The next step is the determination of an initial Air Risk Class (ARC), which is an assessment of the risk to other airspace users, i.e. an assessment of the risk of mid-air collisions with other aircraft. ARC is a qualitative classification of the rate (probability) with which an unmanned aircraft may encounter a manned aircraft in typical general civil airspace. The ARC is an initial assignment of aggregate collision risk for the airspace, prior to the application of mitigation measures. The actual collision risk in a specific local operational area may vary significantly.
With regard to the determination of the initial Air Risk Class (ARC) – as stated in AMC1 to Article 11 to Regulation 2019/947 – SORA uses the operational airspace defined in the ConOps as a reference point for the assessment of airborne collision risk and by defining the Air Risk Category (ARC).
ARC can be reduced by implementing strategic and tactical mitigation measures. The use of strategic approaches reduces the level of ARC. Operating within specific time windows or constraints is one example of a strategic approach to reducing collision risk. Once strategic measures are in place, any airborne collision risk is further minimised by tactical mitigation measures such as detection and avoidance systems (DAA) or alternative methods such as ADS-B, FLARM, U-space or standard procedures.
In the context of the SORA procedure, the unmanned aircraft operator should liaise with the relevant air navigation service provider (in Poland this is the Polish Air Navigation Services Agency) or the provider of U-space services to obtain the required approvals. Regardless of the final results of the risk analysis, the operator of an unmanned aircraft system should focus on features that can enhance the detection of an object in the airspace. Therefore, technologies that enhance the electronic visibility or detection of the UAV are encouraged.
The first step in determining the level of risk in the airspace will be to determine an initial ARC defined from ARC-a to ARC-d, with ARC-a being defined as airspace in which the risk of collision between an unmanned aircraft and a manned aircraft is acceptable without the addition of any tactical mitigation measures, while ARC-b, ARC-c, ARC-d represent areas of airspace with an increasing risk of collision between UAS and manned aircraft.
According to the scheme (decision tree) used in AMC1 to Article 11 of Regulation 2019/947, airspace has been divided into 13 categories of collision risk. These categories have been characterised on the basis of flight altitude, controlled or uncontrolled airspace, flight in or outside the vicinity (environment) of an airport or heliport, airspace over an urban or rural environment and atypical (e.g. segregated) or typical airspace. When operations take place in regions of diverse characteristics, the applicant will be required to undertake an aviation risk analysis for the entire scope of operations in the area. Annex C of AMC1 provides an example scenario for operations in different airspace conditions. It highlights various methods of risk mitigation, such as geographical restrictions on operations, adjusting the timing of operations according to the intensity of air traffic, reducing the period of presence in areas with a higher risk of collision, and applying standard procedures and rules of the airspace.
Thus, if the applicant considers that the preliminary ARC is adequate for the conditions in the designated local operational zone, such ARC is treated as the final ARC. However, if the applicant is of the opinion that this preliminary ARC is excessively high for the specified conditions, it has the opportunity to adjust it. In this situation, the applicant should refer to Annex C, which describes the procedure for correcting the ARC, and apply the recommendations there to adjust the ARC level.
SORA divides mitigation measures into three categories: 1. strategic mitigation methods through the imposition of operational restrictions; 2. strategic mitigation methods through the introduction of general structures and rules; 3. tactical level mitigation measures. Annex C focuses attention on strategic mitigation methods and the preliminary ARC adjustment process.
Further, strategic mitigation methods are divided into: a) reduction methods through operational constraints that are under the control of the UAS operator; and b) reduction methods based on common structures and rules that are not under the direct control of the operator.
The first category of mitigation methods may relate to geographical aspects (such as specific areas or the size of an air zone) or temporal aspects (for example, limiting activities to specific hours).
Annex C discusses in detail the lowering of the preliminary ARC Lowering the ARC should be treated as an exception and must be robustly justified. Moving from a preliminary ARC to a final ARC requires a more sophisticated safety analysis, as this allows the UAS operator to act without additional mitigation measures at the tactical level. If the relevant authority allows the UAS operator to make a final ARC assessment for an operational area in order to comply with the SERA regulation, the operator must provide appropriate means and equipment as an alternative to the ‘visibility and avoidance’ requirement, or the relevant authority must issue an exception to the ‘visibility and avoidance’ and ‘maintain good visibility’ requirements.
In conclusion, it should be stressed that it is the competent authority that makes the final determination of the ARC level in the airspace and there may be situations where a conservative (SORA prescribed) assessment may not be sufficient. In such situations, the competent authority may raise the ARC to a higher level than recommended by SORA.
The next step will be the assignment of Robustness Levels (Final Specific Assurance and Integrity Levels), or so-called SAIL. By comparing the predetermined Ground Risk Level (GRC) and Airspace Risk Level (ARC), the SAIL is determined, representing the level of control over mission safety. Subsequently, the SORA analysis should lead to the identification of operational level safety objectives (Operational Safety Objectives), i.e. the establishment of CSOs.
The various CSOs are grouped according to the threats they help to mitigate, as effective methods to reduce the threats posed by UAVs. The SORA presents in tabular form a qualitative methodology for assessing countermeasures (defences) within an operation – precisely in the form of CSOs – and determining the associated level of resilience. The determination of the CSO level is qualitative, in 4 steps. The levels are as follows: “O” (optional) – the use of a specific barrier is optional and not required, “L” (Low robustness) – the use of a specific barrier is recommended with low robustness, “M” (Medium robustness) – the use of a specific barrier is recommended with medium robustness, “H” (high robustness) – the use of a specific barrier is recommended with high robustness.
As noted in the Easy Access Rules document for Regulation 2019/947, the table indicated above is a consolidated list of common CSOs that have historically been used to ensure safe UAS operations. It represents the combined experience of many experts and is therefore a solid starting point for determining the required security objectives for a specific operation. Additional CSOs for a particular SAIL and the associated level of resilience may be identified by the competent authorities that issue the operational authorisation. Thus, it should be summarised, following the Easy Access Rules document for Regulation 2019/947, that the SORA process provides the applicant, the competent authority and the air navigation service provider with a methodology that includes a number of mitigation measures and safety objectives that need to be considered to provide an appropriate level of assurance that the operation can be conducted safely. These are: (1) mitigation measures used to modify the internal GRC; (2) strategic mitigation measures for the initial ARC; (3) tactical mitigation measures for the remaining ARC; (4) adjacent area/airspace considerations; and (5) CSOs.
