Digital Beamforming with Unequal Tapered Rows in an Airborne Phased Array Radar (APAR)

The Airborne Phased Array Radar (APAR), a next generation airborne radar for meteorological observations of clouds and precipitation and characterization of extreme weather phenomena, is currently under development by the National Center for Atmospheric Research (NCAR), with Ball Aerospace performing design and construction of the radar front-end hardware. APAR is intended to be a C-band dual-polarized active electronically scanned array (AESA) attached to the NSF/NCAR C-130 scientific research aircraft and has an intended 20-year lifetime. Within the APAR aperture, elements are arranged in rows of unequal lengths, with the full array being contained within a circular area. APAR can transmit an elevation-plane fan beam while receiving in multiple simultaneous digital elevation beams. These simultaneous receive beams are achieved using a hybrid beamforming scheme where elements in each row are combined with an analog combiner, but each row is digitized and combined in a digital beamformer. The elements in each row are amplitude tapered for azimuthal sidelobe control and reduced clutter. Due to power conservation in a lossless analog beamformer, the output voltage of each row is scaled down by the square root of the number of elements in that row. The digital beamformer must undo this scaling by applying the appropriate row correction weight to each row prior to summation in order to maximize SNR (Signal-to-Noise-Ratio). This row correction weight has previously been discussed for the case when each row has a uniform excitation. This work extends these studies to the case where an azimuthal taper is applied within each row and an elevation taper weight is applied across the rows. The SNR of an array with such tapers and unequal rows is derived as a function of azimuthal and elevation taper weights. The SNR and pattern characteristics of this array are also studied with MATLAB simulations, to better understand the role of the row-correction weights in hybrid arrays such as APAR to obtain desired pattern characteristics such as low elevation sidelobes while maximizing SNR.