Based on the mission requirements of medium earth orbit (MEO) and low earth orbit (LEO) early warning satellites, the paper focuses on space coverage and target positioning accuracy of MEO and LEO early warning satellites. Firstly, based on ballistic missiles target properties and detectable elements, the mission requirements of limb scanning are induced. Then, according to the geometrical configuration of early warning satellite and ballistic missiles, the mathematical relationship between coverage results of different levels and sensor detection capability is obtained. The coverage ability of early warning satellites in operational scenario is analyzed. Moreover, the positioning principle of double satellites and multiple satellites are researched, and simulation results of positioning accuracy based on early warning satellites trajectory are obtained. The results indicate that the positioning accuracy can satisfy the mission requirements of target guidance in the presence of error.