Techniques for radar imaging using a wideband adaptive array

A microwave imaging approach is simulated and validated experimentally that uses a small, wideband adaptive array. The experimental 12-element linear array and microwave receiver uses stepped frequency CW signals from 2–3 GHz and receives backscattered energy from short range objects in a +/−90° field of view. Discone antenna elements are used due to their wide temporal bandwidth, isotropic azimuth beam pattern and fixed phase center. It is also shown that these antennas have very low mutual coupling, which significantly reduces the calibration requirements. The MUSIC spectrum is used as a calibration tool.; Spatial resampling is used to correct the dispersion effects, which if not compensated causes severe reduction in detection and resolution for medium and large off-axis angles. Fourier processing provides range resolution and the minimum variance spectral estimate is employed to resolve constant range targets for improved angular resolution. Spatial smoothing techniques are used to generate signal plus interference covariance matrices at each range bin.; Clutter affects the angular resolution of the array due to the increase in rank of the signal plus clutter covariance matrix, whereas at the same time the rank of this matrix is reduced for closely spaced scatterers due to signal coherence. A method is proposed to enhance angular resolution in the presence of clutter by an approximate signal subspace projection (ASSP) that maps the received signal space to a lower effective rank approximation. This projection operator has a scalar control parameter that is a function of the signal and clutter amplitude estimates. These operations are accomplished without using eigendecomposition.; The low sidelobe levels allow the imaging of the integrated backscattering from the absorber cones in the chamber. This creates a fairly large clutter signature for testing ASSP. We can easily resolve 2 dihedrals placed at about 70% of a beamwidth apart, with a signal to clutter ratio of 15 dB. In laboratory tests the imaging system has performed very well and has shown good agreement with simulations. Contributions from this work include the validation of an experimental wide angle, superresolution imaging system, the use of discones for arrays and the ASSP algorithm.