Cant receive GPS data

Hi I have tried the following code however it just returns undefined for couple of time and then doesn't output anything
whats the problem here ?

var arDrone = require('ar-drone');
var arDroneConstants = require('ar-drone/lib/constants');

var fs = require('fs');

function navdataOptionMask(c) {
return 1 << c;
}

var navdataOptions = (
navdataOptionMask(arDroneConstants.options.DEMO)
| navdataOptionMask(arDroneConstants.options.VISION_DETECT)
| navdataOptionMask(arDroneConstants.options.MAGNETO)
| navdataOptionMask(arDroneConstants.options.WIFI)
| navdataOptionMask(arDroneConstants.options.ZIMMU_3000)
);

// Connect and configure the drone
var client = new arDrone.createClient();
client.config('general:navdata_demo', true);
client.config('general:navdata_options', navdataOptions);

client.on('navdata', function(navdata){
console.log(navdata.gps);
});