Enhanced Beamforming Algorithm for APAR: Resolving Angular Ambiguity in Hydrometeor Observations

To process complex and high-impact weather cases, airborne phased-array radar (APAR) employs a hybrid analog-digital beamforming (ADBF) architecture within an active electronically scanned array (AESA). This approach balances the increased degrees of freedom in digital beamforming (DBF) design with enhanced receiver sensitivity. DBF enables simultaneous multi-beam reception by leveraging digitized echoes from analog-to-digital converters (ADCs) in each transmit/receive (T/R) module. Conversely, analog beamforming (ABF) uses phase shifters to direct the array’s reception toward a single direction at a time. To cover a spatial volume, an ABF system scans sequentially, while DBF forms multiple adjacent beam angles, using a spoiled transmit beam to illuminate the entire volume in a single scan. This approach preserves correlation integrity by collecting data instantaneously, avoiding errors introduced by time-series sampling.
In a phased array, the direction of arrival (DOA) is determined by phase shifts arising from the spatial path differences relative to each antenna element. When a spoiled beam waveform is applied in DBF, a wide observation angle is assumed. However, in symmetrical phased arrays like APAR, this configuration introduces angular ambiguity when two targets from opposite directions have identical path differences, yielding the same phase value. To resolve this issue, an asymmetric weighting (AW) scheme is proposed.
Due to the ADBF architecture, the row elements are aggregated by RF signal combiners and provide digitized signals for DBF processing in the elevation direction. The AW scheme assigns specific weights to each row’s elements, shifting the beamforming energy center away from a uniform center alignment. This study introduces diagonal and serrated center-offset AW configurations, enhancing row-wise diversity to distinguish targets from opposite directions at the same range. By introducing phase offsets caused by specific weighting centers to each row element, the AW approach enables unambiguous target differentiation. The identical distance objects are calculated in distinguishable phases from each row element owing to a designated offset weighted center to mitigate ambiguity. The attached figure demonstrates the conventional linear center weighting versus the proposed diagonal AW scheme. The AW provides distinguished phase reference through each row element of a target.
A comprehensive Monte Carlo simulation, covering elevation and azimuth angles from -90° to 90°, was conducted using omnidirectional signal sources to emulate real-world hydrometeor scenarios, such as storms, hail, and rain clouds illuminated by a spoiled beam waveform. Results indicate that conventional symmetrical antenna configurations struggle with omni-directional echoes, while AW schemes effectively resolve angular ambiguities in spoiled beam scenarios.
By integrating ABF and DBF structures, APAR is designed to achieve an efficient volumetric coverage architecture, surpassing conventional ABF methods. The proposed AW scheme mitigates inheritance angular ambiguity, enhancing DBF functionality for hydrometeor observations.