The main problem with direct methods is the great computational complexity associated with the estimation and inversion of matrices formed from many degrees of freedom (large number of elements and or pulses). In addition, for methods where formula_9 must be estimated using data samples, the number of samples required to achieve a particular error is heavily dependent on the dimensionality of the interference covariance matrix. As a result, for high dimensional systems, this may require an unachievable number of unambiguous range cells. Further, these adjacent data cells must contain stationary statistics as a function of range which is rarely a good assumption for the large number of cells required (formula_25 for 3 dB SINR degradation from optimal, clairvoyant STAP).