Summary

Air traffic operations above FL 600 are evolving rapidly as various new technologies move out of the experimental stage and into higher volume operations. These include high-altitude, long endurance autonomous aircraft, remotely piloted aircraft, high-altitude balloons and space vehicles. This paper examines the impact of these new vehicles and checks for existing policies for their safe and efficient regulation and control as well as the need for possible new policy.

Introduction

1.1. High altitude operations (HAO) are becoming more and more important, and their number is increasing constantly. Several stakeholders are involved in managing such operations and ATC is one of them. IFATCA was always going for the concept of the human being as the centre point of operations: The ATCO should be the heart of control – even in high altitude airspace.

1.2. The involvement depends on the type of operations but the interface between ATC and all other systems like ETM (Upper Class E Traffic Management) or STM (space traffic management) is essential for the safety of all operations. Trajectory-based operations (TBO) is the technique that is (according to some industry driven programs) envisaged to solve conflicts strategically, before they require tactical manoeuvres at the sector level, thereby minimise the impact on the individual ATCO and to manage possible contingencies (such as emergencies, re-entry, debris etc).

1.3. This paper shows the involvement of ATC and where the challenges presently are located to deal with the increasing number of these HAO, containing of, but not limited to flights into orbit, hypersonic flights, High Altitude Long Endurance (HALE) flights with very low speeds but estimated airborne times of several weeks, space  re-entries, or air launches of space rockets.

1.3.1. An example of this a high involvement of ATC we can look into operations of the (at the end failed) Virgin Orbit Boeing 747 which launched a rocket after departure from Newquay Airport, UK in January 2023. This rocket reached the second stage and a speed of 11.000 miles/hour however, after a serious malfunction the flight ended after reaching space.

1.3.2. However, this project showed that a collaboration of several agencies is not only possible, but vital, for a successful operation in high altitudes and as well within controlled airspace on the way to the launch zone.

1.3.3. In this specific example partners like the UK Space Agency, the Royal Air Force, the Civil Aviation Authority, the US Federal Aviation Administration, ATC and many others planned together to operate as safely as possible keeping in mind other airspace users and valid rules and regulations in this airspace.

Discussion

2.1. Airspace

2.1.1. To clarify what we are talking about we need to define where HAO are taking place:

Figure 1 – Source: Eurocontrol – Echo Project

2.1.2. HAO are mostly, but not only, movements above FL600. This is above all civil and military traffic so far and the upper limit of the airspace. We are discussing the boundary to the space, defined by the Karman line – 100km (or 54 nautical miles) above main sea level.

2.1.3. This altitude is making things easier regarding the existing traffic, however new challenges are developing as this airspace classification is not the same globally.

2.1.4. Some countries have this airspace classified as Class A with all of the associated rules. ATC responsibility and service levels are dependent on the airspace class utilised as well as the navigational requirements for the airspace users.

2.1.5. Other countries have no airspace classification above FL660 but ATC is, in some countries, still responsible for the airspace, however there is no clarification about which rules will apply there at all.

2.1.6. In some countries ATCs responsibility ends with the top flight level of FL660 and all movements happening above are completely unregulated.

2.1.7. To set the basic framework on how to manage this traffic from ATC side we need a global harmonisation of the high altitude airspace.

2.1.8. This is already laid down in existing policy in the IFATCA TPM ATS 3.3:

A regional change of some high altitude airspaces due to pressure of the industry or implementation speed of new technology could lead again to a patchwork of different airspace classes next to each other which is unwanted as IFATCA is aiming for harmonisation.

2.1.9. IFATCA covers this already in the TPM Policy ATS 3.7:

2.1.10. As a further point of discussion, it has to be mentioned that until this time no clear regulation is available regarding what division altitude there is between air law and space law as there is no official boundary between airspace and space. As orientation we can still take the above-mentioned Karman Line. However, this delineation is not internationally accepted.

2.1.11. ICAO itself is still requiring that the possible new airspace users for HAOs fit the present setup of airspace infrastructure – at least for non-segregated airspaces.

2.1.12. The Manual on Remotely Piloted Aircraft Systems RPAS (DOC 10019) clearly states:

ICAO DOC 10019 Manual on Remotely Piloted Aircraft Systems, First Edition, Montréal 2015.

Many planned operations will be conducted by autonomous flights of whatever aircraft and these are not falling under the above statement. A ruleset is not known or in place as these kind of operations are new.

Since these flights need to transit controlled airspace, they are required to fulfil all known requirements for the airspace classes in which they plan to operate.

2.2. Rules of Flight

2.2.1. Directly linked with the question about the airspace classification is the discussion about the applied rules of flight for High Altitude Operations.

As these altitudes cover a wide variety of operational needs and very different types of aircraft it is not possible to comply completely with the basic rules of flight listed by ICAO Standards and Recommended Practises (SARPs) and Procedures for Air Navigation Services (PANS), noted down mostly in ICAO Annex 2.

2.2.2. The expected airspace users are planning different types of operations: flights into orbit, hypersonic flights, High Altitude Long Endurance (HALE) flights with very low speeds but estimated airborne times of several weeks, space re-entries, or air launches of space rockets.

2.2.3. All these operations have special needs that cannot be brought to 100% compliance with the existing flight rules IFR and VFR.

2.2.4. In many parts of the world FL195 is the upper limit of VFR operations. Presently the visual detection is still problematic. Different aircraft sizes and big speed differences make this a real challenge as well for conventional aircraft operating in these altitudes. The immediate deconfliction procedure may take too long to apply, even technology is moving forward to enhance these capabilities.

However, without perfect working “detect & avoid” procedures, the possible new users on their way to high altitude levels are not compliant to existing basic requirements and regulatory framework for VFR flights.

2.2.5. Therefore, new flight rules, beside IFR and VFR, to fit the operational needs of these new entrants (see 2.2.2) and ATM are being researched.

One of these concepts is developed by NASA and is called Digital Flight and Digital Flight Rules (DFR) which tries to facilitate the existing airspace structure and existing ATM infrastructure together with the new airspace users and their needs and requirements together with future technology.

In several steps the DFR concept aims for an integrated solution and not for special, segregated airspaces.

2.2.6. This concept is based on several elements:

• Digital Information Connectivity and Services to maintain a digital model of the operating environment for use by decision-making automation;
• Shared traffic awareness to maintain awareness of relevant traffic for use in conflict management;
• Cooperative practices to govern the behaviour of DFR operations to ensure harmonized use of the airspace;
• Separation Automation for automating the separation function in flight path management.

2.2.7. To summarize:

Digital Flight is an operating mode in which flight operations are conducted by reference to digital information, with the operator ensuring flight-path safety through cooperative practices and self-separation enabled by connected digital technologies and automated information exchange.

Digital Flight Rules are a set of regulations authorizing sustained Digital Flight as an alternative means of separation in VMC and IMC, in lieu of employing visual procedures (i.e., VFR) or receiving Air Traffic Control separation services (i.e., IFR)

(Ref: Digital Flight -A New Cooperative Operating Mode to Complement VFR and IFR – (NASA/TM–20220013225))

2.2.8. EUROCONTROL has as well published a desperate need for new flight rules. “Certain services are provided for VFR traffic only; for VFR and IFR traffic; or for separation between VFR and IFR traffic. These relations need to be defined for UAS and new sets of flight rules established to enable differentiation between them. This document is only concerned with flight rules from the perspective of a UAS flight. However, there are many ATC issues involved with applying VFR and IFR rules to drones since these rules are specifically designed to manned VFR and IFR traffic and do not concern other types of flight operation linked to UAS. New rules must therefore be established for UAS and included in the flight rules procedures for ANSPs to ensure that all players know what to do to ensure separation.”

2.2.9. And they have the very same concerns as we ATCOs do at these times: “A key question is how to deal with regulatory issues without hindering innovation. Rules are necessary to ensure safety, security and fair-play of all players in the UAS value chain, including the public. However, past experience has shown that a more relaxed and supportive approach to technology development and the work of pioneers could speed up the technology development process.”

2.2.10. The NASA concept of DFR is already developed quite far, however the base of all is a high level of automation and the remodelling of the airspace.

2.2.11. Regarding the automation and co-operative separation functionality in this DFR concept IFATCA already has a very detailed policy in place! Just to mention a few policy citations from the TPM:

IFATCA TPM Co-operative Separation (WC 10.2.9):

For all topics regarding automation there is IFATCA policy in place already. This will be mentioned later in the paper.

2.3. Technical issues

2.3.1. Communications

2.3.1.1. Communication to the aircraft may take longer due to the special conditions in high altitudes and for technical reasons.

2.3.1.2. Some RPAS operate with radio uplinks on their side and not with direct Line of Sight (LOS) VHF/UHF radio, these connections can take more time to transmit as the transmission has to be relayed by several stations.

2.3.1.3. The same latency applies for SATCOM connections as well.

2.3.1.4. In addition to the bigger time constraint there are technical limitations of the present UHF/VHF radio used nowadays as these systems are only tested and approved for altitudes up to FL700. In case higher altitudes are intended by the user, tests and further research about the reliability of these systems has to be done so this system can be used in higher altitudes with no operational issues (Existing and Emerging Communication Technologies for Upper Class E Traffic Management (ETM), Regulus Group, March 2020).

2.3.1.5. Additionally, back up procedures should be in place via e.g. landline telephone fixed network.

2.3.1.6. These procedures are working with additional hardware and increases the workload of the ATCO severely as they have to deal with peripheral devices in addition to his working position.

2.3.1.7. What other kind of backup procedures for communication issues can be put in place without increasing the workload of ATCOs and requiring additional external hardware? These are questions that would have to be addressed moving forward.

2.3.1.8. This would be needed to keep out distraction for the ATCO working.

2.3.2. Navigation

2.3.2.1. When planning for traffic in very high altitudes the ATM system must be able to provide procedures for navigation.

2.3.2.2. Actual systems like ground-based VOR/DME/TACAN stations as well as spacebased GPS/GNSS systems are certified and working up to approximately FL600 and therefore cannot be used for navigational purposes during HAO.

2.3.2.3. Only Satellite-based Augmentation Systems (SBAS) and Inertial Navigation Systems (INS) are ready and operational at altitudes above FL600 and although SBAS might be vulnerable to jamming, it is presently almost the only navigational system that can
be used. INS has the danger of gyro drifts, but these small errors could be compensated by new technical procedures.

SBAS is currently not widely available in aircraft, however due to mentioned facts widely recommended for use.

The ATM system should be acknowledging this when designing procedures for operation in these altitudes.

2.3.2.4. It must be observed that the common pressure altimetry systems can also not be used in these altitudes and a common alternative has to be determined (Ref: Existing Navigation Capabilities for Upper Class E Traffic Management (ETM), Regulus Group, Oct 2019).

IFATCA already did research on this problem, available as WP “Concept of GNSS-Based Altitude” (2015).

The Working Paper concluded that “current GNSS technologies do not support the general use of geometric altimetry in aviation and air traffic control. Their precision and reliability are inadequate for these purposes.”

At this time this is the current position, but due to the rapidly changing environment there will the need for further research as the situation evolves.

2.3.3. Surveillance

2.3.3.1. For ATC surveillance there are some limitations as well when looking into HAO.

Only Air Route Surveillance Radar (ARSR) sites are presently capable of reliable surveillance in these altitudes. Commonly used Multilateration, ADS-B and ADS-C are in the present versions not capable of these altitudes, as well as standard ASR radar sites.

ARSR are presently only used by the US Air Force and it is a special long range radar system (Ref: Preliminary Assessment of Surveillance Alternatives for Upper Class E Traffic Management (ETM), Regulus Group, Apr 2019).

2.4. Operational Issues

2.4.1 Flight Operation Modes

The future airspace users are mostly looking for trajectory-based flight paths. This increases the predictability on both sides (operator and ATC) however this comes with several challenges as this modus is based on either full or almost full automation.

Pre-programmed and pre-agreed 4D-trajectories (TBO) will not help the ATM system to be as flexible and dynamic for the presently existing commercial IFR traffic however may open up ways for the possible new users to make their war towards the high airspace. The TBO Concept, featuring IFATCA’s view should be clarified and added to a future working programme.

The existing airspace structure as well as the overall capacity of airspaces will be negatively impacted by saving areas and altitudes for the new users flying their pre-programmed trajectory.

2.4.2 Short notice changes for whatever reason by ATC, or even by the pilot, are not planned and would take a lot of work and more time in comparison to standard, manned aircraft.

2.4.3 Flightpaths planned and submitted via a traffic management system shall be available to every ATCO concerned. The flight plan format presently, in Flight Plan 2012, is not yet capable for detailed 4D-trajectory flight plans in a method easily processible by the ATM system. As there is a shift towards the newer Flight and Flow Information for a Collaborative Environment (FF-ICE) there will be increased integration of the 4D-trajectory concept.

2.4.4 The whole system with the already mentioned automatization moves in the HAO towards a holistic approach (see NASA DFR project details under FLIGHT RULES).

In the idea of the industry the role and tasks of the ATCO may shift as the new systems and technology introduced. However, the ATCO should remain at the heart of the operation though their specific roles may evolve.

If industry had their way traffic would separate itself from each other and any other known traffic (very few in these altitudes), nevertheless there will be communication and coordination with ATC in a certain way, especially for deviations of the flight plan for whatever reasons (technical, emergencies, change of plans, mission control, etc).

As well for planned ascents and descents of balloons or other HALE airspace users must be coordinated and therefore ATC has to be still in charge.

This goes along with the IFATCA policy and principle to keep the ATCO the key element of the ATC system.

An extended version of this policy regarding RPAS operations within special airspaces with segregated operations has to be evaluated.

IFATCA Policy (AAS 1.10 OPERATIONAL USE OF UNMANNED AIRCRAFT (UA)) is:

2.4.5 Coordination and traffic flow control might be needed for certain corridors. The location of alternate descent areas or so called “transition areas” from and to presently used airspaces below to HAO airspaces might differ from the operational areas of these users in order to ease separation to existing traffic flows. This means: In order to be able to descend back after their air work in the high altitudes the user has to change its location in order to get a descent clearance by ATC as the airspace exactly under the air work area might be too busy to facilitate a direct there.

2.4.6 Bringing all this together it will be a challenge. For these kind of free flight operations and 4D-trajectory operations IFATCA has already extensive policy available.

IFATCA TPM – AAS 1.8 4D TRAJECTORY CONCEPTS/MANAGEMENT:

2.4.8 A very central aspect of the ATCO work is laid down in IFATCA’s policy regarding human factors and automation. “The human ATCO shall always be the key element of the ATC system”. This clearly is not the case anymore as the level of automation is planned to go beyond everything existing at this moment.

The policy (IFATCA TPM WC 10.2.5 AUTOMATION / HUMAN FACTORS) states in detail:

Conclusions

3.1. To wrap the topic up we can recall the existing IFATCA policy regarding UAS operations which is valid in HAO as well.

3.2. This policy already includes a lot of the points still being discussed by the authorities, industry and ATC, however, is referring mostly to operations within existing airspaces together with other users.

3.3. The aviation world is moving towards more HAOs and will very likely be shifting for more TBO.

3.4. In this working paper a long list of existing challenges are mentioned where industry is trying to make a fast progress but technical developments and problems as well as regulatory harmonisation is still an issue to organise the integration of high altitude operations into the current ATM system.

Recommendations

4.1. It is recommended that this paper is accepted as information paper.

4.2. It is recommended that the concept of TBO policy is generated in the next working program.

References

ICAO – Manual on Remotely Piloted Aircraft Systems RPAS (DOC 10019).

IFATCA TPM.

Digital Flight – A New Cooperative Operating Mode to Complement VFR and IFR – (NASA/TM–20220013225).

Eurocontrol UAS ATM Flight Rules – Discussion Document – Edition 1.1.

Eurocontrol UAS ATM Integration – Operational Concept – Edition 1.0.

Existing and Emerging Communication Technologies for Upper Class E
Traffic Management (ETM), Regulus Group, March 2020.

Existing Navigation Capabilities for Upper Class E Traffic Management (ETM), Regulus Group, Oct 2019.

Preliminary Assessment of Surveillance Alternatives for Upper Class E Traffic Management (ETM), Regulus Group, Apr 2019.

ICAO- Annex 2, 10th Edition.

Picture 2.1.1. Source: Eurocontrol – Echo Project.