Annex C – GM1 Strategic Mitigation Collision Risk Assessment

CAA ORS9 Decision No. 46

Determining the final air risk class

1. Overview
1.1 The initial ARC is a generalised qualitative classification of a UAS operational collision risk before any strategic mitigations are applied. Strategic Mitigation consists of procedures and operational restrictions intended to control the crewed aircraft type, encounter rates or time of exposure prior to take-off. Strategic Mitigations may be used to adjust the final ARC into the residual ARC, which is then used to define Tactical Mitigation Performance Requirements (TMPR) and the Specific Assurance and Integrity Level (SAIL).
1.2 Strategic mitigations are broadly subdivided into two categories:

(a) Mitigation by operational restriction, which are mitigations that are controlled by the UAS operator, in that they are not reliant on the cooperation of other airspace users to implement an effective mitigation.

(b) Mitigation by common rules and structures, which are mitigations that rely on all aircraft within a certain class of airspace to follow the same structure and rules. All aircraft in the airspace must participate, with the specific ruleset defined by the CAA and / or the ANSP.

1.3 Both of these categories are discussed further below, followed by some generic guidelines on the use of strategic mitigations to reduce an initial ARC assignment to a residual ARC.
Strategic mitigation by operational restriction
1.4 Three types of operational strategic mitigations are considered, each discussed below.
1.5 SM1 - Operational restriction by boundary – Limiting the UAS BVLOS operation to a boundary limited volume enables the use of airspace characterisation (discussed further in Annex C paragraphs 1.30 to 1.35) to adjust the expectation of traffic types, density and encounter rates beyond that in the generalised flowchart. For example, the generalised Class G assumption that results in an initial ARC-c assignment is due to the unknown traffic density and the potential for many types of crewed aircraft to be encountered, including many types of GA, Helicopter Emergency Medical Service (HEMS), Police, SAR, military, pipeline / powerline survey aircraft, etc. However, it may be possible to demonstrate that a specific remote rural location has a significantly reduced traffic density and / or encounter type from the generalised Class G assumption, potentially supporting a reduction in the ARC.
1.6 SM2 - Operational restriction by chronology – Limiting the UAS BVLOS operation to specific times of the day provides a further opportunity for airspace characterisation (discussed further in Annex C paragraphs 1.30 to 1.35) to adjust the expectation of traffic type, density and encounter rates below that expected for the volume as a whole. For example, it may be possible to demonstrate a reduced number of GA VFR flights during the hours of darkness.
1.7 SM3 - Operational restriction by time of exposure – Accepting a higher operational risk only for a limited time. An example of this within crewed aviation is the Minimum Equipment List which allows in certain situations a commercial airline to fly for three to ten days with an inoperative Traffic Collision Avoidance System (TCAS). The safety argument is that three days is a very short exposure time compared to the total life-time risk exposure of the aircraft. This short time of elevated risk exposure is justified to allow for the aircraft to return to a location where proper equipage maintenance may take place. Appreciating this may be a difficult argument for the UAS operation to make, the operator is still free to pursue this line of reasoning for a reduction in collision risk by applying a time of exposure argument. The cumulative impact of such a mitigation must be considered.
Strategic mitigation by common rules and structure
1.8 Several types of operational strategic mitigations are considered, each discussed below.
1.9 SM4 - Special Use Airspace (SUA), including:
1.10 Danger Area (DA) / Temporary Danger Area (TDA) – Airspace of defined dimensions within which activities dangerous to the flight of aircraft may exist at specified times. This structure may be used to provide segregation within Class G airspace and in controlled airspace over the high seas. A TDA typically only lasts 6 months, although under certain circumstances this may be extended up to 12 months.
1.11 Temporary Segregated Area (TSA) – A TSA is a defined volume of airspace, temporarily segregated and allocated for the exclusive use of a particular user during a determined period of time and through which other traffic will not be allowed to transit. This structure may be used to provide segregation within UK controlled airspace.
1.12 Temporary Reserved Area (TRA) – A TRA is airspace that is temporarily reserved and allocated for the specific use of a particular user during a determined period of time and through which other traffic may or may not be allowed to transit in accordance with the air traffic management arrangements notified for that volume of airspace. The use of a TRA for UAS BVLOS is currently enabled by a CAA policy concept as the current approach for trialling a managed form of integration, based on a bespoke ruleset applied by a controlling ANSP, including the potential for equipment carriage, traffic types and traffic density restrictions.
1.13 SM5 - Other airspace requirements, including:
1.14 Transponder Mandatory Zone (TMZ) – A TMZ is airspace of defined dimensions wherein the carriage and operation of pressure-altitude reporting transponders is mandatory (unless operating in compliance with alternative provisions prescribed for that particular airspace by the TMZ Controlling authority that will achieve a cooperative electronic conspicuity environment). Deployment of a TMZ creates a ‘recognised traffic environment’, and assuming appropriate surveillance is available then operation within a TMZ removes non-cooperative traffic from the crewed aircraft encounter set that must be considered by a DAA capability. However, a TMZ alone does not alone require two-way radio communications, provide any control of traffic types or density or imply any form of UTM or air traffic service provision.
1.15 Radio Mandatory Zone (RMZ) – A RMZ is airspace of defined dimension where pilots are required to establish two-way radio communication prior to entry (unless in compliance with alternative provisions prescribed for that area). Operation within a RMZ enables real-time two-way interaction with other air traffic via the appropriate ANSP, which potentially enables strategic mitigation assuming appropriate support agreement from the appropriate ANSP.
1.16 All of the above airspace types are established in accordance with the requirements of the CAA’s Airspace Change Process contained within CAP 1616 and promulgated in the Aeronautical Information Publication (AIP). Where a temporary rather than permanent change to the notified airspace design is required, the procedure in should be followed.
1.17 SM6 - Pre-agreement of any ANSP services to be used in-flight – Several potential options for ANSP support are listed below, each of which require review and approval of operating procedures and any potential changes to the usual ANSP functional system:
1.18 Procedure based segregation – For example approving UAS BVLOS operation when it is known that other aircraft are not within the area.
1.19 A Basic Service – is a service provided for the purpose of giving advice and information useful for the safe and efficient conduct of flights. This may include weather information, changes of serviceability of facilities, conditions at aerodromes, general airspace activity information, and any other information likely to affect safety. The avoidance of other traffic is solely the pilot’s responsibility. Basic Service relies on the pilot avoiding other traffic, unaided by controllers/ FISOs. It is essential that a pilot receiving this service remains alert to the fact that, unlike a Traffic Service and a Deconfliction Service, the provider of a Basic Service is not required to monitor the flight. For these reasons, a DAA system will be required, particularly if this is the sole strategic mitigation.
1.20 Traffic Information - is a surveillance-based service, where in addition to the provisions of a Basic Service, the controller provides specific surveillance derived traffic information to assist the pilot in avoiding other traffic. Controllers may provide headings and/or levels for the purposes of positioning and/or sequencing; however, the controller is not required to achieve deconfliction minima, and the pilot remains responsible for collision avoidance. For these reasons, a DAA system will be required, particularly if this is the sole strategic mitigation.
1.21 Deconfliction Service - is a surveillance-based service where, in addition to the provisions of a Basic Service, the controller will provide specific surveillance derived traffic information and issues headings and/or levels aimed at achieving planned deconfliction minima, or for positioning and/ or sequencing. However, the avoidance of other traffic is ultimately the pilot’s responsibility. For these reasons, a DAA system will be required, particularly if this is the sole strategic mitigation. A Deconfliction Service will only be provided to flights under IFR outside controlled airspace, irrespective of meteorological conditions and, as IFR flight is currently only available to certified UAS, is mentioned for awareness of potential future use only.
1.22 Radar Control Service – is provided to all Instrument Flight Rules (IFR) flights in controlled airspace classes A to E. Radar Control Service is a service under which pilots follow mandatory instructions to enable the prescribed separation minima between Air Systems to be maintained. Such mandatory instructions will generally be associated with essential details of conflicting traffic. Pilots will not change heading or level without prior approval of the Radar Controller (except to ensure the safety of the aircraft). As IFR flight is currently not available to civil UAS, radar control service is mentioned for awareness of potential future use only.
1.23 SM7 - Pre-agreement of any Unmanned Traffic Management (UTM) services to be used in-flight – Several UTM operational concepts have been proposed with the objective to enable safe and efficient UAS operation within a volume of airspace. A UK CAA policy for UTM is currently under development, which may include one or more of the services listed below. Mitigation via UTM services ahead of CAA UTM policy adoption will be subject to CAA scrutiny on a case-by-case basis. Services that maybe considered include:

(i) Geo-consciousness service – Including provision of mapping data, aeronautical information, meteorological data, etc.

(ii) Common altitude reference provision – Ensuring that altitude or level information is in a format that is harmonised and compatible with existing altitude referencing methods.

(iii) Traffic information service – Using ground infrastructure to detect other air traffic and provide a known or recognised traffic environment as defined in CAP1430, UK Air Traffic Management Vocabulary.

1.24 Trajectory deconfliction service – Verifying that the 4D trajectory plans of all aircraft within the area are deconflicted to an appropriate separation minimum. Note that this is distinct from the use of flight plans within crewed aviation, which focus predominantly on airspace capacity and the workload limits of the air traffic controller who provides the required tactical separation and deconfliction services.

(i) Take-off approval service – Validating that an approved deconflicted 4D trajectory is still valid and it is safe to begin the flight.

(ii) Conformance monitoring & alerting service – Based on an approved deconflicted 4D trajectory.

(iii) Conflict monitoring and alerting service – Based on both a surveillance service and an approved deconflicted 4D trajectory.

1.25 Segregation, separation and / or deconfliction instruction or advice service – Using a surveillance capability to maintain separation minima and hence reduce the residual intruder encounter rate.
1.26 SM8 - NOTAM of intended operation – Note that while in some locations value may be gained from this approach it is not considered scalable for routine operations. Therefore, the use of NOTAMs may be limited to specific heights, locations or for new or novel operations.
1.27 SM9 - Military low flying notification – Military low flying occurs in most parts of the United Kingdom at any height up to 2,000 ft above the surface. However, the greatest concentration is between 250 ft and 500 ft and civil pilots are advised to avoid flying in that height band whenever possible. The Low-Level Civil Aircraft Notification Procedure (CANP) as described within the AIP ENR 1.10 FLIGHT PLANNING allows low level civil aerial operators to notify such activity to military low flying units. Before commencing any low flying sortie, military pilots receive a comprehensive brief on all factors likely to affect their flight, including relevant CANP details.
1.28 SM10 - Outreach to local flying clubs and pilots – Airspace characterisation also enables a local flying community in the region of the UAS operational area to be identified, and this may enable coordination and / or direct notification of the UAS operations and vice versa. For example, an agreement could be reached for local flyers to inform the UAS operator of upcoming periods of busier than usual activity, or vice versa.
Description of residual ARCs
1.29 In order to understand the value of different strategic mitigations, a description of the residual ARCs is required. In accordance with the wider SORA methodology, agreement of a residual ARC then results in the assignment of TMPRs that reduce any residual collision risk down to the appropriate target level of safety. Broad descriptions of each residual ARC are as follows:
1.30 Residual ARC-a: Encounter rate with other crewed air traffic demonstrated to be negligible, therefore DAA based tactical mitigation of the air risk is not required.
1.31 Residual ARC-b: Encounter rate with other crewed air traffic demonstrated to be low and exclusively Type-1 (refer to section 1.112(i) for definition), but not negligible. DAA based tactical mitigation is therefore required but must be supported by one or more additional mitigation layers.
1.32 Residual ARC-c: Predominately Type-1 traffic and negligible commercial air transport aircraft, with either an encounter rate that may not be demonstrated to be low enough for ARC-b, or additional supporting strategic mitigations are not available. DAA based tactical mitigation is therefore required and expected to be used routinely rather than occasionally.
1.33 Residual ARC-d: Predominately Type-2 traffic (refer to section 1.112(ii) for definition), therefore subject to the highest level of tactical mitigation due to highest severity consequence and highest safety standard airspace. Specific category operations likely to be exceptions (e.g., via certified DAA system) rather than the normal for this ARC.
Generic guidance on the use of strategic mitigations
1.34 This section provides some generic guidance on the application of the strategic mitigations discussed within paragraphs 1.1 to 1.14. To meet the expectations of the residual ARCs described above applicants are encouraged to assess and make use of these strategic mitigations, or others that may be available. However, each application will still be assessed on a case-by-case basis and may not result in credit being given in the form of a reduced residual ARC. Applicants must also consider making use of additional mitigations to further reduce the safety risk to a level that is "as low as reasonably practicable (ALARP).
1.35 Irrespective of the Air Risk Class (ARC), an applicant must initially consider the expected ruleset of the airspace, Section 6 Airspace Classification, proposing changes only if necessary, and with agreement of the ANSP and authority.
1.36 Regarding strategic mitigation by pre-agreement of the use of ANSP services (SM6), it is worth noting that several different levels of service are currently used by crewed aircraft. Within UK airspace the level of service is in accordance with the classification of the airspace. For uncontrolled airspace and for VFR traffic within Class E a range of Flight Information Services may be available as described within CAP 774, including Basic, Traffic, Deconfliction and Procedural Services. ANSP services within both controlled and uncontrolled airspace typically fall into one of the following categories:
1.37 Separation or deconfliction services – These are used to provide structure to the traffic flow, hence reducing the crewed aircraft encounter rate to below the average traffic density of the operating area. Within crewed aviation an ANSP separation or deconfliction service is supported by a cockpit based ‘see-and-avoid’ layer and hence is not typically a single layer mitigation (unless operating under IMC). A UAS under a normal separation or deconfliction service would therefore generally be required to be supported by a tactical DAA capability, with the performance requirement defined by the encounter types and rates within the operating area.
1.38 Traffic Information services – These are typically used to alert a pilot to the presence of other aircraft, supporting visual acquisition (in support of visual deconfliction) rather than providing real-time intruder tracks for deconfliction. A traffic information service therefore typically only provides a secondary mitigation, alerting a remote pilot to potential traffic, and would therefore need to be supported by a tactical DAA capability, with the performance requirement defined by the encounter types and rates within the operating area.
1.39 It must also be noted that, dependent on the specific class of airspace and other services also being provided, the timeliness of an ANSP service may be affected by the current workload of the Air Traffic Controller or Flight Information Service Officer (FISO). Care must therefore be taken when utilising such services without the cockpit see-and-avoid layer upon which airspace safety is premised. Finally, instructions issued by controllers to pilots operating outside controlled airspace are not mandatory; however, the ATS rely upon pilot compliance with the specified terms and conditions so as to promote a safer operating environment for all airspace users.
1.40 Strategic mitigations suitable for residual ARC-a assignment are as follows:

(i) Segregated airspace, e.g., DA, TDA, TSA.

(ii) Atypical air environment.

1.41 Segregation by procedure. This is not the same as tactical ‘segregation’- i.e. instructions by the Air Traffic Service Unit, issued in order to provide this ‘segregation’ to the UAS. These procedures need to be agreed and promulgated before the operation takes place. An example of segregation by procedure, includes using appropriate operating area surveillance and / or contact requirements to enable UA landing ahead of entry by crewed aircraft into the operating area. Additional requirements may need to be met in order to use segregation by procedure, including requirements for the ANSP that may be triggered as a result of the specific procedure, for example, change of use of airspace..
1.42 However, it should be noted that segregation of UA from crewed aircraft is not considered to be a scalable solution, hence the strategic direction of the CAA, as set out within the Airspace Modernisation Strategy (AMS), is towards integration of UA with crewed traffic.
1.43 Strategic mitigations in support of residual ARC-b assignment include:
1.44 TRA Special Use Airspace, in accordance with CAA’s current BVLOS airspace policy concept this airspace structure is currently required where a DAA capability is present, but the UAS is unable to fully comply within the accepted ruleset. Establishment of a TRA also enables use of a bespoke ruleset for all participants, e.g., requiring mandatory contact, carriage of EC, or potentially carriage of EC-In to support detection and avoidance of UAS with limited visual signature by crewed aircraft.
1.45 Restriction by boundary and / or chronology, using airspace characterisation to validate a default low encounter rate and the presence of only Type-1 traffic.
1.46 Density control of crewed traffic, allowing crewed aircraft encounters to be controlled to the required level, and limited to Type-1 only. Note that this may be enabled via either crewed aircraft access request (e.g., within a TRA) or an UAS Operating procedure that prohibits BVLOS flights when the traffic density is too high, which relies either on suitable surveillance or mandatory contact requirement ahead of entry, as potentially available within a CTR or TRA (assuming this is part of the bespoke ruleset).
1.47 Separation or deconfliction service, providing a level of structure to the traffic within the airspace to reduce the expected rate of crewed aircraft encounters below the mean for the area (which may already have been artificially reduced traffic density control).It may be argued that a structured/orderly flow of air traffic could reduce the encounter rate, compared to a ‘random’ flow of air traffic, outside such a structured environment.
1.48 Traffic information service, alerting the remote pilot to the presence of other aircraft, therefore providing a secondary mitigation and enhancement to self-separation.
1.49 Conflict alerting service, alerting the remote pilot to a potential hazard, therefore providing a secondary mitigation and enhancement to self-separation.
1.50 Promulgation of BVLOS UAS activity, for example via NOTAM, CANP and / or outreach to the local flying community, potentially reducing crewed aircraft encounter rate by increasing awareness of UAS and crewed aircraft activity within a specific region.
1.51 Depending on the specificities of the proposed operating area, one or more of the above mitigations may be required to achieve a residual ARC-b assignment. It should be noted that a residual ARC-b assignment provides a limited form of integration of UAS with crewed aircraft, relying on one or more accommodation measures as defined above. Such measures are required to justify a reduction in tactical mitigation performance requirement for DAA below that required for ARC-c, where DAA based tactical mitigation may be the sole replacement for cockpit based ‘see-and-avoid’.
1.52 Mitigations in support of residual ARC-c assignment (from initial ARC-d) are required to demonstrate the absence of both IFR traffic and Type-2 traffic. This may be achieved using an operational restriction by boundary and / or chronology supported by airspace characterisation. Dependent on the airspace classification some form of pre-agreement of ANSP support may also be required.
Airspace characterisation
1.53 Airspace characterisation data is expected to be used at several stages within the UK SORA air risk model. This section defines what is meant by airspace characterisation data, discusses different levels of data integrity, then provides some examples of the expected use.
1.54 Airspace characterisation data allows an applicant to account for local specificities in the proposed operating area, providing a level of granularity beyond the generalised air risk model. Examples of airspace characterisation data that support the UK SORA air risk assessment process include the following:
1.55 Types of aircraft, e.g., typical airspeeds & equipment carriage, potentially defined by different height bands.
1.56 Surveillance coverage, e.g., primary, secondary, ADS-B, multilateration, etc.
1.57 Traffic activity for each type, e.g., traffic movements, density of traffic in a given area, actual positions / paths, nominal encounter rates, e.g., total or per traffic type, airprox reports, TCAS events etc.
1.58 Given the potentially safety critical implications of the use of airspace characterisation data it is important to understand the associated level of integrity of the data source and any processing. The data integrity requirement may be expected to increase with the associated ARC. Three distinct data sources and associated levels of integrity are expected:

(i) ANSPs, based on actual movement numbers and primary and secondary radar data which can be expected to provide historical 4D trajectory information.

(ii) Crowd sourced organisation, such as OpenSky.

(iii) Qualitative local area surveys, e.g., via contacting the local flying communities and estimating typical traffic types, patterns and rates.

1.59 Example usage of airspace characterisation data within the air risk model includes:

(i) Initial Generalised ARC Flowchart guidance, e.g., demonstrating that a proposed operation avoids known IFR structures and / or known VFR traffic.

(ii) Local estimation of encounter types and rates, e.g., supporting a strategic mitigation of operational restriction by boundary, and / or chronology.

(iii) Definition of intruder aircraft encounter sets, used to navigate the air risk model and to assess tactical mitigations, e.g., DAA systems.

(iv) Quantitative cross check of proposed operation against the Target Level of Safety (TLOS). Quantitative methods are not directly considered within this initial version of the air risk model but will be included in a future update.

1.60 Airspace characterisation should also consider the impact of special events on routine traffic patterns. Such events can expect to be promulgated via NOTAM, but airspace characterisation may allow routine events to be identified in advance.
1.61 Finally, the Air Risk task force within the JARUS Safety and Risk Management group are currently developing an airspace risk characterisation document which will provide guidance for regulators, ANSPs and operators on methods for determining intrinsic air risk via airspace characterisation and encounter rate determination. It is expected that this document may be referenced for further information when available.