Signals from geosynchronous satellites spread out and weaken in power on their journey to a receive site below. While this free space path loss can be easily calculated there are other more unpredictable losses. These include losses due to scattering and absorption by water in various forms, by oxygen molecules and by particulates in the atmosphere. Furthermore, atmospheric turbulence, antenna pointing errors, waveguide losses and ionospheric refraction measurably reduce downlinked signal power. Atmospheric absorption and antenna depointing losses may each typically be in the 0.2 to 0.3 dB range.en skies are clear, a satellite dish detects noise from the warm ground solely through its side lobes and very low temperature galactic noise via its main lobe. However, when rain falls and to a lesser extent when fog, snow and clouds are present, the main lobe "sees" a higher sky temperature than normal via its main lobe. This increase in detected noise temperature is typically about 1.2 dB. The decrease in signal power and increase in noise power lowers the C/N ratio. A receive system that operates near threshold under ideal conditions may fall below threshold during, for example, a rain or wind storm. Ku-band systems are particularly sensitive to rain fading. During a severe storm signal power could fall by as much as 10 dB. System designer must build a fade margin into a satellite reception system that depends upon local atmospheric conditions as well as availability, determined by the minimum percentage of down time. For example, in a very rainy climate to obtain a 99.99% availability at Ku-band a fade margin of 10 dB or more may be required.