Robot_localization gps use only fix messages

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As robot_localization subscribes to NavSatFix message, can we only fuse messages with gps status > 0, meaning only messages with a fix? Or do I need to Code a node which only re-publishes these messages?

navsat_transform_node filters out messages with a status of STATUS_NO_FIX. Are you sure your GPS messages are correctly filling out the status field?

Alternatively, gps driver provides a ublox/navrelposned message which contains the difference between base station and robot in X,Y,Z. Is is possible to fuse these data instead real gps data?

Maybe, though there are gremlins to be found doing that. Those X, Y positions will define your world coordinate frame. So if you move away from the base station at 45 degrees for 1.41 meters, the RTK unit will say that you are at position (1, 1). But your EKF heading may not agree:

• If you are fusing absolute yaw from the IMU, the IMU's 0 orientation needs to match the 0 orientation of your RTK coordinate frame (so if you face the +X axis int the RTK coordinate frame, the IMU would need to read 0).
• If you are only fusing yaw velocity from the IMU, then during that same traversal above, the EKF will think it has a heading of 0, and it will integrate the velocity data from the encoders that way.

In both cases, the net effect will be that your robot will likely look like it is oriented in a different direction than it is travelling. n_t_n handles this issue for you, which is why I recommend it.