8.33 KHz Channel Spacing - What is This?

The radio spectrum, a scarce resort

One of the most basic activities in a cockpit is tuning the radio to the assigned frequency of whoever we want to talk to. Contacting ground control, the tower or one's own company is done by turning a few knobs until the right numbers show in the radio control panel display and we can talk.

Air traffic controllers see the same thing slightly differently. They do not normally have to tune their radios. The proper frequencies for their sector or other working position are pre-set and need no further attention.

With the matter being so pedestrian and the actions so routine, few of us realize that 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.

Many other disciplines have their own radio spectrum and we all guard jealously what we have been given and for good reason. With so many users wanting to use the radio waves, the incumbents better watch or the use it or lose it principle kicks in. Luckily, the frequencies most widely used by aviation (118 - 137 MHz) are not coveted so strongly by others. Our problem is different but not in the least less serious.

VHF fundamentals

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 they can "see" each other (from a radio point of view). If one of the stations is below the horizon of the other station, communications becomes impossible.

Being tuned to the same frequency means that both stations are tuned to the same pre-defined frequency which is within the aviation band. These pre-defined frequencies 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. And herein lies the problem!

You can only pre-define a limited number of frequencies with the required spacing between them if you are to stay within the aviation band. There are many more sectors, towers and other aeronautical stations that need their own, discrete frequencies than there are frequencies available. So what do we do?

The line-of-sight character of VHF radio waves offers a solution of a kind. You can re-use the frequencies if you ensure that the usage areas of each are separated sufficiently so that no interference occurs. Frequencies only used close to the ground can be re-used much more readily than can those used at higher levels. The horizon of these latter is much wider and hence aircraft hundreds of miles away might be heard by a center that has nothing to do with it if the frequency assignment is not done properly.

Reuse not enough? Cut the spacing!

I am not sure who was the first one to bolt a radio on an aircraft, but the idea caught on quickly and soon enough the problem of frequency shortages was born.

Originally the spacing between the frequencies was 200 kHz, providing just 70 channels between 118-132 MHZ as the band was back then (1947). In 1958, he spacing was reduced to 100 KHz, doubling the number of channels to 140.

In 1959 the upper limit of the aviation band was expanded to 136 MHz, giving us another 40 channels, bringing the total to 180.

In 1964, the channel spacing was halved again to 50 kHz, resulting in 360 channels being available.

These dates show not only aviation's ever increasing hunger for frequencies, but also the evolution of aviation radios. In the 1950s no radio set would have been suitable for work with 50 kHz spacing. By 1964, 50 kHz was the standard with more to come...

The channel spacing was further cut to 25 kHz in 1972, doubling the available channels to 720. Seven years later, in 1979, the upper limit of the aviation band was once again expanded, this time to 137 MHz and this delivered another 40 channels, bringing the total to 760.

In 1995, the proposal was made to reduce the channel spacing to 8.33 kHz. Theoretical number of channels: 2280!

This may sound like radio channel nirvana but in real life things are never that simple.

The underlying reasons for the channel hunger

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 chunks to enable controllers to cope. Each new sector needed its own frequency and most of the sectors were in the upper airspace, hence the re-use distance between identical frequencies was very big. This translated into a seemingly insatiable hunger for ever more discrete frequencies.

By the mid-1990s it became clear that the existing VHF system would not be able to make available the required number of frequencies. 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.

Curiously, there seemed to be a mismatch in the magnitude of the problem as seen in the US and in Europe.

While traffic density on the Eastern Seaboard of the US was in fact higher than the busiest areas in Europe, the US frequency managers had no problem satisfying the FAA's demand for new frequencies. At the same time, in Europe, with its lower traffic density, the alarm bells were being sounded that frequency doomsday was nigh.

So what was happening?

To understand this, it is important to remember that frequency managers in European States were part of the communications side of things, often coupled with the old postal monopolies, and they were not really given to international cooperation or worries about aviation's problems outside their own land. That aviation was no longer a purely domestic affair had apparently not really touched them.

Although the States never formally admitted this, most of the frequency shortage was due to poor management of the available frequencies. Valuable frequencies were dormant, never used or simply left there in the dust after the organization originally using it had long disappeared.

The airspace users did raise the issue, brought several examples but to no avail. The local czars of frequency management did not relent and hence there was no other choice but to look at technology based solutions.

The choice between 8.33 kHz channel spacing and VDL Mode 3

While the immediate driver behind the effort to find a solution to the frequency shortage was the fear of skyrocketing delays, experts had been saying since the late 1980s that the complete aviation communications system needed overhaul. The VHF AM voice system and the freshly identified future need for air/ground digital link communications all argued for a common solution that would address the frequency shortage as well as the future communications needs.

Keep in mind that in other areas of communications huge advances were taking place at around the same time while aviation was still trying to make up its mind whether or not to replace a voice communications system that had changed little since the 1940s and which was clearly struggling to keep up with demand.

In the United States a system called VDL Mode 3 was being proposed. This system would have enabled four digital channels to be used on every existing 25 kHz channel and would have provided non-voice data link capability also. There were not many believers outside the US in the feasibility of this technology though and it has still not been implemented anywhere.

In Europe, the splitting of the channel spacing to 8.33 kHz was being put forward as the best solution. Missing a once in a lifetime opportunity, the industry did not examine any long-term alternatives...

The 8.33 decision and what followed

As mentioned earlier, the airspace users were not at all convinced about the need to spend money on aircraft modifications when in their view the frequency shortage was mainly due to poor management of the aviation spectrum.

It was in this ambivalent mood that the industry gathered to attend the ICAO European Regional Air Navigation Meeting (EUR RAN) in 1994 where proposals to address the frequency shortage were also to be discussed and decisions made.

For the current generation of ATM decision makers it may be of interest to mention how most decisions were made back then. Seeking a solution to the frequency shortage, 8.33 kHz was picked up without ever considering possible alternatives and without looking at cost-benefit aspects, user impact or the longer term communications requirements. Clearly not something to bring back... ever.

The airspace users, with the specter of even more serious delays hanging over their heads and with their protests brushed aside, had no choice but to note the mandate: 8.33 kHz in European upper airspace as of 1st January 1998.

The ICAO European Air Navigation Planning Group (EANPG) was charged with organizing the introduction of the new channel spacing. The EANPG in turn requested EUROCONTROL to develop a transition plan and manage its implementation.

This is a very important detail that needs to be remembered. To this day, airspace users tend to blame EUROCONTROL for the whole 8.33 issue when in fact EUROCONTROL was only the agent appointed by ICAO (the States you may say) to carry out the implementation. They did an excellent job and it is not EUROCONTROL's fault that they had to orchestrate the realization of a less than optimal solution. If we consider that EUROCONTROL had to deal with all the ICAO member states in Europe (49) and had to manage the creation of a mixed 25 kHz/8.33 kHz environment, the eventual achievement of the goals is even more laudable.

Mr. Murphy and the 8.33 implementation plan

"If it can go wrong, it will" - states Murphy's first law and this was certainly true of this implementation.

EUROCONTROL, quite correctly, had decided early on to establish a project oriented organization to handle the matter and they also had the good sense of requesting the participation of outside experts from organizations like IFATCA and IATA to ensure direct links to the end-users of the new system.

Right from the start the project was up against a time problem. With the first project steps being taken only in early 1996, the 1 January 1998 deadline was clearly a big question mark. So, the first delay kicked the deadline back to 1 January 1999 and the second delay to 7 October 1999.

Why the delays? The rate of equipage of course was the primary and decisive factor.

In many mandated aircraft equipage scenarios you see the equipage curve rising slowly in the beginning, as only a few aircraft are fitted, then as the deadline approaches, the curve becomes very steep but usually does not reach 100 % before the mandate date. What does this mean?

Obviously, airspace users do not want to spend money too early and fly around with the new equipment without it bringing any benefits. When the time comes and fitting becomes inevitable, there is a mad rush to equip, which in turn can result in a shortage of equipment and an overloading of the shops performing retrofits. In the end, inevitably, there are aircraft left out in the cold, not being able to meet the mandate!

All of this had happened in the case of 8.33 and then more.

When the project started, there were no 8.33 kHz capable radios on the market. A few pre-production samples had been produced, but nothing anyone could buy. In spite of the clear mandate, the presence of the competing VDL Mode 3 system and the fact that 8.33 would only be required in Europe somehow led the manufacturers to slow product development and not produce anything until their customers came with definite orders. The customers on the other hand were reluctant to place orders until closer to the mandate deadline which had to be put off as a result of low equipage rates because of a scarcity of radios! A vicious circle if ever there was one... At times meetings of the 8.33 project team had an air of most participants wishing the whole thing would just go away...

Then there were the aircraft themselves. No matter how advanced the new radios were 8.33 kHz is a very small distance between channels and trials on various aircraft revealed surprising behaviors. Radios on the Boeing 767 for instance worked well while the doors were open but started to produce interference the moment they were closed...

Controllers were fretting about what would happen if pilots regularly mistuned their radios. True, for the first time ever, the numbers seen on the radio control panel do not show the real frequency of an 8.33 spaced channel and this can be confusing.

Issues with the new radiotelephony expressions were also on the agenda for while.

In the end however, the final deadline came and went and the new system worked pretty well. Apart from a few isolated incidents no problems were reported and 8.33 kHz, like any other part of the ATM system, became part of the European scene.

Next steps?

In the meantime, EUROCONTROL has continued to manage the implementation of 8.33 kHz, extending its use also into the lower airspace. They have fulfilled and continue to fulfill the role assigned to them by the EANPG and the benefits specific to 8.33 kHz will no doubt continue to accrue. It is even rumored that the FAA also wants to look into 8.33 kHz channel spacing for introduction in the US.

Did the benefits materialize?

It all depends on how you want to measure the benefits. If the measure is the number of requests for new frequencies that could be accommodated, then the outcome of the exercise is definitely positive. At the very first Frequency Block Planning Meeting held after the introduction of 8.33 kHz channel spacing, 57 of the 59 requests were accommodated, an absolute first. The level of subsequent request satisfactions shows a similar pattern.

It is very likely that a comparison with a "do nothing" scenario would show that investing in 8.33 kHz was not a bad idea.

On the other hand, 8.33 kHz did create the impression that the problem was solved and the motivation to really address the shortcomings of this obsolete communications system has all but disappeared. Back around the time the 8.33 kHz decision was made, it might have been easier to also initiate the development of a new system that would by now provide services to the pilots on a par with what passengers are getting in the near future.

As it is, we are left with a legacy system which will be much more difficult to replace on an industry level now, not least because of the sad shape airlines are in these days.

It is a pity that the EUR RAN meeting in 1994 did not have the vision to look beyond the immediate solution to the problem of frequency shortages.

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