8.33kHz Voice Channel Spacing communications

27-07-2017 09:22:07 - Radio equipment

The fundamental axiom in aviation is: “Aviate, Navigate, Communicate.”  The first priority is to “aviate” - fly the aircraft and keep it safely in the air; secondly, pilots must navigate - fly the aircraft towards the destination.  The final step involves communicating.  Whether this is for talking with other aircraft that share the same airspace, with the relevant Air Traffic Services Units along the way or just for receiving important information about weather or conditions at the destination airport, all these involve using the radio equipment on board. These series of articles will try to elaborate on the principles of radio communication, the need for channel spacing, correct usage of the equipment as well as on the changes to be expected when transitioning to an 8.33kHz voice channel spacing environment.


Chapter 1 - The need for 8.33kHz Voice Channel Spacing explained

VHF fundamentals

To be able to communicate, or receive radio information on board the aircraft the pilot must tune the radio to an assigned frequency. Usually this is done seamlessly by turning a few knobs (or push a series of buttons) until the right numbers show on the radio control panel display. In the same time, on the ground, air traffic controllers have to have their radios tuned to the same frequency to be able to communicate. However, for them, the proper frequencies for the sector or working position are pre-set and usualy do not need further attention.

Worldwide it has been decided for some years that the usual frequencies for communication in Civil Aviation are to be within the VHF (Very High Frequency) band between 118-137Mhz. Being limited to this frequency interval, the ability of pilots and controllers to talk to each other is in fact dependent on one of the scarcest resources in aviation, namely the radio spectrum allocated to aviation use.

VHF is a line-of-site system. This means that two stations can talk to each other assuming that they are tuned to the same frequency and none of them is below the horizon of the other station. Being tuned to the same frequency means that both stations are tuned to the same pre-defined frequency.  These pre-defined frequencies (the ones printed on the VFR charts, available in the AIP, or communicated by ATC) are separated by agreed “spaces”, expressed in kHz. The spaces ensure that communications taking place on adjacent pre-defined frequencies do not interfere with each other. Consequently, within the aviation reserved VHF band (118 - 137Mhz) one can only pre-define a limited number of frequencies with the required spacing between. 

Evolution of communications

In today European environment, there are many more sectors, control towers and other aeronautical stations that require their own discrete frequencies than there are frequencies available. The line-of-sight character of VHF radio waves offers a solution. Frequencies can be re-used if it can be ensured that the usage areas (or operational coverage) of each are separated sufficiently so that no interferences occur. While this is dependent as well on the altitude from which a station is transmitting (i.e. on board the aircraft), only the frequencies used close to the ground can be re-used much more readily than those used at higher levels. Typically, a transmission will cover a range of about 30 miles for an aircraft operating at 1,000 feet above the ground, or about 135 miles with an aircraft operating at 10,000 feet.

Almost since the first use of a radio in an aircraft for communications (though historical information on this subject is contradictory, according to some sources, the first voice communication between an airborne aircraft and the ground was performed as early as 1915), but more likely with the development of commercial flights, it became clearly that there will always be a potential for shortage in the available frequencies.

With the increase of air traffic and therefore the need for communications, the spacing between two consecutive frequencies, which originally was 200kHz in 1947, was reduced gradually to 100kHz in 1958, then  50kHz in 1964 and eventually 25kHz in 1972. With the expansion of the available VHF band for aviation to what is in use today, in 1979 a total of 760 pre-defined frequencies were made available. 

The need for ever more frequencies was driven mainly by the dramatic increase in the number of control sectors in the en-route ATC environment. As traffic grew, air traffic service providers had to split sectors into ever smaller control areas to enable capacity increase. Each new sector required its own frequency and most of the sectors were in the upper airspace, hence the re-use distance between identical frequencies was very large.

By the mid-1990s it became clear that the existing 25kHz spacing based VHF system would not be able to make available the required number of frequencies in Europe. This would put an end to the creation of new sectors, severely limiting the ATC system’s ability to handle the increasing air traffic demand.

While it could be seen as curious, the same magnitude of the issue was not valid in the US. This can be explained by the uneven distribution of the traffic density (and hence of the required number of controlled sectors). Most of the US traffic density is created by flights operating to and from airports located within the coastal area, where the usual constraints in reallocating a specific frequency are not so important (since there are no VHF frequency assignments for controlling the traffic over the Ocean).  In comparison similar traffic densities are found in the European core area (mainly in the airspace over major international airports) where frequency reallocation is not unconstrained as it is dependent on the neighbouring control areas.

Searching for a solution

All proposed solutions to the frequency shortage problem have been analysed. In addition to increasing the number of available frequencies by reducing the channel spacing to 8.33kHz, the main candidates were optimising frequency usage and making use of new technologies. Improvements have been made in the current management of frequencies (especially since the Network Manager has been put in charge of coordinating the frequency assignments), but these are not enough to meet all the forecasted demand for the next years. The new technologies (some still under development under SESAR) are not expected to significantly change the usage of voice VHF communications up to 2030.

The original ICAO decisions concerning the reduction of the channel spacing from 25kHz to 8.33kHz (25kHz divided by 3) were made in 1994 and 1995. The non-binding nature of these decisions meant that certain stakeholders had only partially committed to the implementation, therefore, in 2005 the European Commission started working on a implementing rule on Air-ground Voice Channel Spacing (A-VCS IR) to support the deployment of 8.33kHz in Europe. After consultation with stakeholders it was decided to adopt a phased approach, first addressing the deployment of 8.33kHz above FL195. Provisions for 8.33kHz above FL195 were published in Commission Regulation (EC) No 1265/2007 (the A-VCS IR) on 27 October 2007. The extension of the 8.33kHz voice channel spacing use below FL195 was required by the adoption of the EC Implementing Regulation 1079/2012.

The inability to provide suitable frequency assignments in a timely manner is a constraint to the delivery of operational improvements such as:

  • The creation and modification of sectors to better match traffic flows;
  • The creation and modification of services like approach, tower, ATIS and OPC;
  • The provision of backup services and spare assignments for avoiding interference;
  • Satisfying additional European requirements such as accommodating VHF Data Link (VDL) services

These operational improvements deliver benefits such as reduced delays and increased capacity that would be postponed if the additional frequencies required are not readily available.

Most General Aviation aircraft operating in Europe under VFR remain below FL195 and were unaffected by the initial implementation of 8.33kHz channel spacing. However this second phase, with a deadline for on-board equipment retrofit of 01 January 2018, does impact on private aircraft operators, as it will apply not only to powered aircraft and helicopters but also to gliders, balloons and microlights. While not directly visible, these airspace users will receive small direct benefits from improved access to airspace, reduced delays for the establishment of new flight information services and new GA aerodromes as well as the increased availability of frequencies for air to air communications and for special events (e.g. fly in, airshow, competitions, etc).

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