DISTRIBUTION OF RAIN HEIGHTS AND THEIR IMPLICATIONS ON TERRESTRIAL AND SATELLITE LINKS OVER NIGERIA

Abstract

The use of high-speed wideband internet services has informed the decision to transmit radio waves at Super High Frequency (SHF) and Extra High Frequency (EHF) bands. However, the quality of transmitted signals at frequencies above 10 GHz is usually degraded by atmospheric components such as rain, hail, snow, and atmospheric gases like oxygen, water vapor, etc. These atmospheric components interrupt radio wave signals in the form of attenuation and cross-polarization. Several studies have revealed that attenuation due to rain remains the most severe factor especially in the tropics. Rain height is an important rain parameter that determines the magnitude of rain-induced attenuation along the propagation links. The ITU-R recommended models for the prediction of rain attenuation in the tropics (Nigeria) have been proved to be at variance with actual values obtained by empirical measurements. This work focuses on the estimation and spatial distribution of rain heights in the four geo-climatic regions of Nigeria using precipitation data from Global Precipitation Measurement (GPM) data. Rain height is a level slightly above the Zero Degree Isotherm Height (ZDH) measured from the mean sea level. The rain heights were based on ZDH data extracted from the GPM. The results show that rain height increases from the northernmost (Sahel) part of the country down to the Coastal region with a spike in the Midland Savannah. The average annual rain heights obtained for Sahel, Midland Savannah, Guinea Savannah, and Coastal regions are 5.18 km, 5.22 km, 5.21 km, and 5.21 km, respectively. This shows that the 4.86 km recommended by ITU-R underestimate rain heights with percentage differences of about 6.1%, 6.9%, 6.7%, and 6.7% in the Sahel, Midland Savanna, Guinea Savannah and Coastal Zones, respectively. Comparison of rain heights with atmospheric parameters at the rain height level shows that only air temperature has strong positive correlation co-efficient. Hence, location-dependent relation fittings were deduced for each of theregions for prediction of rain height. The attenuations data from the derived rain heights were compared with the ITU-R recommendations at 99.99% signal availability. The research also reveals that a fade margin of about 38.5 dB is required at the uplink frequency of the Ka band (30 GHz) to achieve 99.99% signal availability in the Coastal zone. This implies that 99.99% signal availability could be achieved on earth-space and inter-terrestrial links operating at Ka band, if the link budget is properly designed to accommodate this fade margin. The overall results will provide vital information on rain-induced attenuation for optimum performance of earth- space links in Nigeria.