The previous analysis for the ACF determination considered only two pulses. To determine the ACF value for different lag times a different pulse separation has to be applied for each lag. This method is called variable-lag double-pulse technique; several pairs of pulses are transmitted with a spacing between pulses that is some integer multiple k of an elemental spacing T. The time between pulse pairs must be sufficiently long that no signals are received from the previous pulse pair while the current pulse pair is being sampled. The time required for a complete set of measurements might be so long that the stationarity requirement for the scatterer becomes questionable. In real radar applications this method is for this reason seldom used.