With the growth of unmanned aircraft systems (UAS) and the increased demand for reliable control and non-payload communications (CNPC) for safe UAS operation, the FCC has established new frequency allocation rules in Part 88 for the Unmanned Aircraft Control Service (UACS). Specifically, the FCC allocated the 5030-5091 MHz band for the UAS Control and Non-payload Communications (CNPC) necessary to support unmanned flights. This article delves into the structure of frequency allocation within Part 88, comparing it to other dynamic frequency management systems like those in CBRS (Part 96), and discussing the safeguards in place to avoid interference with adjacent Radio Navigation Satellite Service (RNSS) and AeroMACS allocations. Learn More about Part 88 (UACS).
Frequency Allocation Structure in UACS
Under Part 88, the FCC introduced a new mechanism called the Dynamic Frequency Management System (DFMS), an automated coordination system designed to manage the shared use of spectrum in the 5030-5091 MHz band specifically for CNPC. According to FCC WT Docket No. 22-323, the DFMS allows UAS operators to submit spectrum requests for temporary, protected use in specific geographic areas for defined time periods. Unlike traditional fixed spectrum licenses, the DFMS enables UAS operations to dynamically access spectrum without requiring long-term, exclusive licensing. The aim is to efficiently support the short-term and dynamic communication needs of UAS while preserving spectrum availability for other users.
Dynamic Frequency Management System (DFMS) in UACS vs. Spectrum Access System (SAS) in CBRS
Both the DFMS for UACS and the Spectrum Access System (SAS) for the Citizens Broadband Radio Service (CBRS) use a dynamic approach to allocate spectrum access on an automated basis. However, there are notable differences between the two systems in scope and implementation. Learn More about CBRS Spectrum Allocation.
Purpose and Band Management:
- DFMS (UACS, Part 88): The DFMS is focused on providing interference-protected access to UAS operators for CNPC in the 5030-5091 MHz band. The system is optimized to support short-term, time-bound UAS operations, where each request corresponds to a specific flight.
- SAS (CBRS, Part 96): The SAS manages the 3550-3700 MHz CBRS band, supporting a tiered user structure including Incumbent, Priority Access License (PAL), and General Authorized Access (GAA) users. CBRS provides managed spectrum sharing between public and private entities with longer-term needs for spectrum access.
Automated Assignment and Interference Protection:
- DFMS: Requests to the DFMS must include operational details such as location, duration, and altitude to prevent interference. The DFMS dynamically adjusts assignments to avoid conflicts based on available spectrum and flight-specific parameters.
- SAS: The SAS system requires periodic heartbeat signals from CBRS devices (CBSDs) to confirm that spectrum assignments are active and interference-free. Through these heartbeat exchanges, the SAS can rapidly reallocate frequencies in response to higher-priority users, such as military radar.
Coordination and Flexibility:
- DFMS: Unlike CBRS, where heartbeats validate ongoing usage, DFMS requires advance coordination for each UAS operation. However, it offers flexibility by not mandating strict “flight plan” information, instead requiring basic details necessary for interference modeling and spectrum reservation. The DFMS tailors frequency assignments to match the geographic, temporal, and spectral needs of each UAS flight, supporting flexible, on-demand access.
- SAS: The SAS can modify frequency assignments based on real-time needs and regulatory priorities but operates without requiring detailed operational parameters.
Protection of Adjacent RNSS and AeroMACS Services
The UACS spectrum allocation and associated technical rules are carefully designed to minimize the risk of interference with critical adjacent services:
RNSS (5010-5030 MHz): Radio Navigation Satellite Service is protected by technical requirements that include limits on out-of-band emissions (OOBE) as specified in RTCA DO-362A standards. Additionally, the separation between UACS (5030-5091 MHz) and RNSS frequencies minimizes the potential for interference, ensuring safe and reliable satellite navigation.
AeroMACS (5000-5030 MHz, 5091-5150 MHz): AeroMACS, a broadband service supporting airport surface operations, is allocated to both lower and upper adjacent bands around UACS frequencies. To prevent interference with AeroMACS operations, UACS deployments must adhere to specific antenna siting and OOBE limits in accordance with RTCA standards. The FAA guidelines recommend that UAS ground station antennas avoid pointing directly toward AeroMACS base stations at airports, reducing the likelihood of interference in critical airport areas. Learn more about AeroMACS.
Guard Bands: While specific guard bands have not been mandated, the FCC has structured UACS spectrum usage and technical requirements to provide adequate buffer zones between RNSS, AeroMACS, and UAS CNPC, limiting potential cross-service interference.
Key UACS Regulatory Parameters for DFMS
Under Part 88, the DFMS is required to implement several regulatory parameters to manage the shared spectrum for UAS operators:
- Frequency Assignments: DFMS assignments are made for limited durations and specific geographic areas, with a maximum flight duration of 24 hours per assignment and a restriction on advance reservation to seven days.
- Automation and Coordination: DFMS administrators must ensure automated responses to spectrum requests and coordinate with any other DFMS systems to prevent conflicting assignments.
- Flexible Request Processing: DFMSs are permitted to adjust the frequency assignment terms based on availability and interference considerations, ensuring that UAS operators receive reliable access without monopolizing spectrum resources.
Conclusion
The FCC’s Part 88 allocation and DFMS framework represent a significant step in supporting the UAS industry by creating a dynamic, automated system to manage frequency access in the 5030-5091 MHz band. Drawing inspiration from the CBRS SAS model, the DFMS provides a tailored approach for UAS operations with time- and location-based frequency assignments. Through careful spectrum allocation, coordination requirements, and built-in protections for adjacent RNSS and AeroMACS services, Part 88 aims to balance the needs of UAS operators and existing users in adjacent bands, allowing for efficient, secure, and reliable unmanned aircraft control communication.
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