Estimation of the Evaporation Duct Refractivity Profile from Modal Analysis and X-Band EM Propagation Measurements

An important application of air-sea interaction research is characterizing the refractivity variations in the marine atmospheric boundary layer (MABL), which are responsible for electromagnetic (EM) ducting. In particular, radar and radio communications near the surface in the maritime environment are strongly influenced by the evaporation duct (ED). The duct can greatly extend the propagation distance, but can also create fading zones. It is therefore of importance to be able to predict ducting conditions from atmospheric and EM propagation models.

While the refractivity profile can be directly measured or calculated from pointwise atmospheric data, EM propagation measurements can also be used to estimate the ED parameters by inverse solution with an EM model. Since the development of high-speed computers and the parabolic wave equation method, long-distance EM propagation over land and sea has mainly been modeled numerically. A large library of propagation data is generated numerically to simulate the measurement using a variable set of ED parameters (primarily the duct height). The parameters corresponding to the best match with the measured data become the inverse solution. However, this brute force numerical approach loses the physical insight offered by analytical methods such as ray tracing and waveguide modal analysis.

In this paper we are interested in modal analysis because the long-distance propagation in the ED is best described in terms of trapped modes. The figure below plots the modal attenuation constants (in dB/km) for the lowest order ED modes as a function of evaporation duct height (EDH). It is observed that the first mode always has the lowest attenuation vs. distance. Therefore, at some distance only one or two modes will be significant. As will be seen in the presentation, this is exactly the behavior of the EM fields in the ED at long distances. Furthermore, the height-gain function of each mode is independent of the transmitter height. These properties make modes ideal for the ED inversion problem by creating a library of the lowest order modes as a function of duct height. Either range sample points or vertical sample points from measurements can be used to compare to the particular attenuation rate or vertical field of the modes, respectively, to deduce the refractivity profile.

The Coupled Air-Sea Processes and Electromagnetic-ducting Research (CASPER) East Coast Intensive Operations Period (IOP) was conducted off the coast of Duck, NC, during October-November of 2015. Measurements were obtained of the one-way propagation loss between the emitters and receiver as a function of range and height. The concurrent meteorological and oceanographic measurements at the ocean surface and within the MABL, such as air temperature, sea surface temperature, relative humidity, air pressure and wind speed, were also measured to infer the refractivity profile using the Navy Atmospheric Vertical Surface Layer Model (NAVSLaM). Inversion results using modal analysis from the CASPER-East experiment and the comparison with the refractivity profile predicted by NAVSLaM will be presented.