Refractivity Estimation Using a Phase-Coherent Vertical Array

This paper explores the capabilities of phase-coherent electromagnetic (EM) data in estimating the lower atmospheric refractivity, specifically the evaporation duct. An evaporation duct is the most common refractive structure over the ocean and it significantly affects the performance of marine radar and communication systems. Refractivity retrieval methods that rely on the EM signal amplitude have been extensively investigated in the literature. Here, we explore the performance improvement in duct estimation when both EM amplitude and phase measured across a vertical array is used instead of just the amplitude. This is done by computing and comparing the information content in amplitude and phase using the Fisher information matrix. The evaporation ducts are retrieved using only the amplitude, only the phase, and both amplitude and phase using both simulated and experimental data. To achieve this, an 8-element phase-coherent, software-defined radio-based vertical array is designed and built. This system is then deployed as part of multiple Coastal Land-Air-Sea Interaction (CLASI) Campaigns, collecting point-to-point phase-coherent propagation data across the vertical array, simultaneously with oceanographic and meteorological sensors measuring the evaporation duct. Phase-coherent inversion results are first compared in simulation to the classical amplitude-only inversion for varying frequency, signal-to-noise ratio, transmitter-receiver distance, number of array elements, overall array length, and ducting conditions. Then these results were verified using the measured array data from two measurement campaigns. The results showed that there is overall less information in phase relative to the amplitude. However, the phase-coherent refractivity inversion algorithm still outperforms the amplitude-only one by up to 33%, depending on the system setup and the ducting conditions.