# Gazebo Simulation

Running the GPS-denied drone simulation with Gazebo.

## Prerequisites

Install Gazebo and ROS-Gazebo bridge:

```bash
./setup/install_ubuntu.sh
source activate.sh
```

## Quick Start

**Terminal 1 - Start Gazebo:**
```bash
source activate.sh
gz sim gazebo/worlds/drone_landing.sdf
```

**Terminal 2 - Spawn drone and start bridge:**
```bash
source activate.sh

# Spawn drone
gz service -s /world/drone_landing_world/create \
  --reqtype gz.msgs.EntityFactory \
  --reptype gz.msgs.Boolean \
  --req 'sdf_filename: "gazebo/models/drone/model.sdf", name: "drone"'

# Start bridge
python gazebo_bridge.py
```

**Terminal 3 - Run controllers:**
```bash
source activate.sh
python controllers.py --pattern circular --speed 0.3
```

## World Description

The `drone_landing.sdf` world contains:

| Object | Description |
|--------|-------------|
| Ground Plane | Infinite flat surface |
| Sun | Directional light with shadows |
| Landing Pad | Green box with "H" marker at origin |

## Drone Model

Quadrotor drone with:

- **Body**: 0.3m × 0.3m × 0.1m, 1.0 kg
- **Rotors**: 4 spinning rotors
- **IMU**: Orientation and angular velocity
- **Camera**: 320x240 downward-facing sensor
- **Odometry**: Position and velocity

### Gazebo Plugins

| Plugin | Function |
|--------|----------|
| MulticopterMotorModel | Motor dynamics |
| MulticopterVelocityControl | Velocity commands |
| OdometryPublisher | Pose and twist |

## Camera System

The drone has a downward-facing camera:

| Property | Value |
|----------|-------|
| Resolution | 320 x 240 |
| FOV | 60 degrees |
| Format | Base64 encoded JPEG |
| Update Rate | 30 Hz (Gazebo) / ~5 Hz (in telemetry) |
| Topic | `/drone/camera` |

## Gazebo Topics

| Topic | Type | Description |
|-------|------|-------------|
| `/drone/cmd_vel` | `gz.msgs.Twist` | Velocity commands |
| `/model/drone/odometry` | `gz.msgs.Odometry` | Drone state |
| `/drone/camera` | `gz.msgs.Image` | Camera images |
| `/drone/imu` | `gz.msgs.IMU` | IMU data |

## GPS-Denied Sensors

The `gazebo_bridge.py` converts Gazebo data to GPS-denied sensor format:

| Sensor | Source |
|--------|--------|
| IMU | Odometry orientation + angular velocity |
| Altimeter | Odometry Z position |
| Velocity | Odometry twist |
| Camera | Camera sensor (base64 JPEG) |
| Landing Pad | Computed from relative position |

## Headless Mode

Run without GUI:

```bash
gz sim -s gazebo/worlds/drone_landing.sdf
```

## Using the Launch File

For ROS 2 packages:

```bash
ros2 launch <package_name> drone_landing.launch.py
```

## Troubleshooting

### "Cannot connect to display"

```bash
export DISPLAY=:0
# or use headless mode
gz sim -s gazebo/worlds/drone_landing.sdf
```

### Drone falls immediately

The velocity controller may need to be enabled:

```bash
gz topic -t /drone/enable -m gz.msgs.Boolean -p 'data: true'
```

### Topics not visible in ROS

Ensure the bridge is running:

```bash
python gazebo_bridge.py
```

### Model not found

Set the model path:

```bash
export GZ_SIM_RESOURCE_PATH=$PWD/gazebo/models:$GZ_SIM_RESOURCE_PATH
```

### Camera image not in telemetry

Ensure PIL/Pillow is installed:
```bash
pip install pillow
```