Evaluation of daily tropospheric radio refractivity, refractivity gradient and geoclimatic factor over Otukpo, Benue State, Nigeria

Abstract

Radio refractivity is a key parameter for reliable terrestrial microwave-link design because the tropospheric propagation of radio waves is strongly affected by atmospheric refractive properties. Daily meteorological data spanning 23 years (2000--2022) were analysed to characterise surface radio refractivity (N_s), estimate refractivity at 100 m altitude (N₁₀₀), determine the radio refractivity gradient (RRG), and compute the geoclimatic factor (K-factor) over Otukpo, Benue State, Nigeria (7.22^oN, 8.15^oE; 91 m altitude). Surface refractivity was calculated using the International Telecommunication Union--Radiocommunication Sector (ITU-R) P.453-14 three-term equation with temperature, relative humidity, and surface pressure data from the National Aeronautics and Space Administration Prediction of Worldwide Energy Resources (NASA POWER) reanalysis database. N₁₀₀ was estimated with the ITU-R exponential refractivity profile, and the RRG and K-factor were computed using ITU-R P.530-18. Seasonal and interannual variability were assessed from monthly and annual averages. The grand mean surface refractivity was 374.21 N-units, about 61 N-units higher than the height-adjusted ITU-R reference value (approx 313 N-units). Refractivity showed a distinct bimodal seasonal pattern, with the highest value in May (384.32 N-units) and the lowest value in January (350.44 N-units) during the Harmattan season. The mean RRG and mean K-factor were estimated as -50.57 N-units km^-1 and 1.4757, respectively, corresponding to an effective Earth radius of about 9,402 km. These values were derived from observed surface refractivity climatology using the ITU-R model and are not direct measurements of vertical atmospheric structure. The results show that refractivity over Otukpo is generally higher than standard ITU-R conditions, indicating that locally derived propagation parameters are more representative for terrestrial microwave-link planning. These estimates should be confirmed in future studies using upper-air or high-resolution atmospheric profile data.