8.33kHz Voice Channel Spacing communications
27-07-2017 10:01:44 - Équipement radio
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 2 – Communications in 8.33kHz Voice Channel Spacing
Communications in an 8.33kHz VCS environment
By reducing the channel spacing between consecutive frequencies in the VHF aeronautical communication band, 8.33 channel spacing adds two additional channels for every 25kHz. In such a way, as an example, in addition to 118.025Mhz the following frequencies can be used as well 118.0333MHz and 118.0167MHz (in fact by dividing 25 by 3 we obtain 8.333333… and therefore the new frequencies should be 118.03333333… and 118.016666666… but for simplification the rounded values are considered)
In a today’s 25kHz spaced VHF radio communication, the digits selected (dialled) on the radio equipment interface on board the aircraft correspond to the frequency used for transmission and reception (frequency which can be found on the charts or indicated by air traffic controllers). This is valid for most of the radio equipment installed on board the aircraft. For some radio models, however, it may happen that only 2 digits are available for selection after the decimal – this means for example that for frequencies such as 121.625MHz the selection on the display will show 121.62Mhz, but the equipment will actually transmit and receive on 121.625MHz – It can also be noticed that in some parts of the globe, in an environment using 25kHz spacing controllers will only mention the first two digits after the decimal regardless of the frequency to be dialled (e.g. "Contact 121.62")
In an 8.33kHz channel spacing environment, while the frequencies used involve at least 4 digits after the decimal, to ease the communications between controllers and pilots but also to reduce the potential for errors, by convention, a channel has been allocated for each frequency. In such a way only 3 digits after the decimal are required, simplifying a lot the phraseology as well as the manipulation of the radio equipment on board.
Consequently, as an example, in order to communicate on 118.0333Mhz a pilot will have to dial 118.035 on his 8.33 capable radio equipment interface – the system is designed in such a way that when dialling this channel the radio will be tuned to 118.0333MHz. The allocation of a new channel is valid as well for the old 25kHz frequency, regardless the fact that the carrier frequency is the same. As an example, in both 25kHz and 8.33kHz spacing the frequency 118.025Mhz is used, but the for an 8.33kHz channel spacing environment the frequency has been “coded” as 118.030 channel. The new channel is allocated in order to distinguish on the radio interface between the 25kHz spacing and 8.33kHz spacing modes (i.e. in an 8.33 environment in order to transmit on 118.025Mhz, pilots will have to dial 118.030 on the radio interface)
The following table is provided as an example of the pairing between channels and carrier frequencies, for both 25kHz and 8.33 spacing.
|
Old 25kHz radio |
New 8.33kHz radio |
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|
|
Dial selection |
Real TX/RX frequency |
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|
Frequency |
Dial |
25kHz Frequency |
8.33kHz Channel |
Frequency (Mhz) |
Spacing (kHz) |
|
118.000 |
118.000 |
118.000 |
|
118.0000 |
25 |
|
|
118.005 |
|
118.005 |
118.0000 |
8.33 |
|
|
118.010 |
|
118.010 |
118.0083 |
8.33 |
|
|
118.015 |
|
118.015 |
118.0167 |
8.33 |
|
118.025 |
118.025 |
118.025 |
|
118.0250 |
25 |
|
|
118.030 |
|
118.030 |
118.0250 |
8.33 |
|
|
118.035 |
|
118.035 |
118.0333 |
8.33 |
|
Etc….. |
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Radio equipment
A new 8.33kHz spacing capable radio will operate on 25kHz or 8.33kHz depending on the user selection. As an example, if the user selects a 25kHz spaced frequency such as 132.000, the new equipment will operate just like the old radio, but if the 132.005 channel is selected, the same frequency will be used but the radio will be working in an 8.33kHz spacing mode. In most of the cases, these radios are provided with a switch to enable them to operate in either 25kHz or 8.33kHz mode. It is essential that the radio is configured on the correct mode to enable the selection of the desired channel or frequency.
It is worth noting that some frequencies will remain designated as 25kHz (such as the emergency frequency 121.5MHz); during the transition period untill all aircraft will be equiped, for safety reasons, it is also expected that some of the frequency assignments mostly used for information (e.g. ATIS) will remain in 25kHz spacing.
As an example, in practical terms, to instruct a flight to change the communication frequency to 132.0083MHz the phraseology used by air traffic controller will be "Cessna 123 contact London 132.010” ("One tree two decimal zero one zero") Flight crews need to be aware that there will be no specific indication in the phraseology if the new frequency is an 8.33channel or a 25kHz frequency.
Inadequate use of radio
Communications problems caused by mistuning VHF channels are an important safety and security risk for air traffic management. Given the difference between 25kHz spaced frequencies and the 8.33kHz channels, it is very important that flight crews understand the correct operation of their radio equipment and tune their radios accordingly. Pilots have to understand the difference between frequencies and channels and they should not attempt to tune their 25kHz radio to match an 8.33kHz channel.
It worth noting that operating a 25kHz radio in a sector in which communications are performed in 8.33kHz channel spacing, even if the transmission is performed on the central frequency (i.e. identical carrier frequency in both 25kHz and 8.33kHz cases) this may create interference on adjacent 8.33kHz channels which could be in use by another control centre. It is also possible that reception of 25kHz transmissions will be poor on the ground and ATC may not be able to hear the communications.
It is recommended that flight crews, in preparation of the flight, especially after 01/01/2018, consult relevant local aeronautical publication in order to identify the correct equipment requirements for the airspace in which they will operate. Pilots need to be aware that internet available flight information services may not have the correct information regarding the frequencies in use for a particular service and/or airspace, especially in the transition period and after 31/12/2018. Correlating this information with certified sources (such as the local AIP) is recommended before operating.
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