Aircraft Communications

A look at analog and digital communications.

It used to be that a wing wave or rapid tail deflection was all that was needed by the pilot of an aircraft to acknowledge a visual queue from a person on the ground. The evolution of electronic communications equipment is almost as dramatic as the evolution of the aircraft. In fact the radio age came about at almost the same time as the Wright Brothers took to the air. (Sorry but I could not resist the pun.)

Traditional aircraft communications are based on analog voice on either a Very High Frequency (VHF) or High Frequency (HF) radio waves. In the mid 1980s the use of data-based communications became a reality. Airspace management is transcending into the computer age and as new requirements evolve and the choice of communications technologies expand, regulating the world’s air traffic flow can safely become more automated. Aircraft are currently being equipped with communications technologies that transport data via satellite plus while they are on the ground; mobile communication and in some cases broadband networks can receive or broadcast strategic information regarding aircraft situation and even maintenance trends.

Aircraft communications are being expanded; in fact, in recent years a new abbreviation has surfaced. CNS ATM stands for “Communication, Navigation, and Surveillance and Air Traffic Management” which was created to support modernization of the dated and overload prone Air Traffic Control system.

Aircraft that are intended to transport passengers are equipped with radios that enable analog voice communications. This is currently and will be for the foreseeable future the primary means for pilots to communicate withdifferent entities of the Air Traffic Control (ATC) system.

ITU radio spectrum allocations

The allocation of radio spectrum is defined by the International Telecommunications Union (ITU) and relates the use of a frequency to a specific service. In the case of civil aviation there are separate ITU allocations for communications, navigation, and surveillance. Such differentiation between functions corresponds to the safety requirements for Air Traffic Control. The ITU has assigned frequencies for use by aircraft analog voice dialogue in parts of the “High Frequency” (3-30 MHz) band and in the 118-137 MHz section of the wider “Very High Frequency” range. Aircraft can use radios operating in the HF radio band for long-range communications as the signals are reflected by the ionosphere. Unfortunately when using HF the link audio quality is very poor due to this long propagation of the wave. Aircraft can use radios operating in the VHF band to communicate with other radios in line-of-sight coverage. These signals do not reflect off the ionosphere or penetrate obstacles such as mountains or buildings. The advantage of VHF over HF is that the link quality is much better and there is greater reuse of the frequency channel. The use of the word “analog” in relation to voice radio communications means that the changes in the sound of the voice are converted by the transmitter into corresponding variations in the radio signal and converted back by the receiver.

This analog system is simpler than more recent digitized voice systems that periodically measure the sound of the voice, convert the sound into a number in a predefined range, and send the numbers over the radio link. Aircraft VHF analog radios can use channels of varying width and the minimum width depends on the precision of the technology.

Aircraft have been using VHF radios for the past six decades and advancements in electronics have enabled the minimum channel width to be reduced from 100 kHz down to 8.33 kHz, which gives an exponential increase to the number of usable frequencies.

Aircraft Communications Addressing and Reporting System (ACARS)

Aircraft began to be equipped with computers in the 1970s and this led to the development in 1978 of a data communications system called the “Aircraft Communications Addressing and Reporting System” (ACARS).

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