tidal stream radial velocities

I briefly helped Koposov this morning on an observing proposal to follow up his statistical measurement of the proper motion of a cold tidal stream with stellar radial velocities. The combination of transverse and radial velocities with distance and angular information means that if this proposal is accepted, Koposov will have not only full 6d phase-space information, but he will have that along the length of a long stream. This permits extremely precise orbit modeling, or, as Rix would say, a direct measurement of the acceleration due to gravity (velocity of the stream and curvature of the stream makes acceleration of the stream).