This paper investigates a robust adaptive beamforming technique for two dimensional antenna array. Robustness is needed against imperfect knowledge of desired signal direction of arrival as well as against other errors. Erroneous estimation of source direction is especially true in air surveillance systems because aircraft are moving in high speed. Need for two dimensional antenna arrays is also substantial as aircraft are often at the same azimuth but at different elevation angles relative to

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... round system. Proposed algorithm is based on Generalized Sidelobe Canceller and uses Discrete Prolate Spheroidal Window or often refered to as Slepian window function. Only Slepians corresponding to large spectral concentration are used in both steering and blocking branches.