The proposed earth rotation service and the related efforts to monitor the terrestral reference frame will employ instrumental and computational techniques of widely different kinds. It has already been demonstrated that the results produced by these techniques are sensitive to systematic errors occurring at the level of a few millimeters. As a result it will be extremely important to verify the performance of the different systems by making comparative observations at the same sites. Only mobile laser-ranging systems are currently in a position to perform this kind of service. To demonstrate the status of laser ranging during co-location, this paper presents a proposal for a controlled collocation experiment involving U.S. and European stations of the IRIS network.