How to generate different random cmd_vel messages?

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Hello everyone,

I'm trying to have two (or several) robots receiving a random cmd_vel message. I was expecting that each robot would present a different velocity (I'm using Gazebo) but instead what I get is that every robot has the same velocity (however random, it is still the same for each one)

publisher pubvel is below as well as my launch file where I launch both the robot model and the publisher within the same namespace. One namespace for each robot.

    #include "ros/ros.h"
    #include "std_msgs/String.h"
    #include <geometry_msgs/Twist.h>
    #include <stdlib.h>


    int main(int argc, char **argv)
    {
    // Initialize the ROS system and become a node .

      ros::init(argc, argv, "publish_velocity");

      ros::NodeHandle nh;

    // Create a publisher object .
      ros::Publisher publisher = nh.advertise<geometry_msgs::Twist>("cmd_vel", 1000);

    // Loop at 2Hz until the node i s shut down.
      ros::Rate rate(2); 

    while (ros::ok())
      {
       //srand(time(0));

            geometry_msgs::Twist msg;

            msg.linear.x=double(rand())/double(RAND_MAX);
            msg.angular.z=double(rand())/double(RAND_MAX)-1;
            publisher.publish(msg);

            ROS_INFO_STREAM("Sending random velocity command:"<<" linear="<<msg.linear.x<<" angular=" <<msg.angular.z);

            rate.sleep();
      }  
      return 0;
}

<launch>

<group ns="robot1">

<param name="tf_prefix" value="robot1_tf" />

<include file="$(find random_vel)/launch_files/one_robot.launch" >

    <arg name="init_pose" value="-x 0 -y 2 -z 0" />

    <arg name="robot_name"  value="Robot1" />

    <arg name="namespace_arg" value="robot1"/>

</include>

<node pkg="random_vel" type="pubvel" name="pubvel" output="screen" />

</group>

<group ns="robot2">

<param name="tf_prefix" value="robot2_tf" />

<include file="$(find random_vel)/launch_files/one_robot.launch" >

<arg name="init_pose" value="-x 0 -y -2 -z 0" />

<arg name="robot_name"  value="Robot2" />

<arg name="namespace_arg" value="robot2"/>

</include>

<node pkg="random_vel" type="pubvel" name="pubvel" output="screen"/>

</group>

</launch>

Thank you very much in advance,

Hugo