Purpose: For several years following its publication in 1961, ARINC Characteristic 546 served to describe the basic VHF communications transceiver system of the airline industry. Towards the end of the sixties, however, the airlines began planning for the implementation of a VHF SATCOM system to provide long-range pilot-to-controller and company operational voice communications. As part of that activity, AEEC produced ARINC Characteristic 566.

ARINC Characteristic 566 described two versions of an airborne VHF SATCOM terminal. In the first of these, an ARINC 546 transceiver was supplemented with other units (modulation adapter/modem, preamplifier/switching unit and power amplifier) to form the terminal. In the second, a more sophisticated transceiver absorbed the functions of the modulation adapter/modem to eliminate this unit from the equipment complement. This transceiver became known as the "ARINC 566" transceiver.

25 kHz channel spacing in the aeronautical mobile communications environment was foreseen as inevitable by the airlines even before the preparation of ARINC Characteristic 546. As part of that activity, therefore, they laid careful plans to ensure that ARINC 546 transceivers would not be made obsolete when the need to adopt the closer spacing arose. Adequate transmitter frequency stability was specified, as were provisions for the easy modification of IF and audio bandwidths. All these features were carried over into the ARINC 566 transceiver specification when it was written some half-dozen years later.

It was not until April 1972, at the 7th Air Navigation Conference of the International Civil Aviation Organization, that the international standardization of system parameters for 25 kHz channel spacing was finalized. Reviewing this event a month later at the Spring 1972 AEEC General Session in Montreal, the airlines decided that a new ARINC specification should be written to replace Characteristic 546. They noted that many ARINC 546 radios now in service would reach retirement age before the 25 kHz implementation dates set by ICAO, and that no operational justification would exist on many aircraft for replacing them with the sophisticated and expensive ARINC 566 SATCOM transceivers. They determined that the new Characteristic should describe a simple, "no-frills", 25 kHz VHF transceiver for terrestrial use only. It should, they decided, supplement Characteristic 566 without downgrading the stature of that document as the airline industry specification for VHF SATCOM hardware.

ARINC Characteristic 566A sets out to fulfill this function.

Section 3.0 is applicable to the 25 kHz channel-spaced voice and data modes of operation. Section 4.0 is applicable to an optional 8.33 kHz channel-spaced voice mode of operation and defines only the characteristics specific to the 8.33 kHz channel-spaced mode of operation, such as the channeling, the frequency selection, the selectivity, the frequency stability, the frequency response, and the transmitter occupied spectrum.

The characteristics in Section 3.0 are also applicable to the optional 8.33 kHz channel-spaced voice mode of operation unless otherwise specified in Section 4.0.

COMMENTARY: The 8.33 kHz channel-spaced mode of operation for voice communications, if implemented concurrently with the 25 kHz channel-spaced modes of operation, would require dual-bandwidth receivers to ensure inter-operability of the airborne transceivers with the current (25 kHz) and proposed European narrowband (8.33 kHz) VHF voice communications systems. The narrowband mode of operation is limited to voice communications.