Warns Possible Radio Altimeter Anomalies Beginning December 5

The impending rollout of 5G networks across the United States has been a concern for industry experts, with worries that the licensed frequency ranges used in the next generation network has some overlap with frequencies used in radar altimeters.

The FAA has released their bulletin outlining their concerns, including a request for industry action to aid in testing and evaluation. The agency needs wide-ranging data to establish the warranted level of caution going forward. With the aviation industry at odds with telecom giants, proper study is needed to evaluate real-world impact before establishing regulations and limitations on placement for the equipment. The new bands will be active from the scheduled start date of December 5, 2021 in 46 markets. 

The FAA states that mobile broadband rollout begins in the 3700-3980 MHz,  and notes that countries around the world have already implemented wireless networks from 3300-4200 MHz with varied regulations to prevent signal contamination in the terminal airport environment. Some solutions have included mitigations like temporary proximity and power restrictions for networks operating in the frequencies in question, but widespread study has not been thoroughly completed.

To date, previous developments have not led to known issues, though their overlap with radar altimeters is less likely given their lower active frequency range.

The bulletin outlines the FAA’s requests for information from possibly affected manufacturers. Radio altimeter manufacturers are requested to submit receiver RF selectivity, interference tolerance masks, and baseline operational specifications for each model number in production or in use, and approximate numbers of the same in service. Data may be marked as proprietary if needed, with directions for submission found in the SAIB. Manufacturers of aircraft equipped with similar equipment are requested to complete analysis or testing to determine the susceptibility in their systems to interference from fundamental emissions in the 3700-3800 Mhz band, as well as potential spurious signals in the 4200-4400 MHz band. 

The FAA has also requested that operators and pilots review 14 CFR Part 5 for Part 121 certificate holders where appropriate. Other certificate holders may use the Safety Management Systems to assess risk for their aircraft configuration.

Pilots should be aware of the potential degradation of the radio altimeter capabilities, and plan to compensate for in-flight anomalies.

Caution should be exercised for erroneous readings as well as loss of function. Trust in dependent aircraft systems should be reassessed, with backup equipment consulted to confirm positioning. If an anomaly is encountered, ATC should be notified as soon as practical, with as much detail as possible to help identify the source of the error. 

Further detail can be found in SAIB AIR-21-18. 

FMI: www.faa.gov, www.fcc.gov