9.3 Radio Noise Effects

The major sources of radio noise affecting satellite communications are shown in the figure 25.

Figure 25: Minimum expected external noise from natural and man-made sources 10 MHz to 100 GHz [Ref 9].

The major effects are:

the Sun (but this is only important if you are pointing at or close to it [note that the sun occupies 32 minutes of arc and is not a point source]). Also note that the radio noise from the sun varies over time and is related to the sunspot cycle. Values of 10,000K or more may be encountered. This will knock out satellite communications during sun transits.

• Other celestial objects such as the moon (200K and 32 minutes of arc), planets and radio stars also contribute radio noise but these are much less than the sun and are generally ignored for small earth stations. (Though stable radio stars can be used to calibrate the gain performance of large earth stations)
• rain (and heavy cloud). Contribution to system noise temperature due to rain or clouds is given by the expression:

Note the system performance loss resulting from the system noise temperature increase adds to the system loss due to rain attenuation.

• atmospheric water vapour and oxygen. For all except very low elevation angles and very high frequencies (or frequencies near the OH- absorption lines eg 22.3 GHz) this is generally only a few K.
• cosmic background noise around 2.7K (usually considered negligible).

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