47 CFR § 87.421 - Frequencies.
The Commission will assign VHF frequencies after coordination with the FAA. Frequencies in the following bands are available to control towers and RCOs. Channel spacing is 25 kHz.
(a) The frequency 123.100 MHz is available for use by control towers and RCOs at special aeronautical events on the condition that no harmful interference is caused to search and rescue operations in the locale involved.
(b) Frequencies in the bands 200.0-285.0 and 325.0-405.0 kHz will normally be assigned only to control towers and RCOs authorized to operate on at least one VHF frequency. The Commission may assign frequencies in these bands to entities that do not provide VHF service in cases where granting such an application will not adversely affect life and property in the air.
(c) Frequencies listed in the introductory paragraph of this section are available to control towers and RCOs for communications with ground vehicles and aircraft on the ground. The antenna heights shall be restricted to the minimum necessary to achieve the required coverage. Channel spacing is 25 kHz.
(d) 121.500 MHz: emergency and distress only.
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