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

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 (Ns), 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 (7.22oN, 8.15oE; 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.