It is shown that adaptive canceling arrays which track interference by regular updates of the beamformer weights can introduce a spectral null at the excised interference frequency. This PSD estimation bias effect we call "spectral scooping" is most prominent for narrowband interference (i.e., occupying only a few spectral bins at the desired PSD estimation resolution). Scooping is problematic in radio astronomy where bias in either the weak signal or noise floor spectra can corrupt the observation. The mathematical basis for scooping is derived, and an algorithm to eliminate it is proposed. Both simulated and real data experiments demonstrate the effectiveness of the proposed algorithm.
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