Radio Wave Ducting

Ever since Marconi's trans-Atlantic telegram in 1901, over-the-horizon (OTH) propagation of radio waves has drawn interest. Today, the radio science community understands that there are numerous mechanisms for OTH propagation. Of particular relevance to modern maritime radars and communication networks is a mirage-like OTH mechanism called "ducting." Much is known about ducting from decades of research and field observations, yet it remains an oft-misinterpreted and difficult-to-measure phenomena. This apparent contradiction is resolved by understanding the technical nuances of ducting: the sensitivity of radio waves to small changes in humidity; the waveguide-like responses of different kinds and sizes of ducts to different radio frequencies; the fact that ducting occurs in boundary layer flow, with complex fluid dynamics that are difficult to measure or model; and the interplay of air and ocean waves, which adds to the complexity of both fluid flow and electromagnetic scattering. Furthermore, despite a great deal of empirical investigation at frequencies below 20 GHz, technological advances and a desire for more bandwidth push system designs to higher radio frequencies. This, in turn, pushes engineering past the bounds of validated numerical models. This talk attempts to frame both the well-understood and the fruitful unknown regarding radio ducts. This includes recommendations for growing the role of numerical weather prediction (NWP) in investigating and predicting ducting events.