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Although radio spectrum spans hundred of billions of frequencies, which are measured in hertz, the lowest 1 percent of frequencies-those that fall below 3.1 gigahertz-account for 90 percent of the spectrum's use. Only a fraction of this region is set aside exclusively for government use, which is shared among DoD, NASA, the FAA, FBI, FEMA and other agencies.

Source: Defense Department and General Accounting Office