A method is presented for target mode identification and sensor placement on a phase-by-phase basis for sequentially assembled large space structures. At each phase, a Craig-Bampton representation is generated by constraining the structure at the actuator locations and at the interface to the next phase. Fixed interface modes are computed and ranked according to dynamical importance using an absolute measure called effective excitation mass. Modes with large excitation mass are strongly excited by the actuators during a current phase modal survey, or will be strongly excited by inputs from the next-phase structure. Inclusion of fixed interface modes in the Craig-Bampton representation which are strongly excited by the next-phase structure allows sensors placed during the current phase to do a better job of anticipating the dynamics which will be important in the next phase. Finite element model modes which are accurately predicted by the Craig-Bampton representation are used as target modes for sensor placement. Sensors are placed such that the target mode partitions are as spatially independent as possible. In a numerical example, the sensor configurations placed using the proposed method provided superior Fisher Information matrix determinants and condition numbers for initial and intermediate phases when compared to results using sensor configuration subsets placed optimally for the full structure.