Monday, February 2, 2015

Rain Fade

Rain fade refers to the effect of moisture in the atmosphere along the line of site of wireless transmission, whereby the moisture absorbs some of the radio energy, increasing the path loss.

Absorption is a function of radio frequency, and involves molecular interactions.

The amount of atmosphere along a given transmission site line is relative to the elevation angle, in that a steering angle to the horizon will intersect much more than an angle to the overhead zenith.

In order to provide for continuing coverage, a margin must exist to overcome the variable path loss due to rain fade.

The margin can be accounted for statistically using a model for a given region.

Some regions receive a lot more rain, and heavy rain showers are the most feared, albeit localized.

The margins can be based on assuming adequate performance for the majority of the time, that being 99% or 99.9% of the time.

Here is an example for two locations, one in the UK and one in Singapore.

Because Ka-band radio energy is absorbed more readily than Ku-band, if accepting a margin that is inadequate 1% of the time shows a modest difference in the UK, but a 9 dB difference in Singapore.

 If protecting to 99.9% of the time, the UK margin is 5 dB higher for Ka, and in Singapore about 41 dB higher.

It is reasonable to suggest the 0.1% Singapore case would be unreasonable for Ku as well.

Every satcom link involves uplink and downlink.

The feeder links through the Teleport are as susceptible to rain fade as the service links to the remote terminal.

Teleport locations may not be ideally situated from rain fade, but geographic diversity through a second teleport can provide a highly reliable combination.

Significant rain fall is generally quite localized.

Teleports can utilize methods to boost uplink power levels to account for uplink rain fade.

Airplanes flying above the weather will suffer less from rain fade.

Airplanes operate at low altitude and on the ground as well, subject to rain fade.

Airplanes moving through the atmosphere will encounter varying amounts of rain fade, adding an extra variable to any signal measurements.

Peter Lemme

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