How do you implement GPS using robot_localization

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Have you seen this page? If you have and something isn't clear, please let me know and I'll update it.

EDIT: You might benefit by reading up a little more on roslaunch. One of the points of roslaunch is to save you from having to use rosrun and entering a ton of parameters by hand, e.g.,

rosrun amazing_package amazing_node param_a:=100 param_b:=true param_c:=-999 param_d:=false

Instead, you just create a launch file that kicks off the node and passes in all those parameters. Then you can just launch your launch file instead, e.g.,

roslaunch amazing_package amazing_launch_file.launch

The state estimation nodes in robot_localization do not require an odometry source. navsat_transform_node converts GPS data to an odometry message so that ekf_localization_node or ukf_localization_node can consume it. However, if you want to fuse GPS data, you will need an IMU with an earth-referenced absolute heading.