How to define a new frame of Reference for world and model ?

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I don't think that there is a way to create new frame of reference. But we can transform entity, like force, velocity, etc., such that entities are transformed into NED frame

To transform entity into NED frame, we need to rotate about any one axis of frame in gazebo by 180 degrees or Pi radians. This is will make sure that other two are oriented in such a way that it becomes NED frame.

For example, consider my X-axis is oriented with X-axis of NED frame. then rotate Pi radians about X-axis of frame in gazebo. It rotate will other two remaining axes in such a position that entity are in NED Frame.