8.33 kHz Communications #
ICAO PANS-ATM (Procedures for Air Navigation Services – Air Traffic Management) Doc. 4444 and Annex 10 Volume 2 (Communications Procedures) of the Convention on International Civil Aviation serve as the foundation for the protocol for communication between pilots of aircraft and air traffic control. As voice communications are handled between 117.975 and 137 MHz, frequencies are a scarce resource in the aircraft radio spectrum. The existing and anticipated demand for communication channels is particularly high in the congested European skies. VHF 8.33 kHz channel spacing was consequently adopted for airspaces above FL 245 in 1999.
Following this, the European Commission regulated the airspace below FL 195 in regulation No. 1079/2012, and the airspace down to FL 195 in regulation No. 1265/2007. On January 1st, 2018, this went into effect. From that point forward, in order to comply with the rules, every aircraft operating in European airspace must be outfitted with these 8.33 kHz channel radios. In order to conform to the increased channel spacing, many frequencies have been modified practically since 2018.
Voice Channel Spacing #
Since 1972, the COM portion of the aviation band has had access to about 760 channels for communication thanks to the venerable 25 kHz channel spacing. Prior to then, there were two occasions when the channel width was decreased from 200 kHz to 100 kHz in the 1950s and to 50 kHz in the 1960s. Better channel separation is now achievable because to advancements in radio technology. The next technological step would be to move voice communications away from the tried-and-true Amplitude Modulation (AM) format and toward a more contemporary digital one.
Why 8.33 kHz? #
Only 5 kHz of bandwidth is required for comprehensible voice communications. Divide 25 by 3, and you get 8.33333, which is enough space to prevent next channel leakage. Thus, choosing 8.33 kHz was not too difficult. As a result, there are now 2280 channels instead of 760, a threefold increase. Additionally, thanks to smarter radio station channel assignment and improved electronics, adjacent channel interference can be avoided. In reality, several of the original 25 kHz channel assignments are not usable because of nearby frequencies.
Frequencies & Channels #
On traditional 25 kHz radios, the number you dialed corresponded to the precise frequency. The decision was made to introduce display 5 kHz channel steps because to the radio’s display limitations, the 8.33 kHz gap, and the need to keep communication simple.
