One approach would be: you're moving along a straight line, perpendicular to a large body that attracts you through gravity.
Now "down" is towards the gravity, "up" is the opposite direction, and "left" and "right" are perpendicular, but which one is left and right, if you cannot rely on handedness?
That helps if you meet them in person, but if you communicate through radio waves, they'll have no way to know how to decode signal into an image with the intended left/right and up/down orientation
One approach would be: you're moving along a straight line, perpendicular to a large body that attracts you through gravity.
Now "down" is towards the gravity, "up" is the opposite direction, and "left" and "right" are perpendicular, but which one is left and right, if you cannot rely on handedness?