HIRF Avoidance for VTOL/AAM Vehicles

The need to protect air vehicles from High-Intensity Radiated Fields (HIRF) has increased substantially in recent years due to several factors: greater reliance on digital electronics, reduced inherent shielding in modern airframe designs, higher data and processor speeds, broader frequency spectrum usage, and the growing density of transmitters. Common HIRF sources include radars, broadcast towers, satellite communication uplinks, and microwave links. Traditional approaches to HIRF protection require costly shielding and filtering measures, particularly for rotorcraft and advanced air mobility (AAM) vehicles, which operate closer to ground-level transmitters. This paper proposes a cost-effective alternative: a HIRF avoidance strategy using geographic “HIRF-maps”. Instead of designing vehicles to meet the stringent HIRF tolerance levels defined by rotorcraft standards, this approach allows for lower vehicle tolerance levels -provided they maintain safe standoff distances from known transmitter sources.

The proposed method calculates these avoidance distances based on each vehicle's electromagnetic tolerance and the power output of nearby transmitters. The concept was demonstrated using MATLAB and government transmitter databases to generate HIRF-maps highlighting risk zones for flight planning. These maps can be integrated into navigation tools to guide AAM operations away from high-intensity fields.

This approach is well suited for civilian AAM applications in complex urban airspaces and also applicable in non-urban areas. It prioritizes affordability, flexibility, and safety by relying on informed avoidance rather than overengineering. Field data from major cities such as New York indicate that some urban environments significantly exceed standard HIRF levels, highlighting the need for customized tolerance thresholds and avoidance protocols.

A key contribution of this paper is the recommendation of urban tolerance thresholds to support AAM vehicle certification and flight planning. Based on transmitter data and maps from twelve major U.S. cities, a minimum tolerance level is proposed that enables vehicles to withstand most common transmitters, limiting the need for avoidance maps to only the most powerful emitters. Outside urban areas, HIRF avoidance is generally simpler due to lower transmitter density; however, vehicles may still encounter high-power pulsed sources such as weather, aviation, and military radars. The paper also recommends a peak vehicle tolerance level and corresponding stand-off distances based on emitter locations.

The proposed avoidance framework offers a software-based mitigation strategy that reduces the need for heavy shielding while supporting compliance with HIRF certification requirements. It can be integrated into flight control and navigation systems to help plan HIRF-aware routes in real time or during pre-flight planning. This innovation improves operational flexibility, reduces costs, and enhances safety for AAM operations, particularly in dense urban environments where electromagnetic congestion is highest.