The measurement model of tracking a ballistic target of coast phase in the LEO space-based infrared constellation was constructed. A suitable accurate target motion model was developed. Considering the robustness of the nonlinear filter, a square root version of the UKF was included. It was corroborated in the theoretical analysis. The simulation shows that the square root UKF (SRUKF) has good performance when tracking a ballistic target in the LEO space-based infrared constellation.