Atmospheric noise data for tropical regions


  • S. V. C. Aiya


The known unsatisfactory nature of atmospheric noise data based on pre-1964 measurements for tropical regions is due to the inadequacy of the dynamic range of the measuring equipment in recording local storm noise. This high-amplitude, highly impulsive noise arises from the electrical discharges inside the cloud at heights varying from about 3 to over 10 km above the mean sea level in the tropics. Direct ray reception of the radiation from such discharges is expected for source distances of the order of 300 km. The number of local storm days is experimentally found to be about three times the recorded number of thunderstorm days and can exceed 50% of the days of a season in the tropics. Electrical discharges corresponding to one lightning flash give rise to one noise burst in the receiver. The structure of such noise bursts and their amplitude and time characteristics have been studied. Highest amplitude noise present for 10% of the time in a short period of time like 5 min is the portion-relevant for interference purposes, and this arises from the noise bursts described. Its short-term characteristics are log normal. The long-term characteristics of the short-term mean values are also log normal. Consequently, noise data are furnished in terms of mean values and standard deviations. Along with such noise data, the required signal-to-noise ratios for different types of service are also given.