sons: (1) in bands shared with terrestrial services such as the popular 4-GHz band to ensure noninterference with those services, and (2) in the satellite itself, which can derive power only from solar cells. It takes a great number of solar cells to produce the RF power necessary; thus the downlink, from satellite to earth, is critical, and received signal levels will be much lower than on comparative radiolinks, as low as - 150 dBW. A third problem is crowding. The equatorial orbit is filling with geostationary satellites. Radio-frequency interference from one satellite system to another is increasing. This is particularly true for systems employing smaller antennas at earth stations with their inherent wider beamwidths. It all boils down to a frequency congestion of emitters. It should be noted that by the year 2000, we can expect to see several low earth-orbit (LEO) satellite systems in operation. These satellites typically orbit some 500 km above the earth. 9.3.4 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) 12 These two bands are intended mainly for military application. CONCE