In our previous blog, we explored Step 2 of SORA, where we determined the Intrinsic Ground Risk Class (iGRC) by evaluating the aircraft's physical characteristics, operational footprint, and population density. Now, in Step 3, we examine how strategic and tactical mitigation measures can reduce the iGRC to determine the Final Ground Risk Class (GRC). This step is essential for ensuring that drone operations adhere to safety regulations while minimizing risks to people and property on the ground.

Understanding the Final Ground Risk Class (GRC)

The Final Ground Risk Class (GRC) represents the adjusted level of ground risk after implementing mitigation measures. The goal of this step is to reduce the likelihood and consequences of an unmanned aircraft (UA) impacting the ground by applying strategic and tactical mitigations.

Key Principles of Step 3

• The intrinsic risk of a UAS operation can be mitigated before and during flight.

• 
  

  
  

• Risk reduction is achieved through strategic mitigations (e.g., operational restrictions, controlled ground areas) and tactical mitigations (e.g., onboard safety systems, parachutes).

• 
  

  
  

• The Final GRC is determined by subtracting the mitigation credits from the iGRC.

• 
  

  
  

• If the Final GRC is greater than 7, the operation falls outside the scope of SORA and may require certification.

Mitigation Strategies for Final GRC Determination

1. Strategic Mitigations (M1)

These mitigations reduce ground risk before the flight takes place and typically involve operational and procedural restrictions.

M1(A) Sheltering

• Assumes people spend most of their time indoors and that structures provide a level of protection against drone impacts.

• 
  

  
  

• Sheltering mitigations cannot be used for operations over open-air assemblies or large open spaces without cover.

• 
  

  
  

• Integrity levels:

  • Low: Operator declares the presence of sheltering structures.

  • 
    

    
    

  • Medium: Uses time-based restrictions to maximize sheltering effectiveness.

  • 
    

    
    

  • High: Requires substantiated structural integrity analysis.

M1(B) Operational Restrictions

• Limits operations to areas where fewer uninvolved people are present.

• 
  

  
  

• Methods include scheduling flights during low population density periods and avoiding highly populated regions.

• 
  

  
  

• Integrity levels:

  • Medium: Justifies lower population density based on data analysis.

  • 
    

    
    

  • High: Requires third-party validation of population density reductions.

M1(C) Ground Observation

• Requires the use of ground observers or onboard sensors to monitor the operational area in real time.

• 
  

  
  

• Allows the remote pilot to dynamically adjust flight paths to avoid uninvolved persons.

• 
  

  
  

• Integrity levels:

  • Low: Ground observation conducted by remote crew members.

  • 
    

    
    

  • Medium: Incorporates sensor-based detection and automated alerts.

  • 
    

    
    

  • High: Uses AI-based tracking with real-time avoidance capabilities.

2. Tactical Mitigations (M2)

These mitigations reduce the effects of a ground impact and apply during flight.

M2(A) Impact Dynamics Reduction

• Involves design features or safety equipment to reduce the impact energy of a falling drone.

• 
  

  
  

• Examples include parachute deployment systems, autorotation capabilities, and frangible materials to lessen impact severity.

• 
  

  
  

• Integrity levels:

  • Medium: Reduces impact risk by 90% (one order of magnitude).

  • 
    

    
    

  • High: Reduces impact risk by 99% (two orders of magnitude).

• 
  

  
  

• Assurance levels:

  • Medium: Requires simulation-based validation and operational experience.

  • 
    

    
    

  • High: Requires third-party validation, testing, and flight demonstrations.

Critical Areas and Maximum Characteristic Dimensions

The critical area is the projected ground impact zone of an unmitigated UAS crash. It is directly related to the maximum characteristic dimension of the UA. The larger the aircraft, the larger the critical area, and thus the higher the ground risk.

Operators can apply M2 mitigations to reduce the critical area by demonstrating the effectiveness of parachutes, energy absorption designs, and impact reduction techniques.

Applying the Mitigation Measures

Mitigation measures must be applied in numerical order to determine their cumulative effect on reducing ground risk. The table below summarizes the credit values assigned to each mitigation type:

Determining the Final GRC

1. Identify the iGRC based on Step 2 calculations.

2. 
   

   
   

3. Apply applicable mitigation measures sequentially.

4. 
   

   
   

5. Subtract the cumulative credit from the iGRC to obtain the Final GRC.

6. 
   

   
   

7. Assess whether further mitigations are required or if the operation should be reassessed under a different risk category.

Conclusion

Step 3 of the SORA process allows drone operators to implement effective mitigations to reduce the ground risk of their operations. Understanding which mitigations apply to your operation is critical for ensuring regulatory compliance and operational safety.

At AirHub Consultancy, we support enterprises and public agencies in conducting risk assessments and integrating appropriate mitigations. Our AirHub Drone Operations Platform provides tools for analyzing operational environments, identifying population densities, and simulating risk mitigation strategies.

Stay tuned for our next blog, where we will explore Step 4 of SORA: Determination of the Initial Air Risk Class (ARC)!

Need help with your SORA application? Contact AirHub Consultancy for expert guidance on navigating the SORA process and ensuring compliance with UAS regulations.