Estimation of background noise in HF-band

This document reports on how to estimate background noise (Gaussian Noise) in the HF-band. The daily medians are affected by various kinds of noise. To obtain basic noise, hourly APDs (Amplitude Probability Distribution) graphs are plotted on a Rayleigh graph, and then the lowest Gaussian approximated lines are drawn. New medians are then calculated using the approximated lines. The result shows that the noise figures are proportional to logarithmic frequencies, as the ITU-R report described. The parameters of proportion mostly agree with those from three observations, but the values are a little larger than those recommended by ITU-R.