Analysis of Fade Duration Along Earth-Space Paths in the Tropics

A database of fade duration statistics was built as part of a study aimed at the analysis of propagation characteristics in the tropics. Not unexpectedly, the number of statistics available for the tropics is small and the periods of measurement are short compared to those for temperate regions. Almost all the tropical data in the database was collected at Ku-band. Fade duration has been frequently modelled as a combination of two functions, one of them taking account of shorter durations associated with tropospheric scintillation and the second accounting for longer durations caused by rain. Three of the combinations are: power law+log-normal, exponential+log-normal, and a double log-normal. Still other models have proposed a sum of two or three negative exponentials. Fits to Ku-band distributions for several tropical locations showed good agreement between measurements and the sum of three exponentials. The best fits to Ka-band distributions from ACTS data collected at Tampa, Florida, were produced by a double log-normal. Based on this analysis, the performance of five prediction models was tested: the ITU-R, COST 205, Dissanayake-Haidara (D-H), Braten, and Lekkla. The best predictions for the tropics were produced by the COST 205 and the D-H models. The quality of predictions for tropical and temperate locations was comparable. A second model test was carried out using only statistics derived from radiometric measurements. Finally, some potential improvements to the models were identified and are indicated at the end of the paper.