47 CFR § 87.107 - Station identification.
(a) Aircraft station. Identify by one of the following means:
(1) Aircraft radio station call sign.
(2) The type of aircraft followed by the characters of the registration marking (“N” number) of the aircraft, omitting the prefix letter “N.” When communication is initiated by a ground station, an aircraft station may use the type of aircraft followed by the last three characters of the registration marking. Notwithstanding any other provision of this section, an aircraft being moved by maintenance personnel from one location in an airport to another location in that airport may be identified by a station identification consisting of the name of the company owning or operating the aircraft, followed by the word “Maintenance” and additional alphanumeric characters of the licensee's choosing.
(3) The FAA assigned radiotelephony designator of the aircraft operating organization followed by the flight identification number.
(4) An aircraft identification approved by the FAA for use by aircraft stations participating in an organized flying activity of short duration.
(b) Land and fixed stations. Identify by means of radio station call sign, its location, its assigned FAA identifier, the name of the city area or airport which it serves, or any additional identification required. An aeronautical enroute station which is part of a multistation network may also be identified by the location of its control point.
(c) Survival craft station. Identify by transmitting a reference to its parent aircraft. No identification is required when distress signals are transmitted automatically. Transmissions other than distress or emergency signals, such as equipment testing or adjustment, must be identified by the call sign or by the registration marking of the parent aircraft followed by a single digit other than 0 or 1.
(d) Exempted station. The following types of stations are exempted from the use of a call sign: Airborne weather radar, radio altimeter, air traffic control transponder, distance measuring equipment, collision avoidance equipment, racon, radio relay, radionavigation land test station (MTF), and automatically controlled aeronautical enroute stations.