Evaporation ducts have a significant impact on electromagnetic (EM) wave propagation. The accurate prediction of EM propagation loss (PL) is important for the practical application of EM systems. However, the random weather processes and interaction of air–sea variables will cause complex spatial and temporal fluctuations in PL, especially for an over-the-horizon link. The physics prediction models show some limitations in practice. This paper proposes a data-driven PL prediction method based on the Gated Recurrent Unit network (GRU-PL). An X-band EM propagation system was developed and deployed for one month to collect PL of a 53 km link in the South China Sea for model training and testing. The GRU-PL model uses the air–sea variables as inputs and output the predicted PL. The root-mean-square error (RMSE) of the GRU-PL model is 7.04 dB, decreasing by 57.2% compared with the classical method. Furthermore, a method which uses the inversion evaporation duct heights (EDHs) as the output to train the GRU network, and then uses the parabolic equation model to predict the PL based on the GRU predicted EDHs, is also discussed. The RMSE is 7.3 dB. This method shows good potential in PL prediction of dynamically changing EM propagation links.

This article presents the measurement results of maritime wireless propagation channel at 8GHz in the South China Sea region. Large-scale path loss and small-scale fading of channels between a marine observation platform and a moving vessel are investigated. For large-scale path loss, comparisons are conducted between the free space model, two-ray model, parabolic equation model and log-distance model with the measured path loss data. The results show that the log-distance model agrees well with the general trend of the measured data, whereas the parabolic equation model has a better ability to capture the variations of the interference nulls. In addition, the free space model and two-ray model have larger deviation from the empirical data, implying that they are not suitable for the desired scenario. For small-scale fading, various distributions are considered to fit the fading. Based on the criteria of Kolmogorov–Smirnov statistics, the Rician distribution has the best goodness of fit, compared with the Rayleigh, Nakagami-m, Welbull, or Lognormal distributions. These research can provide good guidance for efficient designing and future deployments of distributed buoy networks in the South China Sea region.

A passive broadband source localization approach in the deep ocean is proposed based on the coherent matched-field processing combined with a Riemannian distance. The spatial coherence between hydrophones over frequency bandwidth is utilized for eliminating the unknown source spectrum. Three Riemannian distances are introduced for the measurement of the similarity between the data and replica matrices. The experimental results indicate that the proposed processors with a short array can effectively suppress the sidelobe and improve the localization performance. The source localization influenced by the factors such as the aperture, number of hydrophones, and desired bandwidth is also demonstrated.

Evaporation ducts, which form as a result of evaporation from the ocean, have a significant impact on electromagnetic (EM) propagation in coastal and marine environments. Theoretically, typhoons would substantially increase the mass, momentum, and heat exchanges at the air-sea interface and thus inevitably influence these EM propagation environments, including the duct conditions and propagation boundary conditions. However, the effects of typhoons on EM propagation have seldom been reported. In this study, anomalous propagation in the evaporation duct induced by Typhoon Kompasu (202118) was recorded by using an EM propagation system and automatic weather station. The length of the propagation link was 53 km, which was beyond the horizon. The evaporation duct's impact on EM propagation during Typhoon Kompasu was studied based on model simulation results. Propagation far beyond the horizon indicated that ducting was present during the typhoon passage. Furthermore, the approaching of the typhoon caused a decrease in path loss of about 20 dB. This was because the abnormal marine meteorological conditions induced by typhoons result in a high evaporation duct, which leads to a small path loss for EM propagation.

An evaporation duct, which is often occurs in the air–sea boundary layer, have a considerable impact on the remote sensing detection capability of electromagnetic (EM) systems. Tropical cyclones (TCs) substantially increase the transfer of water vapor and heat in the air–sea boundary and inevitably affect EM propagation environments. However, observations of duct environments and their effects on EM propagation during a TC process have seldom been reported. In this paper, X-band EM propagation systems that can operate continuously in the TC environment were specially developed, and observations of multiple over-the-horizon propagation links during TC Mulan (202207) were presented. The path loss (PL) of five links, and the surface variations both in Mulan’s area of high wind speed (WS) and eye were recorded. The average PL of a 53-km link was approximately 155 dB in the high WS area, while a 30 dB increase in PL was detected in Mulan’s eye. The inversion and simulation results show that the evaporation duct height (EDH) was high in the high WS area, which led to low PL in EM propagation; conversely, EDH was low inside Mulan’s eye, resulting in high PL.

Maritime high-speed over-the-horizon wireless communication is realizable through evaporation ducts. Detailed measurement, analysis, and modeling of duct channels are essential for application of this communication technique. In this paper, X-band electromagnetic (EM) wave propagation systems were developed and deployed for a 133-km over-the-horizon microwave link in coastal areas of the Yellow Sea. The propagation length was 7.7 times the line-of-sight length. Measurement results including the path loss (PL) and meteorological data were obtained during a 54-day period in autumn 2021. The long-term channel results were analyzed on the basis of statistical analysis and model simulations. Results showed that our measurement system, with a maximum measurable power loss of 200 dB, had connected with a probability of 56.2% during the measurement period. Model simulation showed that evaporation duct environments are not ideal in autumn, with an average evaporation duct height (EDH) of 10.6 m. The land breeze in autumn introduced dry and cold air to the link, which could promote evaporation of seawater and reduce PL by approximately 40 dB. Annual spatiotemporal characteristics of EDH showed that evaporation ducts are most suitable for over-the-horizon communication in spring, especially May.