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FIFTH ANNUAL
ASAP '97 WORKSHOP


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Clutter Mitigation in an
Airborne Radar Based
Upon Phase-Weight
Perturbation

K.R. Akbulut, B.N. Suresh Babu, R.M. Davis, and J.D.R. Kramer
The MITRE Corporation
202 Burlington Road
Bedford, MA 01730

Abstract In an airborne surveillance radar, the sensitivity for detecting low velocity targets is often limited by ground clutter that fills the low Doppler frequency filters. In order to reduce the clutter, it is necessary to reduce the antenna sidelobes where the clutter is most significant. In this paper, we describe a low cost sidelobe canceller that minimizes the clutter by using a phase perturbation algorithm. The antenna is modeled as a rotating planar array. The adaptive phases are applied to the row phase shifters. The same phase is applied on both transmit and receive and the phase settings remain fixed during a coherent processing interval. The environment seen by the canceller will be non-stationary due to the rotation of the antenna and motion of the aircraft. The canceller adaptively modifies the phase shifter settings to lower the elevation sidelobes, thereby, suppressing sidelobe clutter. Orthogonal binary sequences are used to provide the phase perturbations to those rows that are made adaptive. A gradient based algorithm updates the adaptive phases. An estimate of the gradient is obtained by cross-correlating the binary codes with the instantaneous output power. Only the output power needs to be measured. The phase-only perturbation algorithm eliminates the need for separate receivers on the adaptive channels and supports low cost retrofitting of adaptive nulling on phased arrays using conventional beam-steering circuits to apply the adaptive weights. This paper describes the perturbation algorithm and the simulation being developed to evaluate its ability to reduce sidelobe clutter.

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