Frequency Bands Desirable and Available

The most desirable frequency bands for commercial satellite communication are in the spectrum 1000-10,000 MHz. These bands are:

3700-4200 MHz (satellite-to-earth or downlink) 5925-6425 MHz (earth-to-satellite or uplink) 7250-7750 MHz (downlink)12 7900-8400 MHz (uplink)12

These bands are preferred by design engineers for the following primary reasons:

• Less atmospheric absorption than higher frequencies

• Rainfall loss not a concern

• Less noise, both galactic and man-made

• Well-developed technology

• Less free-space loss compared with the higher frequencies

There are two factors contraindicating application of these bands and pushing for the use of higher frequencies:

1. The bands are shared with terrestrial services.

2. There is orbital crowding (discussed earlier).

Higher-frequency bands for commercial satellite service are:

10.95-11.2 GHz (downlink) 11.45 -12.2 GHz (downlink) 14.0 - 14.5 GHz (uplink) 17.7 - 20.2 GHz (downlink) 27.5-30.0 GHz (uplink)

12These two bands are intended mainly for military application.

Above 10 GHz rainfall attenuation and scattering and other moisture and gaseous absorption must be taken into account. The satellite link must meet a BER of 1 x 10-6 at least 99.9% of the time. One solution is a space-diversity scheme where we can be fairly well assured that one of the two antenna installations will not be seriously affected by the heavy rainfall cell affecting the other installation. Antenna separations of 4-10 km are being employed. Another advantage with the higher frequencies is that requirements for downlink interference are less; thus satellites may radiate more power. This is often carried out on the satellite using spot-beam antennas rather than general-coverage antennas.

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