STUDY OF TROPOSPHERIC PROPAGATION MECHANISM AND THEIR IMPACT ON LOS PROPAGATION IN A TROPICAL REGION

Abstract

To achieve good design on the Line-of-Sight (LOS) propagation, the knowledge of the structure of the radio refractive index of the lower atmosphere (Troposphere) is required. In this study,the measurement of the meteorological parameters namely: atmospheric pressure, temperature and relative humidity at the ground surface and at altitudes 0.5 km and 1.5 km were utilized based on 2 years (2010-2011) satellite data. The data utilized are those obtained from the NASA Goddard Earth Science Data and Information Service center website.The data were used to deduce the diurnal, seasonal and vertical profiles of the radio refractivity over Lagos (3.32°E, 6.4°N), a tropical region in the Southwestern part of Nigeria. The results show that refractivity are of high values in the wet season months (June, July, August and September) and low values in the dry season months (January, February, November and December). Diurnally, the refractivity values vary between 260 N-units and 288 N-units for the selected months in the dry season (February and November) while for the selected months in the wet season (July and August), refractivity values are between 270 N-units and 302 N-units for the year 2011. Meanwhile, the refractivity values for the dry season months (February and November) is between 262 N-units and 277 N-units, while the refractivity values in the wet season months (July and August) is between 276 N-units and 305 N-units for the year 2010. High radio refractivity values are observed in the early morning hours and late in the evening hours. The result also shows that the radio refractivity values decrease with increasing altitudes and vary seasonally with height. Further results also show that the strong variation in humidity may be responsible for the large values of refractivity gradient and this were observed more in the year 2011 at altitude of 1.5 km. The results also show that the propagation condition in the study location at the altitude 0.5 km and 1.5 km, have varying degree of occurrence but prevalently super-refraction condition takes the lead.That is super-refraction condition is prevalent both in the dry and wet season months at altitude 1.5 km for the two years of study. Sub-refraction condition also dominated some of the dry season months and then very low percentage time of occurrence of ducting condition which was observed in November 2010 at an altitude of 1.5 km. The implication of the prevalence of super-refraction and low percentage of ducting to propagation is that, radio stations around Lagos can have high radio signal strengths far beyond the line-of-sight (at a very long range). This could cause interference between widely separated radio stations operating on the same frequency. On the other hand, sub-refraction reduces radio horizon. It can therefore be recommended that more investigation should be carried out at more higher altitudes by considering more tropical regions in Nigeria.