Refractivity Inversions From Point‐To‐Point X‐Band Radar Propagation Measurements

Abstract

Dynamic refractive environments within the marine atmospheric boundary layer (MABL) pose difficulties in the prediction of X‐band radar wave propagation due to natural phenomena such as evaporation ducts (ED). This study utilizes a unique data set collected during the Coupled Air‐Sea Processes and Electromagnetic Ducting Research (CASPER)‐East field campaign, including multiple refractivity estimation methods and twelve point‐to‐point (PTP) electromagnetic datasets, to assess the efficacy of PTP inversion techniques for remote sensing of atmospheric refractivity within the MABL. Comparison of refractivity between the inverse and other refractivity methods show reasonable evaporation duct height estimates by the inversion, and inverse‐based propagation predictions are also shown to be more accurate than propagation based on other refractivity prediction methods: numerical weather prediction, theory, and in situ atmospheric measurements. These results propose the effectiveness of a PTP metaheuristic radar inversion to remotely sense refractive environments from radar propagation measurements in stable and unstable atmospheric conditions.

Relative Influence of Sea State and Mean, Turbulent, and Heterogenous Refractivity on X-Band Propagation

“…Furthermore, at this transition between evaporation and mixed layers, smoothing is performed to ensure a continuous vertical gradient along the profile at the transition from logarithmic (duct) to linear (mixed layer) refractive gradients. The potential refractivity gradient [26], 𝑐 / , allows modification of the shape of the refractivity profile within the evaporation layer for a specified duct height [9], [12], [14]. The evaporation layer extends from the surface to 𝑧 1 ≡ 2𝑧 0 .…”

“…While this model may not incorporate all recent modifications to sea state models for radar remote sensing, it accounts for most, if not all, ocean wavelengths (swell to capillary) influencing propagation in a marine environment. Penton and Hackett [12] and Pastore et al [9] implement ∅ 8 showcasing the fidelity of the wave model in estimating propagation for low-grazing angle (and evaporation ducting) scenarios. Moreover, Pastore et al [9] demonstrate incorporation of ∅ 8 for retrieval of refractivity over a rough surface via RF inversions utilizing RF measurements.…”

“…EM propagation predictions are commonly estimated via parabolic wave equation (PWE) methods, a physics-based technique that accounts for EM wave refraction, forward scattering, and attenuation due to atmospheric and surface boundary conditions [1]. PWE is widely employed as an EM propagation prediction method within the MASL (e.g., [2]- [9]).…”

“…Description of the atmosphere within PWEs is commonly prescribed as modified refractivity (𝑀) distributions, which are a function of the prevailing meteorological conditions [13]. Commonly, parametric refractivity models are implemented to model refractivity profiles [2], [6], [9], [12], [14], where model parameters generally include refractive characteristics that are known to affect propagation -such as evaporation duct height (EDH), duct curvature, and mixed layer slope [3], [9], [11], [14]- [16]. Parametric models such as these typically neglect turbulent fluctuations and spatial heterogeneity in the refractive environment, which can lead to discrepancies between measured and modeled EM propagation [4], [5], [17], [18].…”

“…While the atmospheric and sea-surface estimation methods discussed are widely verified within PWEs [2], [6], [9], [12], [14], [22], differences between measured and modeled propagation persist. These differences stem from shortcomings of these estimation methods and can potentially be attributed to: (i) not accounting for refractive fluctuations due to turbulence, (ii) inaccurate representation of the sea-surface, and (iii) not accounting for spatially heterogenous environments in which evaporation duct height (EDH) and strength can vary with range [5], [17], [18], or any combination thereof.…”