47 CFR § 90.1333
Restrictions on the operation of mobile and portable stations
February 16, 2021
(a) Mobile and portable stations may operate only if they can positively receive and decode an enabling signal transmitted by a base station.
(b) Any mobile/portable stations may communicate with any other mobile/portable stations so long as each mobile/portable can positively receive and decode an enabling signal transmitted by a base station.
(c) Airborne operations by mobile/portable stations is prohibited.