I want to track the state of the AR Drone. For that, I have created a Subscriber, but for some reason, it doesn't go into my callback function. This is my code:

#include "ros/ros.h"
#include "std_msgs/Empty.h"
#include "std_msgs/String.h"
#include <iostream>
#include <ardrone_autonomy/Navdata.h>
using namespace std;
using namespace ros;
using namespace std_msgs;

//from ardrone_autonomy.msg import Navdata

ardrone_autonomy::Navdata data;
int state;

void receiveNavdata(const ardrone_autonomy::Navdata& data)
{
    cout<<"\nIN NAV"<<endl;
    cout<<"\nState: "<< data.state<<endl;   
    state=data.state;   
}

int main(int argc, char **argv){

  init(argc, argv, "talker");
    NodeHandle rospy;

    Subscriber navData = rospy.subscribe("/ardrone/navdata", 1, receiveNavdata);


while( state == -1){
    ros::spinOnce();
    ros::Duration(0.1).sleep();
}   

Publisher takeoff = rospy.advertise<Empty>("/ardrone/takeoff", 1);

while(takeoff.getNumSubscribers() < 1){
    ros::Duration(0.1).sleep();
}

cout<<"\nTakeOff"<<endl;

takeoff.publish(empty);
ros::Duration(5).sleep();

    return 0;
}