New 5G Plan Could Jam Radar Altimeters


The Radio Technical Committee for Aeronautics (RTCA) is warning that a plan to allow 5G cell signals in radio spectrum adjacent to the frequencies used by radio altimeters could threaten flight safety. The private, not-for-profit organization has submitted a technical study to the Federal Communications Commission saying that 5G signals, including those coming from cellphones aboard aircraft, can interrupt radio altimeter operations, causing a cascading series of errors in aircraft flight control systems. “The results presented in this report reveal a major risk that 5G telecommunications systems in the 3.7–3.98 GHz band will cause harmful interference to radar altimeters on all types of civil aircraft—including commercial transport airplanes; business, regional, and general aviation airplanes; and both transport and general aviation helicopters,” RTCA says in its executive summary of the data it compiled. “The results of the study performed clearly indicate that this risk is widespread and has the potential for broad impacts to aviation operations in the United States, including the possibility of catastrophic failures leading to multiple fatalities, in the absence of appropriate mitigations.”

In December, the FCC is planning to auction off a portion of radio spectrum (3.7-3.98 GHz) for “flexible use” including 5G. That slice of radio spectrum is adjacent to the 4.2-4.4 GHz portion assigned to aeronautical equipment all over the world. Radar altimeters use that band to measure the distance between the aircraft and the ground in real time, vital information for a host of systems and warnings on thousands of aircraft. “… Failures of these sensors can therefore lead to incidents with catastrophic results resulting in multiple fatalities,” the executive summary says. Future radar altimeters will be hardened against 5G interference but there are tens of thousands of legacy units in aircraft that are at risk of failure, the RTCA says. “Given the extent to which the safe interference limits are exceeded and the breadth of the impacts to aviation safety, the risk of harmful interference to radar altimeters cannot be adequately mitigated by the aviation industry acting alone,” the report says. It calls on the mobile industry, aviation and radio spectrum regulators “to work together to ensure that safety-critical aviation systems will continue to be protected for the purposes of public safety.”

Russ Niles
Russ Niles is Editor-in-Chief of AVweb. He has been a pilot for 30 years and joined AVweb 22 years ago. He and his wife Marni live in southern British Columbia where they also operate a small winery.

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  1. The people who generate these study summaries never seem to write plainly enough. This one suggests that “mitigations” should be considered, without offering the most obvious mitigation which is “Don’t do it.”

    • RTCA doesn’t practice plain English. Have you read the full report? I haven’t. But, having worked with RTCA’s protocols I suspect they are all saying, “Don’t do it as presently planned.”

      And, keep in mind, the field tests are yet to come before the FCC’s signs off.

      • This is the Lightsquared/Ligado fiasco repeated. What is it about the physics of RF propagation and detection the FCC doesn’t seem to feel is pertinent? With Lightsquared the FCC allowed the purchase of Satband low power orbit to earth comm frequencies in the GPS guardband. Then allowed Lightsquared and Phil Harbin to “supplement” orbital comm signals with “terrestrial towers” to “fill in the gaps” with high powered terrestrial signals which put first order harmonics into the GPS bands. The initial low powered tests disrupted GPS at 10,000 ft for 1500 nm. Finally Lightsquared went bankrupt, now Ligato arising from the ashes is singing the same song: It’ll work this time and here’s the lightsquared data to prove it, so just give it to us.

        And the story repeats… with radio altimeters? Glad I have my altimeter correction card glued to my forehead.

  2. The US Defense Department had already spoken up about this bleed through and I believe NOAA/NASA had also expressed strong concerns about the auctioning off these frequencies because they impact satellites from measuring ground temperatures which is important for weather reporting and collecting data for climate change.

    Impact to Aviation, Check.
    Impact to Military Defense, Check.
    Impact to NOAA, GPS, and weather reporting, Check.

    They’ll sell it anyway because…Profit!! and wait to clean up the fallout when planes crash and GPS becomes more unreliable.

  3. This makes a lot less sense to me than the GPS issue. These 4.2G band is 200+MHz away from 5G (which is HUGE), and the new transmitter (5G) can easily be required to have fairly tight filtering, just like it already does in the LTE bands. In addition, radar typically interferes with narrowband signals, not vice-versa.

    In the GPS case, the problem is the millions of GPS receivers that were fielded had very little interference rejection filtering, so even a tightly filtered Ligado will still interfere with all the poorly filtering GPS receivers. In other words, the fault is not Ligado’s, but the fact GPS receivers were poorly designed, widely fielded, and there’s no going back now.

    If this is a real issue, please enlighten me. Otherwise, this sounds more like an attempt to tie a trivial issue to a critical problem which only waters down the urgency of the critical issue and elevates the significance of the irrelevant one.