How to get the target when in real experiment?

[XXLiu-HNU]

In the simulation, I find the target is get by the car_state, the Odometry of the car is easy to know, so in the code is very simple:

void Sim_detect::car_state_cb(const nav_msgs::Odometry& car_state){
    if(!has_map) return;
    Eigen::Vector3d car_pos;
    car_pos<<car_state.pose.pose.position.x,car_state.pose.pose.position.y,car_state.pose.pose.position.z;
    // if((car_pos-drone_pos).norm()>3.0) return;
    if(!is_block(car_pos,drone_pos)){
        nav_msgs::Odometry state_pub = car_state;
        state_pub.header.stamp = ros::Time::now();    
        detection_pub.publish(state_pub);
    }

}

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But how to detect in the real world, because the odometry the target_people is not know?