Studies on the refractive index structure in Singapore

The statistical analysis of the atmospheric refractivity gradient in Singapore, which is in the vicinity of the equator, is presented. The results are obtained from the radiosonde observation (RAOB) data over 6 years, measured at 50-m height interval as opposed to the commonly used standard pressure levels. This is done considering that it is useful to look into the refractive index structure using more detailed refractivity data. This analysis is part of a project concerning the study of the propagation characteristics of a 1.9 GHz link. In the analysis, the refractivity gradient over the lowest 100 m and the first 1 km are computed and compared with the values given in the ITU-R recommendation. The statistical distribution of the refractivity gradient for Singapore is compared with the well-known normal-exponential distribution model of Akiyama (1971). Subsequently, the correlation between the monthly statistics of the refractivity gradient for the first 1 km and monthly mean surface refractivity are found. The results can be used to estimate the distribution of the gradient using only surface refractivity information. This is important due to the fact that surface weather data is easier to obtain at different locations along the radio link. In order to predict the occurrence of anomalous propagation, the duct occurrence probabilities are also studied.