Rigid covariance as a natural extension of Painlev\'e--Gullstrand   space-times: gravitational waves
  The group of rigid motions is considered to guide the search for a natural system of space-time coordinates in General Relativity. This search leads us to a natural extension of the space-times that support Painlev\'{e}--Gullstrand synchronization. As an interesting example, here we describe a system of rigid coordinates for the cross mode of gravitational linear plane waves. 
Xavier Ja\'en and Alfred Molina
gr-qc