robot_localization: fuse GPS and Odometry without heading

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So, I want to use the robot_localization package to fuse GPS (GNSS) data with odometry data with a UKF.

Reading the docs I stumbled across the navsat_transform_node to transform the GPS data to the wordl_frame, it is stated that the navsat_transform_node does need 3 inputs to do so:
- coordinates
- heading
- position estimate

The problem now is: I don't have a heading - reading from my GPS as I only use 1 antenna.

Can I fuse GPS and Odometry in a different way without using navsat_transform_node?

Thanks!