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A method for finding best channels as the auxiliary channels to cancel the interference components in the main channel in reduced-dimension (RD) space-time adaptive processing (STAP) is proposed. Motivated by the cross-spectral metric (CSM) which evaluates the significance of eigenvectors; the significance of each angular-Doppler channel to output signal-to-interference-noise ratio (SINR) is evaluated and taken into consideration for the auxiliary channels selection in RD STAP. The proposed algorithm can achieve the best output SINR performance by selecting the best channels as auxiliary channels when the degrees of freedom (DoFs) of a STAP system are fixed. It is demonstrated that the proposed algorithm allows the dimensionality of the STAP to be reduced much less than the rank of clutter without significant SINR loss. The SINR loss will be less than 3 dB when only one channel is selected as the auxiliary channel under the condition of known clutter covariance matrix. Generally, 3~5 channels are enough even when the clutter covariance matrix is unknown. Consequently, the proposed approach can reduce the requirement of the sample support dramatically.