2. Microwaves and rainfall:

The Ku band microwave signals arriving from the satellites have a very short wavelength, about 2.4 cm, slightly less than one inch. During periods of heavy rain, the raindrops between the satellite and the receiving antenna can act like small antennas, absorbing some or most of the signals, an effect known as rain fade.

For similar reasons, the satellite operators need to carefully monitor their transmit or uplink signal strength (from earth to the satellites) because these signals are similarly attenuated when passing through rain. However, when there is rain fade in the uplink, the attenuation affects all subscribers. The solution to this is to increase the uplink transmit power by an amount sufficient to overcome the fade effects.

The approximate rainfall attenuation at 12.5 GHz, assuming that one mile or 1.6 Km of the path from satellite to receiver passes through rain, is given in the table below. Rainfall is measured as depth per hour:

Downlink Attenuation

To help put things into perspective, attenuation is measured in dB or decibels, a base-10 logarithmic relative-power scale. This means that 3.0 dB attenuation or loss equates to a loss of one-half the signal power while 10 dB attenuation is a reduction to one-tenth the normal power.

Even slight rainfall attenuation would affect the quality of the received signals except that DBS uses an error reduction technique known as forward Error Correction (FEC) described in the next section.

C O N T E N T S

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