A mobile mapping system (MMS) is a combination of several modern spatial techniques, including laser scanning, digital photogrammetry, global positioning system, and inertial navigation system. It is one of the most powerful systems capable of acquiring meticulous spatial information in a fast and automatic manner. However, the degradation of positional quality in areas with very limited satellite visibility is a constant issue disturbing the reliability of such a system. In this letter, a rigorous registration approach for MMS-collected point clouds is proposed. By making use of a time-variant reference transformation model and a few calibration points, most positioning errors due to the loss of satellite signals can be significantly eliminated. Consequently, the applicability of the MMS technique can be further extended to areas where current methods are limited (e.g., in a tunnel or urban area).