Simulation Analysis and Experimental Study on the Echo Characteristics of High-Frequency Hybrid Sky–Surface Wave Propagation Mode

In high-frequency hybrid sky-surface wave radar, the echo characteristics of hybrid sky-surface wave propagation channel are extremely complex in spatial, time, and frequency domains, which seriously affect radar performance. The mechanisms and echo characteristics are theoretically and experimentally analyzed in this paper. Based on the hybrid sky-surface propagation model, the signal-to-noise ratio equation of the channel echo and ionospheric direct wave are initially derived. The simulation method is developed, which considers bistatic layout, ionospheric propagation, sea surface propagation, and radio noise. The echo characteristics by sweep frequency detection are simulated and analyzed. Using the newly developed hybrid sky-surface wave experimental system, the experimental results by sweep frequency detection are first achieved in China and abroad, which are consistent with the theoretical analysis, and the validity of the simulation is verified. Then, the characteristics of first-order sea clutter in different azimuths are theoretically and experimentally analyzed, and it is found that the bistatic layout and ionosphere play a significant role in the Doppler broadening and path distribution of the sea clutter. Finally, the experimental results by joint sweep and fixed frequency detections are analyzed. The echo characteristics are extremely complex with frequency. Specifically, owing to multimode echoes overlap, the spread of the sea clutter and ionospheric direct wave signals would seriously affect ship target detection in part of the operating frequency band. It is shown that the detection and diagnosis for hybrid sky-surface channel in spatial, time, and frequency domains are crucial for target detection performance. The research results on the propagation mechanisms and echo characteristics could provide important supporting function for layout design, frequency selection, target detection, and location of the radar.