It is necessary to determine camera postures in inertial coordinate systems, when the mapping camera is used in a photogrammetry for a transport stereo mapping satellite. In determining the camera postures, the postures of star sensor measuring system in the inertial coordinate system were measured by star sensors, then, the postures of the mapping camera measuring system in the inertial coordinate system were obtained by the transition matrix between star sensor measuring system and the cubic prism in star sensor and the transition matrix between the two cubic prisms in the star sensor and mapping camera. This paper introduces the definition of coordinate systems. By using the measuring systems of four theodolites, we establish coordinate systems on two cubic prisms respectively and a transition matrix for the coordinate systems. The calibration method of two coordinate systems is presented. The measuring results from many experiments show that the maximum calibration error is 1.011 6″,which is better than 2″(1σ) and satisfies the precision requirement of the stereoscopic mapping.