Architecture

System architecture for GPS-denied UAV/UGV navigation.

Overview

The system uses vision-based navigation with GPS reserved only for geofencing.

Vision Sensors (Cameras)
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Visual Odometry & Optical Flow
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Position Estimator (EKF)
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ArduPilot Flight Controller
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Motor Control

Components

Perception

Forward Camera: Visual odometry, feature tracking
Downward Camera: Optical flow, ground plane detection
IMU: Angular velocity, acceleration

Localization

Visual Odometry: Estimates motion from camera images
Optical Flow: Velocity estimation from downward camera
EKF Fusion: Combines all sensor inputs into position estimate

Waypoint Navigation: Relative coordinates (meters from origin)
Path Planning: Collision-free paths using local map
Position Hold: Maintain position using vision

Control

ArduPilot: Flight controller firmware
MAVROS: ROS interface to ArduPilot
Velocity/Position Control: Low-level motor commands

Safety

Geofencing: GPS-based boundary checking (safety only)
Altitude Limits: Maximum flight ceiling
Failsafe: RTL on signal loss or boundary breach

Coordinate Frames

Frame	Description
body	Attached to vehicle, X forward, Y right, Z down
odom	Origin at takeoff, accumulates drift
map	Fixed world frame (may be corrected)

All navigation commands use local coordinates relative to takeoff point.

Data Flow

Camera Images
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Feature Detection (ORB/SIFT)
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Feature Matching (frame to frame)
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Motion Estimation (Essential Matrix)
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EKF Update (fuse with IMU)
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Position Estimate (x, y, z, roll, pitch, yaw)
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ArduPilot (external position input)
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PID Control -> Motor PWM

ArduPilot Integration

ArduPilot receives external position via MAVLink:

Visual odometry → VISION_POSITION_ESTIMATE
EKF uses external source (EK3_SRC1_POSXY=6)
GPS disabled for navigation (GPS_TYPE=0 or EK3_SRC1_POSXY!=1)

Geofencing

GPS is only used for safety boundaries:

GPS position → lat/lon
Check against polygon/circle boundary
If outside: trigger RTL or LAND

Navigation continues using vision regardless of GPS status.

ROS 2 Topics

Topic	Type	Description
/uav/camera/forward/image_raw	sensor_msgs/Image	Forward camera
/uav/camera/downward/image_raw	sensor_msgs/Image	Downward camera
/mavros/local_position/pose	geometry_msgs/PoseStamped	Current position
/mavros/setpoint_position/local	geometry_msgs/PoseStamped	Target position
/mavros/imu/data	sensor_msgs/Imu	IMU data

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