Abstract: In the process of ballistic missile defense, the key to carry out missile intercept is whether the phased array radar could capture the target rapidly under the guidance of early warning satellite. Some key problems are studied in this paper, aiming at the problem of search area definition of precision tracking radar under the guidance of low earth orbit (LEO) early warning satellite. Taking the space transportation system (STSS) of the USA as the study object, the detecting precision of multi-satellite is analyzed firstly, the CramérRao Bound of position error is taken to express detection precision of tracking satellite, and then the expression and transmission of measurement error under different coordinates system are discussed. Finally, the search area of phased array radar under pointing coordinate system is proposed and deduced when searching type of phased array radar is taking into account. The simulation results indicate that when the method is adopted, the search area of phased array radar is reduced, making radar searching more purposeful.