The type of mission a satellite program is designed to accomplish has a large influence on the selection of frequency bands it uses. The required bandwidth must be a fraction of the operating frequency, so the more bandwidth required, the higher the operating frequency. The user type and location (e.g., fixed site vs. ship) determine the kind and size of equipment to be used.
Spectrum allocations associate portions of the electromagnetic spectrum with various radio services. A satellite system must select frequencies that are within the allocations that match its mission. Furthermore, a space system must select frequencies allocated for space use. The spectrum environment—the number of other users and the interference they would cause—has an influence on the frequency selection.
Equipment performance also affects frequency selection. The performance of antennas is a good example. For an antenna of a given size, beamwidth is inversely proportional to frequency, so the higher the frequency, the narrower the beam that can be formed. On the other hand, the accuracy required on the antenna's surface tightens with increasing frequency. In contrast, satellite amplifier efficiency (useful output power divided by input prime power) is relatively insensitive to frequencies under 15 gigahertz, then gradually decreases as frequency increases.
The disturbances and attenuation caused by the atmosphere and ionosphere influence frequency selection. Ionospheric disturbances are high at low frequencies and drop with increasing frequency; they are generally insignificant above 2 gigahertz. Atmospheric attenuation is low at low frequencies and increases significantly above 10 gigahertz.
Military satellites operate at a variety of frequencies. Those serving mobile users operate below 1 gigahertz in a band long used for terrestrial military communications. DSCS operates at 7 and 8 gigahertz for several reasons: the allocation is appropriate, the higher frequencies support the wider bandwidths needed for the mission, and building the equipment was possible in the 1960s, when the system began.
Milstar uses 20 and 44 gigahertz because, as in the DSCS example, the allocation is appropriate, the bandwidth is wide, and building the equipment is possible. In addition, the propagation disturbance caused by nuclear explosions is lower at these frequencies than it would be at lower frequencies. Atmospheric attenuation, although high, is tolerable. Milstar crosslinks use frequencies near 60 gigahertz because the very high atmospheric attenuation between 55 and 65 gigahertz isolates the crosslinks from terrestrial attacks and because the allocation is appropriate.