RADIO PHYSICS AND RADIO ASTRONOMY

Subject and Purpose. Th is paper addresses the challenge of measuring the angular coordinates of over-sea low-altitude targets and strives to improve the measurement accuracy of radar systems that employ high-resolution algorithms implemented through active phased array antennas (APAAs). The primary purpose is to substantiate the eff ectiveness of off-axis target illumination in the context of sea clutter and multipath. Off-axis target illumination reduces the depth of the probing signal interference fading over the radar-to-target path. The investigated illumination method focuses on mitigating errors in target angle measurements by multi-channel methods based on high-resolution algorithms, such as root-MUSIC. One of the main spoilers of measurement accuracy when using those algorithms is the noise in the radar receiving channels. Off-axis target illumination helps avoid a deep fade of the signal, thereby enhancing the signal-to-noise ratio (SNR) in the receiving channels of APAA-equipped radar systems, which contributes to target tracking stability despite destructive multipath interference.

Methods and Methodology. The study employs computer simulations within a multipath radio-wave propagation model for a rough sea surface, allowing for both specular and diffuse reflections. The power distribution of diffuse reflections and their mapping along the propagation path are evaluated using the empirical glistening-surface model, the most advanced to date.

Results. Quantitative assessments of the effectiveness of off-axis target illumination have been obtained in the context of fading conditions produced by multipath radio wave propagation over the sea surface. The optimal illumination angles have been identified. They help avoid deep fades over a wide range of sea states. These optimal angles are confined within 0.6 to 0.75 of the beamwidth of the radiation pattern. The lower end of the indicated range refers to the diffuse sea clutter, and the upper end corresponds to the specular reflection of the probing signal. In the most adverse cases, a gain in the SNR reaches 6—8 dB.

Conclusions. It has been confirmed that off-axis illumination allows decreasing the depth of the illumination signal fades, which ensures more stable target tracking by angular coordinates. Recommendations for selecting off-axis illumination angles have
been formulated.

Key words: illumination level, off-axis illumination, active phased array antenna (APAA), low-altitude target, rough sea surface, multipath propagation, elevation angle, antenna pattern, computer modeling

Manuscript submitted  15.06.2025

Radio phys. radio astron. 2025, 30(4): 250-257

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