Space-time adaptive processing (STAP) has been proven to be optimum in scenarios where an airborne phased-array radar is used to search for moving targets. The STAP requires the inverse of the covariance matrix (ICM) of undesired signals. The computation of the real-time ICM is impractical at current computer speeds. Proposing two Theorems, this report indicates that the ICM is approximately invariant if radar and platform parameters remain unchanged. A pre-built STAP (PSTAP) processor is then proposed. Both the simulated data from a generic airborne phased array radar model and real data collected by the multi-channel airborne radar measurement (MCARM) system are processed to verify the processor. Results indicate that the performance of the proposed PSTAP processor is the same as that of the real-time STAP processor.