How can I know the AR.Drone's height in order to control it?

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I believe that you can get altitude data even if it is not in flight by subscribing to the /ardrone/navdata_altitude topic. You will have to modify the ardrone.launch file so that the enable_navdata_pressure_raw parameter is set to true. Just copy and paste this:

<param name="enable_navdata_pressure_raw" value="true" />

and add it between the <node> and </node> tags of the launch file. In order to use this data you will have to include the navdata_altitude message/datatype and access it in your callback function using data.altitude_raw.