Risk Assessment and SORA Methodology
One possible way to operate in the specific category is to obtain an operational authorisation issued by the competent authority. As Article 11 of Regulation (EU) 2019/947  indicates, the application for operational authorisation is correlated with a safety risk assessment.
EASA recommends the Specific Operations Risk Assessment (SORA) methodology, developed by the Joint Authorities for Rulemaking on Unmanned Systems (JARUS), as AMC (Acceptable Means of Compliance) to comply with the said article .
SORA provides a risk-based model that guides the operator and the authority having jurisdiction in determining whether an operation can be carried out safely. The model considers the threats of different natures for a specific hazard and the mitigation methods for the latter.
The SORA introduces the robustness term to determine the level at which the different mitigations or OSOs (Operational Safety Objectives) should be applied. These levels are divided into low (L), medium (M) and high (H). They are obtained as a combination of an integrity level (Integrity), defined as the safety gain provided by each mitigation, and a guarantee level (Assurance) on how to demonstrate the effectiveness of such mitigation. Each robustness level can be obtained according to the following combinations:
Figure 2. Levels of robustness
Source: JARUS guidelines on Specific Operations Risk Assessment (SORA) 
The SORA process is divided into eleven steps that follow a logical sequence to analyse the proposed concept of operations (see figure below). This methodology takes into account the risks associated with the ground and air parts, including the applicable mitigations to reduce them. In relation to a certain level of risk (ground and air), a list of OSOs that must be met with a specific level of robustness is provided. The result of the SORA process is the recognition that the proposed concept of operations can be carried out with an acceptable level of risk. The following is a general explanation of the different steps of SORA:
Figure 3. The SORA process
Source: UAV Navigation-Grupo Oesía
- Prior Evaluation: consists of a previous step in which the operator must verify the operation’s viability and ensure that it is not subject to any exclusion by the competent authority.
- Step 1 – Description of Concept of Operations (ConOpS): This step collects and provides technical, operational, and human (training) information regarding the intended use of the UAS in your operation.
- Step 2 - Obtaining the Intrinsic Ground Risk Class (GRC): The intrinsic GRC refers to the initial unmitigated risk that a person may be hit by a UAS (in case of loss of control). This parameter is linked to the characteristics of the UAS (such as MTOM, speed, etc.), the conduct of the flight, and the characteristics of the ground zone. The GRC is evaluated quantitatively as a level from one to ten (from lowest to highest risk).
- Step 3 - Obtaining the Final GRC: this consists of reducing the intrinsic GRC by applying mitigation barriers. The final GRC must be equal to or less than 7 to continue with the SORA process.
- Step 4 - Obtaining the Initial Air Risk Class (ARC): The initial ARC refers to the risk associated with a mid-air collision with a manned aircraft. Its determination is made through the corresponding Airspace Encounter Category (AEC) based on the type of airspace in which the operation takes place. The ARC is qualitatively assessed at four levels (from lowest to highest risk): ARC-a, ARC-b, ARC-c, and ARC-d.
- Step 5 - Application of Strategic Mitigations: this consists of reducing the initial ARC through the application of strategic mitigations that demonstrate that the risk of collision with manned aircraft in the operational volume is really lower than that obtained through the AEC. The result of these mitigations is called residual ARC.
- Step 6 - Tactical Mitigation Performance Requirement (TMPR) and robustness levels: this consists of reducing the residual ARC by applying tactical mitigations. They take the form of technical requirements that the applicant must demonstrate that it meets a certain level of robustness. A distinction is made between requirements in the form of Sense and Avoid (SAA) for VLOS (Visual Line Of Sight) operations and Detect and Avoid (DAA) for BVLOS (Beyond Visual Line Of Sight) operations.
- Step 7 - Determination of SAIL: this step is determined by the value of the final GRC and the residual ARC, respectively obtained in steps #3 and #6. The SAIL is quantitatively assessed at levels one to six (from lowest to highest risk).
- Step 8 - Identification of the OSOs: The OSOs are the requirements to guarantee the safety and viability of a UAS operation. These requirements are divided into 24 OSOs, which must be met at a certain level of robustness depending on the SAIL. The OSOs are divided into the following areas: UAS technical issues (OSOs #1- #10), deterioration of the external systems that support the operation of the UAS (OSOs #11 - #13), human error (OSOs #14 - # 20) and adverse operating conditions (OSOs #21 - #24).
- Step 9 - Adjacent Area/Airspace Considerations: This step assesses the risk associated with loss of control of the UAS with consequent infringement of adjacent areas on the ground and/or adjacent airspace. Depending on the characteristics of the adjacent airspace/area, the operator must guarantee a specific level of containment.
- Step 10 - Safety Report: in this last step, the UAS operator prepares the documentation, including the risk assessment and the corresponding evidence, which will be sent to the competent authority to obtain operational authorisation.
The SORA structure presented in this document is based on SORA version 2.0 . JARUS has produced an update corresponding to version 2.5, which is under review and is expected to be published in the coming months.
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