DISTRIBUTION OFRAINF ALL RATES OF SHORT INTEGRA TION TIME AND THEIR APPLICATIONS TO MICROWAVE PROPAGATION OVER NIGERIA

Abstract

In this study, measurement of long-term point and line rainfall rates carried out over a period of 30 years (1975 to 2004) in some locations in Nigeria, namely: Calabar, lkeja. Akure, Minna and Kano (representing different climatic zones) have been analyzed to investigate tropospheric effects on microwave and millimeter wave propagation. Three different types or rainfall data were used namely; 30-year surface data from measurements made at Meteorological stations using rain gauge, 10-year satellite data from Tropical Rainfall Measurement Mission (TRMM) over the locations and I-year data from vertical looking Micro Rain Radar data of one-minute integration time in Akure (data validation). The results arc presented in a form suitable for the prediction of microwave attenuation due to rainfall. The Moupfouma model in conjunction with Chebils model prediction was found to give the best prediction accuracy among the tested models for Nigeria. The rainfall rate exceeded for 0.01 % of the time (i.e. approximately one hour in a year) Rool, obtained for each of the locations, was found to be 106.8, 105.5, 129.8, 100.2 and 92.7 mmlh for Akure, Ikeja, Calabar, Minna and Kana, respectively. These results are in good agreement with the results from other tropical locations. Comparison of these results with ITU model shows that the ITU model underestimated the rainfall rate exceeded for 0.0 I% or the time in the locations. One-minute rain rate maps for 0.1 %, 0.0 I% and 0.001% of the time were then generated. The results from these maps present significant and interesting tools necessary for obtaining a better estimate of the rain attenuation in Nigeria. They are also of interest and value as contributions to worldwide statistical data for the tropical regions of the world. The rain-induced attenuation over the terrestrial line-of-site links for path lengths ranging 1-20 km was also tested using different existing attenuation models such as Crane global, ITt JRand Moupfouma model for terrestrial path at the lower frequency of 12 GHz and at the higher frequency of 30 GHz for 0.1 % and 0.0 I% of time over each or the locations. From the rain attenuation obtained at these frequencies and for a path length of 20km, the lTU-R model gives the lowest attenuation values in all the locations studied in Nigeria, while the path attenuation predicted by the Moupfouma model was the highest. The relative performance of a number of attenuation prediction models along the Earthspace path was also examined over three elevation angles of 23°, 42.50 and 55° for Ku-and Kafrequency bands. The comparisons show that ITU model proves to be a success in predicting the attenuation exceeded using the appropriate meteorological parameters from this region. The study further revealed that the rain-induced attenuation obtained over the different elevation angles, particularly at Ku- and Ka-band frequencies and for the entire INTELSAT satellite footprints over the Indian Ocean will be higher than rain attenuation for satellite footprints over the Atlantic Ocean Region (AOR). The same was (rue of the parked NIGCOMSAT-I. The results of the rain attenuation obtained were also used to develop contour maps for 0.1%, 0.01% and 0.001% of the time. The information provided in the maps are very useful lor preliminary system design most especially for the proposed Nigerian Communication Satellite -2 (NIGCOMSAT-2) to be launched in 2010. It will also serve as useful tool for planning and design of regional or hemispherical VSA T broadband-access' initiatives and for rural telecommunication networks in Nigeria. The result of the estimation of XPD at varIOUS elevation angles and at frequencies ranging from 10-35 GHz shows the dependence of XPD on frequency, co-polar attenuation and rainfall rates. The orthogonal channels at the same frequency situated at the coastal region is prone to higher amount of unwanted signal mostly at tbe elevation angle of 23° (for • communication links over the Indian Ocean) than at other elevation angles considered in this report.