Abstract: Capacity of repetitive observation of remote sensing satellite sensors is a key indicator of the system's dynamic monitoring ability. In the paper, the algorithm of orbit computing for realizing repetitive observations was presented based on the analysis of repetitive observation conditions under two imaging modes: nadir and side-looking; selective orbits for repetitive observations in particular areas were also described taking the HJ-1A/1B satellites (the optical satellites of the small satellite constellation for environment and disaster monitoring and forecasting) for example. The work is focused on the orbits of HJ-1A/B satellites. Before computation and analysis of the orbits, the Area of Interest (AOI) should be defined. Considering the optical sensors, in this paper, only the orbits from north to south were computed, yet orbits from south to north were passed over. The capacity of repetitive observation is defined as the overlap of two satellite scenes. By analysis of the nadir imaging and the side-looking imaging mode, the paper draws the conclusion of how to choose the best orbit to realize repetitive observation. Finally, impacts of the satellite constellation consisting of two satellites on the repetitive observations as well as that of latitude were discussed.