The legal framework also defines the types of radio emissions that may be transmitted on aviation frequencies. This is important because transmitting an unauthorized type of emission can constitute misuse.

Radio emissions are classified by the mode of modulation and the type of signal:

A3E (Amplitude Modulation with Fully Modulated Double Sideband): This is the standard emission type for aeronautical VHF voice communication. All aircraft radios and all ATC stations use A3E. The "A" indicates amplitude modulation; the "3" indicates analog voice modulation; and the "E" indicates double sideband. In practical terms, A3E is what you hear on 121.5 MHz and all VHF aeronautical frequencies.

A1A (Amplitude Modulation with Keyed On-Off): This is morse code transmission. Morse is used for navigation aids (VORs, NDBs) and for some older emergency communications, but not for voice ATC in modern aviation.

F3E (Frequency Modulation with Fully Modulated Voice): This is the standard for UHF military aviation and some civil UHF bands. It is not used for standard ATC communication in the VHF band.

J3E (Single Sideband with Reduced Carrier): This is used for high-frequency (HF) communications over long distances, such as transoceanic flights. It is not relevant to VFR operations in European airspace, which is VHF-based.

As a VFR pilot, you will only transmit A3E (voice on VHF). You are not authorized to transmit other emission types. If your radio somehow transmits a different emission type, or if it transmits a spurious emission (an unintended signal on an unintended frequency), this is a technical violation and the radio must be taken out of service immediately.