A recording disk contains at least two types of servo sector format, including a first type having a relatively larger track identifying field containing the track number, and a second type having an abbreviated track identifying field identifying a track within a range of tracks, the range being less than the full range of tracks on the disk surface. The object of the invention is to provide an active roll stabilization system for ships that can be used both with ships which are underway and with ships that are at anchor. The user or the device terminates the first user session and subsequently receives an input of the user to initiate a second user session. The first additional reflection surface can be disposed lower than the top end portion and in the front direction from the back end edge.