# Gazebo Simulation Guide

Running the GPS-denied drone simulation with Gazebo Ignition Fortress on Linux/WSL2.

## Quick Start (Two Terminals)

**Terminal 1 - Launch Gazebo + Bridge:**
```bash
source activate.sh
ros2 launch gazebo/launch/drone_landing.launch.py
```

**Terminal 2 - Run Controllers:**
```bash
source activate.sh
python run_gazebo.py --pattern circular --speed 0.3
```

Both the drone AND rover will move!

## Command Options

```bash
python run_gazebo.py --help

Options:
  --pattern       stationary, linear, circular, square, random
  --speed, -s     Rover speed in m/s (default: 0.5)
  --amplitude, -a Movement amplitude (default: 2.0)
  --no-rover      Disable rover controller
```

## How It Works

1. **Gazebo** runs the physics simulation with:
   - Drone with `VelocityControl` plugin (responds to `/drone/cmd_vel`)
   - Rover with `VelocityControl` plugin (responds to `/rover/cmd_vel`)

2. **ros_gz_bridge** connects Gazebo topics to ROS 2

3. **run_gazebo.py** starts:
   - `GazeboBridge` - converts ROS topics to telemetry format
   - `DroneController` - your landing algorithm
   - `RoverController` - moves the landing pad

## GPS-Denied Sensors

The `GazeboBridge` provides the same sensor interface as PyBullet:

| Sensor | Source |
|--------|--------|
| IMU | Gazebo odometry orientation |
| Altimeter | Gazebo Z position |
| Velocity | Gazebo twist |
| Camera | Gazebo camera sensor (if enabled) |
| Landing Pad | Computed from relative position |

## Topics

### ROS 2 Topics (your code uses these)

| Topic | Type | Direction |
|-------|------|-----------|
| `/cmd_vel` | `Twist` | Input (from DroneController) |
| `/drone/telemetry` | `String` | Output (to DroneController) |
| `/rover/telemetry` | `String` | Output (rover position) |

### Gazebo Topics (bridged automatically)

| Topic | Type | Description |
|-------|------|-------------|
| `/drone/cmd_vel` | `Twist` | Drone velocity commands |
| `/rover/cmd_vel` | `Twist` | Rover velocity commands |
| `/model/drone/odometry` | `Odometry` | Drone state |
| `/drone/imu` | `IMU` | IMU sensor data |
| `/clock` | `Clock` | Simulation time |

## Headless Mode (WSL2 / No GPU)

Run Gazebo without GUI:

```bash
# Server mode only
ign gazebo -s gazebo/worlds/drone_landing.sdf
```

Then run the bridge manually:
```bash
ros2 run ros_gz_bridge parameter_bridge \
  /drone/cmd_vel@geometry_msgs/msg/Twist]ignition.msgs.Twist \
  /rover/cmd_vel@geometry_msgs/msg/Twist]ignition.msgs.Twist \
  /model/drone/odometry@nav_msgs/msg/Odometry[ignition.msgs.Odometry
```

## World File Details

The world file `gazebo/worlds/drone_landing.sdf` includes:

- **Drone** at (0, 0, 2) with:
  - `VelocityControl` plugin for movement
  - `OdometryPublisher` plugin for telemetry
  - IMU sensor

- **Landing Pad (Rover)** at (0, 0, 0.15) with:
  - `VelocityControl` plugin for movement
  - Visual H marker

## Troubleshooting

### Drone falls immediately

The drone should hover with the controller running. If it falls:
1. Check that `run_gazebo.py` is running
2. Verify the bridge shows "Passing message from ROS"
3. Check `/drone/cmd_vel` topic: `ros2 topic echo /drone/cmd_vel`

### Rover doesn't move

1. Check that `/rover/cmd_vel` is bridged
2. Verify RoverController is publishing: `ros2 topic echo /rover/cmd_vel`

### Model not found

Set the model path:
```bash
export GZ_SIM_RESOURCE_PATH=$PWD/gazebo/models:$GZ_SIM_RESOURCE_PATH
export IGN_GAZEBO_RESOURCE_PATH=$PWD/gazebo/models:$IGN_GAZEBO_RESOURCE_PATH
```

### "Cannot connect to display" (WSL2)

Use headless mode:
```bash
ign gazebo -s gazebo/worlds/drone_landing.sdf
```

Or ensure WSLg is working:
```bash
export DISPLAY=:0
```

### Plugin not found

For Ignition Fortress, plugins use `libignition-gazebo-*-system.so` naming.

Check available plugins:
```bash
ls /usr/lib/x86_64-linux-gnu/ign-gazebo-6/plugins/
```

## Launch File Options

```bash
ros2 launch gazebo/launch/drone_landing.launch.py use_sim_time:=true
```

| Argument | Default | Description |
|----------|---------|-------------|
| `use_sim_time` | `true` | Use Gazebo clock |