodic Reset: Return to known position

Geofencing

GPS is only used for safety boundaries:

geofence:
  enabled: true
  use_gps: true
  fence_type: polygon
  action: RTL
  max_altitude: 50

If drone crosses boundary, triggers return-to-launch.

Coordinate System

All waypoints use local NED coordinates:

• X: North (meters from origin)
• Y: East (meters from origin)
• Z: Down (negative for altitude)

Example mission:

waypoints = [
    {"x": 0, "y": 0, "z": -5},    # Takeoff to 5m
    {"x": 10, "y": 0, "z": -5},   # 10m north
    {"x": 10, "y": 10, "z": -5},  # 10m east
    {"x": 0, "y": 0, "z": -5},    # Return
    {"x": 0, "y": 0, "z": 0},     # Land
]

Limitations

• Drift accumulates over distance/time
• Requires visual features (fails in featureless environments)
• Requires sufficient lighting
• Performance degrades with fast motion or blur