Challenges in any characterization of the radio refractivity structure of the marine boundary layer

Summary form only given. It is a well-known property of the marine boundary layer (MBL) that refractive structure within this layer can significantly impact the propagation of radio frequency (RF) waves. This has implications for radio communications and radar system design in the UHF, SHF and EHF frequency bands (roughly 300 MHz to 100 GHz). There are many electromagnetic field solvers, such as parabolic-equation and raytrace codes, that can predict the impact of the MBL on RF propagation if given refractivity as a function of range and height within the MBL. Refractivity, in turn, can be computed from a deceptively simple, algebraic equation that is itself a function of temperature, air pressure and water vapor partial pressure. Despite the simplicity of this equation, proper characterization of temperature and water vapor profiles for input to the RF refractivity equation is currently the most challenging aspect of modeling RF propagation within the MBL. This difficulty arises from both the typical geometries of interest, and also the length scales that impact RF propagation. Geometries for low-elevation, over-water RF links or horizon-search radars are such that energy interacts with the MBL throughout the entire propagation path. Such geometries accentuate the fact that RF propagation is sensitive to relatively small gradients of water vapor within the MBL. This presentation provides insights into these difficulties by drawing on past work (e.g., Dockery et al, USNC-URSI NRSM 2010) on requirements for field-test temperature and humidity measurements for post-test reconstruction, which is the most demanding application. We also show how far these stringent requirements can be degraded while still retaining a qualitative “situational awareness” level of assessment. Particular emphasis is placed on characterization of evaporation ducts versus other radio-significant layers in the MBL, and the respective differences in accuracy requirements and cha- acterization procedures. This includes difficulties arising from the turbulent nature of evaporation ducting layers, which makes them difficult if not impossible to characterize with a single vertical sounding. Measured data and notional examples are shown, with the goal of providing general rules of thumb that apply to any application where RF propagation conditions in the MBL must characterized.