code based on ubuntu 16.04 & ros kinetic & Hector quadrotor project.
Change camera in hector model into downward orientation
(1) Put all packages (apriltags2_ros & hector_quadrotor & hl_controller) into ~/catkin_ws/src.
(2) cd ~/catkin_ws, catkin_make
(3) source ~/catkin_ws/devel/setup.bash
(1) roslaunch hector_quadrotor_demo hl.launch
(2) roslaunch hl_controller hl_controller.launch
(3) rostopic pub -r 10 /hl_change_mode std_msgs/String "data: 'tracking'"