Summary

Frequency congestion is a major topic in today’s ATM environment. This working paper looks at various technical and procedural solutions proposed. It also reviews implementations of new Datalink messages and concepts aimed at reducing voice frequency congestion. New IFATCA policy is proposed.

Introduction

1.1 The concept of using radiotelephony for providing air traffic services to aircraft has been used ever since air traffic services themselves were put into operation. In the beginning, separation was established by solely relying on pilot reports about aircraft position, altitude and flight path obtained via radiotelephony.

1.2 Meanwhile, technology has evolved, moving from High Frequency (HF) to Very High Frequency (VHF) and Ultra High Frequency (UHF) and further on to Datalink (Controller Pilot Data Link Communications – CPDLC). IFATCA has identified a need to study frequency management, in particular the case of an aircraft instructed to leave one control frequency and monitor the next control frequency without establishing communications. This is something done by voice at least in Europe, and a Datalink equivalent is being sought – however the practice itself should also be reviewed by IFATCA. Pilots are also required to sometimes establish radio contact with two different ATC units at the same time in order to receive an onward clearance, and again the Datalink message set is currently being modified to allow for this kind of situation.

1.3 A related item is the IFATCA “Statement on the Future of Global Air Traffic Management” which makes reference to removing housekeeping tasks such as frequency transfers, on which we currently do not have any Policy.

Discussion

2.1 R/T technology

2.1.1 Radiotelephony remains the main technical means of interaction between air traffic control and aircraft. This is especially true for high density airspace, where exchange of voice messages over VHF and UHF radio bands are used almost exclusively for tactical ATC messages. Datalink has been in use for a couple of years for example at the Maastricht Upper Area Control Centre (MUAC), which covers the airspace over the Netherlands, Belgium, Luxemburg and north-western Germany above FL245, as well as for transmission of pre-departure clearances at various airports.

2.1.2 In remote and oceanic airspace, HF communications remains one of the main technologies in operation, increasingly being replaced by Datalink both for pilot-initiated position reports (Automatic Dependant Surveillance – Contract “ADS-C”) and requests as well as air traffic control clearances that are not time critical. The main driving forces behind the move to Datalink are the possible unreliability of long range voice communication – as the distance between the aircraft and the ground station may well exceed several thousand miles – as well as the user-unfriendliness of HF communications with very tiring background noise and compared to VHF radio or satellite telephone, poor sound quality. CPDLC comes without background noise but with built-in error protocols to protect against messages being corrupted while being transmitted.

2.1.3 With several aircraft tuned to the same voice frequency, there is always a danger of an aircrew reacting to a message not directed at them. CPDLC protects against this scenario. However, neither technology can protect against a message erroneously addressed by ATC at a wrong aircraft.

2.1.4 R/T technology does not allow for simultaneous transmissions by several stations. Where radio transmissions do cross, there is always a danger of information getting lost in the process. At best, the stations who tried to initiate a transmission will have to repeat their calls, resulting in a capacity loss. At worst the crossing of transmissions will not be detected, which might lead to a separation loss. There exist documented cases of a clearance read back by both the intended aircraft as well as another aircraft. Due to technical reasons the double read backs could not be detected, or much less corrected, by ATC. Subsequently, both aircraft acted upon the instructions received which then led to a loss of separation between the aircraft which had erroneously reacted to the clearance and a third aircraft.

2.1.5 The number of radio frequencies available to aviation is limited. Apart from ATC units, frequencies are allocated to airlines and small uncontrolled aerodromes that provide aerodrome flight information service (AFIS) and sometimes even operate pilot-activated runway lighting systems that are switched on by several successive clicks of the aircraft transmitter button on an AFIS frequency. Radio frequencies are therefore being reused, i.e. the same frequency might be allocated to an ACC sector in Tanzania and in the London TMA. It takes a major concerted effort to make sure that multi-used frequencies are protected within a certain geographical area. Where large airspace sectors are combined, it often becomes necessary for the controller to switch aircraft between frequencies even though they remain within his or her area of competence simply because using a single frequency for the whole combined airspace could interfere with the next unit that uses the same frequency. Also, transceivers on the ground might not be rigged with both frequencies. Datalink can eliminate this sort of frequency switching; the aircraft remains logged onto the Datalink network as it passes through several adjacent sectors.

2.2 Airspace

2.2.1 Whether the airspace is surveyed or unsurveyed impacts on the communication requirements. In airspace without surveillance available to the controller, ATC relies exclusively on pilot reports about aircraft position, altitude, speed, and projected flight path. The reason for the lack of surveillance is usually that the installation and – more importantly – subsequent maintenance of ground-to-air transceivers is either economically not viable (because demand for the airspace is low) or downright impossible.

2.2.2 As stated above, the lack of surveillance results in a higher demand for communication bandwidth per aircraft, as all information normally derived from surveillance such as aircraft position and trajectory has to be communicated to ATC. Also, ATC providers will usually strive to use their controller workforce to maximum benefit of the aviation system. Therefore sectors in low density airspace will usually be very large in order to bring the controller-to-aircraft ratio to an acceptable level. For instance, New Zealand oceanic airspace features a single sector 4000 NMs wide. In such large nonsurveillance sectors, frequency load might then become an issue even though traffic density is low.

2.2.3 In surveillance airspace on the other hand, there is usually no need to pass position information or other data such as wind conditions to ATC through communication since ATC has access to such information through surveillance. The resulting reduction of frequency usage however is generally offset by a higher traffic density (usually the reason to provide surveillance in the first place). So in surveillance airspace, frequency load could also cause problems. There are busy terminal areas where frequency load is so high that pilots do not read back clearances at all because the controller would not have sufficient air time left to work all his or her aircraft. Of course such a scenario is unacceptable on safety grounds alone.

2.2.4 In Amendment 5 to ICAO Document 4444 (PANS-ATM), the following procedures for frequency changes were laid down:

4.11.3 Radiotelephony procedures for air-ground voice communication channel changeover

4.11.3.1 When so prescribed by the appropriate ATS authority, the initial call to an ATC unit after a change of air-ground voice communication channel shall contain the following elements:

1) designation of station being called;

2) call sign, and for aircraft in the heavy wake turbulence category, the word “Heavy”;

3) level, including passing and cleared levels if not maintaining the cleared level;

4) speed, if assigned by ATC; and

5) additional elements, as required by the appropriate ATS authority.

There is a documented incident at a major European airport which, in order to reduce frequency load, has procedures in place that require pilots to check into a frequency “callsign only” without reporting the cleared and passing level(s). In one particular case an aircraft was erroneously descending to a lower level than what the controller was expecting, resulting in a loss of separation. With a proper initial call, the controller would have been able to spot the error.

2.2.5 A related scenario is a sector where frequency congestion is not the limiting factor, but controller workload may be. The actual number of transmissions might not saturate the available air time, but there might be so much intra-controller coordination going on offfrequency that even though the frequency itself is not busy, the controllers might well be.

2.2.6 Whenever either frequency or sector load become too high, the traditional solution is to split sectors. Apart from the fact that you need extra controllers to man the newly split positions, this poses also several technical, frequency protection and procedural problems.

2.2.6.1 As stated in 2.1.5, the number of available voice frequencies is limited. Additional frequencies might not be available without a major reshuffling.

2.2.6.2 ATC sectors do not work below a certain size. The smaller a sector is, the less aircraft it can accommodate, and the more coordination is required between sectors. At same stage there simply isn’t enough room left to solve conflicts within your own sector.

2.2.6.3 The number of frequency transfers increases, resulting in a reduction of time available for tactical transmissions for separation purposes.

2.3 Increase of number of available frequencies

2.3.1 There are two technical solutions to frequency shortage. The most obvious one is to increase the number of frequencies available. This can be achieved by widening the frequency spectrum available to aviation, or by reducing the steps between frequencies, and indeed this is what has happened already. The option of encoding transmission data to achieve a more efficient use of spectrum has not been implemented.

2.3.2 The number of available VHF assignments was increased by optimising frequency reuse (improved coordination and possibly confining VHF assignments to smaller areas), using more spectrum (118 to 136 MHz increased to 137 MHz), and splitting the radio spectrum into narrower band widths (50 kHz to 25 kHz channels).

2.3.3 In 1994 it was decided to introduce a further channel split from 25 to 8.33 kHz. Subsequently, 8.33 kHz was introduced above FL245 in the ICAO EUR Region from October 1999 and above FL195 from the 15 March 2007. At the time of writing, 8.33 kHz channels have been implemented in the airspace of over 20 ICAO EUR Region States, and Europe is reviewing the need for 8.33 kHz below FL195, as well as other measures to alleviate VHF congestion.

2.3.4 There are two main drawbacks of this way forward. New radios have to be fitted on all aircraft that want to use 8.33 airspace, and at the moment there is no plan beyond 8.33.

2.3.5 8.33 does not alleviate controller overload, and it carries all the drawbacks of human error such as misdelivery of instructions, mishearing, or failure to recognise read back errors.

2.4 Alternatives to voice communications

2.4.1 The other option is to reduce frequency load by other technical means. Instead of simply splitting frequencies further and further, emphasis could be placed on a more efficient use of available air time.

2.4.2 As stated above, a lot of air time is presently being used for routing messages like frequency changes and initial calls. There already exist some concepts to shift some of the routine message traffic to Datalink, but at the moment at least, there is an increased focus on recreating all kinds of voice techniques and instructions “verbatim” in the Datalink message set instead of recognising and developing the strengths of the Datalink concept.

2.4.3 Two examples of where questionable voice concepts are now being recreated in CPLDC by describing new message sets are downstream clearances and the use of MONITOR.

2.4.3.1 The term “downstream clearance” refers to the concept that an aircraft is in contact with two ATC units at the same time. One is the unit in whose airspace the aircraft is presently operating in, and the other is a unit further down the route that provides by Datalink instructions for when the aircraft will enter that unit’s airspace. The technical implementation was reviewed in WP 90 at the 2009 IFATCA Annual conference, and the subject of downstream clearances as such is being reviewed in the Working Paper Study Route Clearances and Associated requirements that is also part of the 2010 Punta Cana Technical Working Program.

2.4.3.2 MONITOR is an instruction to an aircraft to change over to another frequency but NOT report in on that frequency. Instead, pilots are asked to remain silent until the controller explicitly addresses them. The main reason for this technique is to reduce frequency congestion. But again, similar to the example provided in 2.2.3, controllers lose the safeguard to spot that an aircraft might not be at the assigned level or climbing/descending to a level different from what the controller is expecting, or on final approach to a different runway than originally assigned. What makes matters even worse is that, until the accepting controller actually establishes communication with the aircraft in question, neither the controller nor the pilot will know whether the aircraft is actually on the correct frequency or not. Assuming that the controller addresses the aircraft for tactical reasons, the sudden realization that the aircraft is not on frequency might leave the controller without options to solve a possible conflict.

2.4.4 Alternative applications for Datalink should be explored to relieve the pressure on existing voice communication concepts. Besides mimicking voice (the CPDLC message set is quite exhaustive), Datalink will offer new concepts such as tailored arrivals, paired approaches, graphical taxiing, required time over a distant point (4DTRAD) with a mix of surveillance (ADS-B for air-to-air surveillance, ADS-C for ground surveillance with the still-in-development Extended Projected Profile) and communications (voice or new CPDLC messages). Voice is still a quick and efficient tool but future and more complex procedures will require CPDLC for identification phases or complete route instructions.

2.4.4.1 Modern ATC systems increasingly require controllers to document instructions to aircraft by entering them directly into the ATC system. Such systems then use the information entered to automate inter-centre coordination. Some systems, for example in the UK, even compare the instruction that the controller has recorded to the selected level that the aircraft is broadcasting by Mode-S downlink, and alert the controller in case of a mismatch.

2.4.4.2 Things like frequency changes could also be automated. An example: The transferring controller uses the internal handoff feature to transfer the aircraft to another sector. As soon as the accepting controller accepts the handoff, the system automatically sends a Datalink message to the aircraft with the accepting sector’s frequency. The aircraft then automatically changes to the new frequency, and the ATC system compares things like level passing and selected level from the Mode-S broadcast with its own data. The transferring controller could see the aircraft label turn from green (his or her own aircraft) to yellow (upon initiating the transfer) to blue (other controllers’ aircraft), while the accepting controller would see the sequence the other way around. If, for example, the cleared level stored in the ATC computer and the selected level transmitted by Mode-S did not match, the label could turn to red instead of green.

2.5 Human factors

2.5.1 Typically CPDLC is not connected to the ATC ground system. However, controllers have to document all instructions internally anyway. By interfacing the ATC and CPDLC systems, CPDLC messages could be automatically generated from such controller actions and uplinked to aircraft without any voice communication from ATC at all.

2.5.2 There would still be a need for voice communication for anything that is NOT recorded by the controller as a routine task, such as traffic and weather information and other messages that are not an actual instruction. While it reduces frequency congestion, trying to transmit such information via Datalink could increase controller workload.

2.5.3 It remains unclear if a complete or partial removal of the voice environment would decrease pilots’ and controllers’ situational awareness. To pilots, a busy frequency is an indication of controller workload. Also, the plotting of the traffic situation around one’s own aircraft from listening to other radio traffic has been cited in the past as an advantage of voice communication. ADS-B IN will increase pilots’ situational awareness as the traffic will be visual to them on the CDTI.

Conclusions

3.1 Voice communication has its limitations, both in the frequency range available and the technology itself. Datalink can mitigate or at least alleviate many of these limitations.

3.2 8.33 kHz spacing provides additional channels for voice communication but does not address any other limitations. The number of channels for voice communication is still limited.

3.3 Recreating voice techniques and instructions “verbatim” in the Datalink message set was the initial objective of CPDLC but there is now a need to recognise and develop the strengths of the Datalink further.

3.4 Further integration of ATC computer systems and Datalink could make CPDLC more flexible, reliable and easier to use than voice communication.

3.5 It remains unclear how a complete or partial removal of the voice environment affects pilots’ and controllers’ situational awareness.

Recommendations

4.1 IFATCA Policy is:

IFATCA recognises the need for, and supports the reduction of, voice communication workload of controllers. However simply omitting items without alternative methods of accomplishing essential checks compromises safety.

and is included on page 3 2 4 6 of the IFATCA Technical and Professional Manual.

4.2 IFATCA Policy is:

IFATCA supports Datalink concepts that improve frequency management provided that they demonstrate an identical or better level of safety and efficiency compared to voice communication.

and is included on page 3 2 1 11 of the IFATCA Technical and Professional Manual.