robot_localization: Potential transformation error

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I am trying to fuse GPS with an IMU and am running into an issue I believe is some sort of transformation error. The transformations between the links and the base_link are specified in my URDF file and echoing tf_static shows this.

I have 2 gps units mounted to my robot to give me a position and heading and an IMU which outputs a yaw rate.

I have a node which converts the 2 gps information into an odometry message where the x,y position are the UTM coordinates of the rear gps and the heading angle is a straight line extending from the rear gps to the front gps. The frame_id in this message is odom and the child_frame_id is rear_gps_link.

When the odometry message is the sole input to the EKF the filtered output closely resembles the odometry message (i.e. the x,y position closely tracks the x,y position from the odometry msg), however, adding the IMU as a source results in the filtered output staying at position 0,0 (only the angular information updates) and I have no idea why.

I'm not able to attach the examples here but I can send them individually if needed.

Thanks, Ben