Section 90.281 - Restrictions on operational fixed stations in the 421-430 MHz band(a) Except for control stations, operational fixed facilities will not be authorized in the 421-430 MHz band. This does not preclude secondary fixed tone signaling and alarm operations authorized in § 90.235 .(b) Control stations associated with one or more mobile relay stations will be authorized only on the assigned frequency of the associated mobile station. Use of a mobile service frequency by a control station of a mobile relay system is subject to the condition that harmful interference shall not be caused to stations of licensees authorized to use the frequency for mobile service communications.52 FR 6158, Mar. 2, 1987, as amended at 54 FR 38681, Sept. 20, 1989