INVESTIGATION OF RADIO REFRACTIVITY PROFILE OVER WEST AFRICA CLIMATE USING ERA INTERIM DATA

Abstract

Antenna design and orientation have important effects on radio wave propagation. The existence of abnormal propagation events (ducts, super and sub refraction) offers valuable information about the propagation of radio waves and propagation conditions in the atmosphere. Atmospheric properties determining peculiarities of radio propagation and propagation conditions are characterized by the radio meteorology parameters. The analysis of radio wave propagation behaviour is carried out with the aim of improving radio wave communication service in tropical and sub tropical regions of West Africa. The meteorological parameters such as pressure, temperature and relative humidity, at the surface, 100 meters altitude and planetary boundary layers across the West Africa at resolution of 0.25o × 0.25o were obtained from the archive of the European Centre for Medium-Range Weather Forecasts (ECMWF) Era interim. ERA-Interim is produced at T255 spectral resolution (about 80 km) and covers the period from January 1979 to date, with product updates at approximately one month delay on real-time basis. The time resolution of the products was 6 hours per day (00 UTC, 06 UTC, 12 UTC and 18 UTC). Radio refractivity, refractivity gradient and geoclimatic factor were computed from the atmospheric parameters obtained. Polynomial regression equations were developed to model geoclimatic factor from surface temperature and relative humidity. The results of the computations were used to investigate spatial propagation conditions across West Africa region. The result showed negative relative humidity difference between surface and 100 meter altitude across West Africa with higher negativity gradient in the north, while temperature difference increases northward with positive gradient in the north and negative gradient in the southern part. Likewise, radio refractivity values for the rainy season months are higher than that of the dry season months. Similarly, maximum values of radio refractivity were obtained at midnight while minimum values were obtained at mid-day. The spatial distribution of anomalous conditions obtained from refractivity gradient shows the occurrence of ducting at mid night and early morning at Soudano Sahel region during the dry season and prevalence of ducting at coastal region throughout the season. Low percentage of super refractive conditions were observed in part of coastal and low altitude area during the rainy season. The occurrence of sub refractive conditions were discernable in most of high altitude areas at mid-day. The result of the gradient at planetary boundary layers shows that super refraction is only the anomalous propagation in this region. The implication of these is that radio signal will experience multipath fading and even blackout condition in ducting and super refractive environment which enhance and increase radio horizon. In sub refractive condition radio signal will experience diffraction loss which reduces radio horizon and causes radio outage. The geoclimatic factor which cater for geographical and climatic conditions in multipath fading distribution exhibits diurnal, monthly, seasonal and yearly variations. Validation and testing of the developed model for geoclimatic factor yield encouraging results in most of the locations.