ROS 2 Location Publisher-Subscriber

This project is a ROS 2 setup where a publisher node receives location data via a TCP socket server and publishes it as a geometry_msgs/Vector3 message. A subscriber node then listens to this topic and prints the received latitude and longitude.

Table of Contents

• Overview
• Getting Started
  • Prerequisites
  • Setup
• Usage
  • Running the Publisher Node
  • Running the Subscriber Node
• Node Details
  • latlon_publisher.py
  • latlon_subscriber.py
• Topic Information

Overview

The latlon_publisher.py node acts as a bridge between an external data source (via a TCP socket) and the ROS 2 ecosystem. It listens for incoming JSON messages containing latitude and longitude. Once received, it converts this data into a geometry_msgs/Vector3 message (using x for latitude and y for longitude) and publishes it on the /location_data topic.

The latlon_subscriber.py node simply subscribes to the /location_data topic and logs the received latitude and longitude values to the console.

Getting Started

Prerequisites

Before you begin, ensure you have:

• ROS 2 installed: This project is built for ROS 2. If you don't have it, follow the official ROS 2 installation guide for your operating system.
• Python 3: ROS 2 relies on Python 3.
• rclpy and geometry_msgs: These are standard ROS 2 Python libraries and message types, so they should be available with a typical ROS 2 installation.

Setup

1. Create a ROS 2 Workspace (if you don't have one):

   mkdir -p ~/ros2_ws/src
   cd ~/ros2_ws/src

2. Create a new ROS 2 package:

   ros2 pkg create --build-type ament_python your_package_name --dependencies rclpy geometry_msgs

   Replace your_package_name with a descriptive name for your package, e.g., location_monitor.

3. Place the Python files: Navigate into your package directory (e.g., ~/ros2_ws/src/location_monitor/). Create a folder named your_package_name (e.g., location_monitor) inside the src subfolder (e.g., ~/ros2_ws/src/location_monitor/location_monitor). Place latlon_publisher.py and latlon_subscriber.py inside this second your_package_name folder.

   Your directory structure should look something like this:

   ros2_ws/
   ├── src/
   │   └── your_package_name/
   │       ├── your_package_name/  <-- Place your Python files here
   │       │   ├── latlon_publisher.py
   │       │   └── latlon_subscriber.py
   │       ├── package.xml
   │       └── setup.py
   

4. Modify setup.py: Open setup.py in your package directory (~/ros2_ws/src/your_package_name/setup.py). You need to add the entry points for your executables. Modify the entry_points section to include your publisher and subscriber:

   from setuptools import setup
   
   package_name = 'your_package_name' # Make sure this matches your package name
   
   setup(
       name=package_name,
       version='0.0.0',
       packages=[package_name],
       data_files=[
           ('share/' + package_name, ['package.xml']),
           ('share/' + package_name + '/resource', ['resource/' + package_name]),
           ('share/ament_index/resource_index/packages', ['resource/' + package_name]),
       ],
       install_requires=['setuptools'],
       zip_safe=True,
       maintainer='Your Name', # Replace with your name
       maintainer_email='[email protected]', # Replace with your email
       description='ROS 2 package for publishing and subscribing to location data.',
       license='TODO: License declaration',
       tests_require=['pytest'],
       entry_points={
           'console_scripts': [
               'latlon_publisher = your_package_name.latlon_publisher:main',
               'latlon_subscriber = your_package_name.latlon_subscriber:main',
           ],
       },
   )

   Remember to replace your_package_name with the actual name of your package.

5. Build your package:

   cd ~/ros2_ws
   colcon build --packages-select your_package_name

   Replace your_package_name with the actual name of your package.

6. Source your workspace:

   source install/setup.bash

   You'll need to run this command in every new terminal you open before running your ROS 2 nodes.

Usage

Running the Publisher Node

The publisher node (latlon_publisher) creates a TCP socket server on 0.0.0.0:8080. It waits for a client connection and then continuously receives data, parses it as JSON, and publishes it.

Open a new terminal and run:

ros2 run your_package_name latlon_publisher

Replace your_package_name with the actual name of your package.

You should see messages like:

[INFO] [latlon_publisher]: Waiting for client connection...

Running the Subscriber Node

The subscriber node will listen for messages on the /location_data topic and print them.

Open another new terminal and run:

ros2 run your_package_name latlon_subscriber

Replace your_package_name with the actual name of your package.

You should see output similar to this as data is sent to the publisher (e.g., from a connected Flutter app or another client sending JSON):

[INFO] [latlon_subscriber]: Received Location: Latitude=34.0522, Longitude=-118.2437
[INFO] [latlon_subscriber]: Received Location: Latitude=40.7128, Longitude=-74.0060
[INFO] [latlon_subscriber]: Received Location: Latitude=51.5074, Longitude=0.1278

Node Details

latlon_publisher.py

• Node Name: latlon_publisher
• Purpose: Receives JSON-formatted latitude/longitude data over a TCP socket and publishes it as a ROS 2 geometry_msgs/Vector3 message.
• Socket Server:
  • Address: 0.0.0.0
  • Port: 8080
  • Listens for a single client connection.
• Published Topic:
  • Name: /location_data
  • Type: geometry_msgs/Vector3
  • Message Mapping:
    • Vector3.x maps to latitude
    • Vector3.y maps to longitude
    • Vector3.z is set to 0.0 (unused in this context but required by the message type)

latlon_subscriber.py

• Node Name: latlon_subscriber
• Purpose: Subscribes to the /location_data topic and prints the latitude and longitude received from the latlon_publisher.
• Subscribed Topic:
  • Name: /location_data
  • Type: geometry_msgs/Vector3

Topic Information

You can inspect the /location_data topic using ROS 2 command-line tools:

• List topics:

  ros2 topic list

You should see /location_data in the list.

• View topic type:

  ros2 topic info /location_data

This will show Type: geometry_msgs/msg/Vector3.

• Echo topic messages (for debugging):

  ros2 topic echo /location_data

  This will print the raw geometry_msgs/Vector3 messages.