I am experiencing a problem while flying with two AR drones. Sometimes the ultrasound sensor is sending incorrect values. I am controlling each drone with one separate PC. As suggested in issue #193 I tried changing the `ultrasound_freq` to 7 for one drone and to 8 for the other one. I am using [this launch file](https://pastebin.com/phtAJgSY). In one config the parameter ultrasound_freq ist set like this `<param name="ultrasound_freq" value="8" />` and in the other one it is set to 7.<br> The problem that I am experiencing is that even though I am setting these values in my launch file they are not correctly set in `data/config.ini` of the AR drone. Whenever I try to set them to 8 the config.ini still reads 7. When I now edit the config.ini file and set `ultrasound_freq = 8` manually, then start roslaunch again, the value will be overwritten to 7 while it should remain at 8. The weird thing is that if I set my ultrasound_freq to 9 in my launch file and then start it, it will get overwritten in the config.ini and then reads `ultrasound_freq = 9`. When I change my launch file back to 8 again and then run it, the config still reads 9. It seems that all values are accepted except for 8. I even tried the example configs where ultrasound_freq is set to 8 but the config.ini still reads `ultrasound_freq = 7`. The output when running `roslaunch start.launch` ``` $ roslaunch start.launch ... logging to /home/void/.ros/log/ad0ddd28-86d8-11e8-83c6-955de12afcbf/roslaunch-void-2627.log Checking log directory for disk usage. This may take awhile. Press Ctrl-C to interrupt Done checking log file disk usage. Usage is <1GB. started roslaunch server http://void:46855/ SUMMARY ======== CLEAR PARAMETERS * /ardrone_autonomy/ PARAMETERS * /ardrone_autonomy/altitude_max: 1500 * /ardrone_autonomy/altitude_min: 50 * /ardrone_autonomy/bitrate: 4000 * /ardrone_autonomy/control_vz_max: 2000 * /ardrone_autonomy/control_yaw: 1.75 * /ardrone_autonomy/cov/imu_av: [1.0, 0.0, 0.0, 0... * /ardrone_autonomy/cov/imu_la: [0.1, 0.0, 0.0, 0... * /ardrone_autonomy/cov/imu_or: [1.0, 0.0, 0.0, 0... * /ardrone_autonomy/euler_angle_max: 0.18 * /ardrone_autonomy/looprate: 200 * /ardrone_autonomy/max_bitrate: 4000 * /ardrone_autonomy/navdata_demo: 0 * /ardrone_autonomy/realtime_navdata: False * /ardrone_autonomy/realtime_video: True * /ardrone_autonomy/ultrasound_freq: 8 * /rosdistro: kinetic * /rosversion: 1.12.13 NODES / ar_track_alvar_bottom (ar_track_alvar/findMarkerBundlesNoKinect) ar_track_alvar_front (ar_track_alvar/findMarkerBundlesNoKinect) ardrone_autonomy (ardrone_autonomy/ardrone_driver) auto-starting new master process[master]: started with pid [2640] ROS_MASTER_URI=http://localhost:11311 ```