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So, here's what's actually happening in a nutshell... The FCC allocated a specific band for radar altimeters to transmit in. The FCC only specified rules on tr
by tjohns 5y ago
So, here's what's actually happening in a nutshell...
The FCC allocated a specific band for radar altimeters to transmit in. The FCC only specified rules on transmission, and the radar altimeters are 100% compliant with the FCC rules and only transmit within their assigned band.
The FAA published TSO-87C, which is the technical spec for the radar altimeters. But this also just covers transmission and performance. There's nothing in this spec about rejecting external interference. The radar altimeters are compliant with this spec as well.
The problem is on the unregulated receive side. All radio signals have some amount of bleed-over into adjacent bands due to harmonic interference. Historically, the adjacent frequencies were used for low-power satellite communication, and the manufacturers added enough filtering to protect against the harmonic interference from these low-power stations. They did not anticipate a high-power (5G) signal in those adjacent bands, and nothing in the FAA/FCC specifications protected against it either.
So it's not that the radio altimeters are "using" the adjacent bands, but the receive filtering is (in some cases) insufficient to account for the new (louder) users of the adjacent bands.
- dataflow 5y agoWhat I'm confused about is, why are the FCC and FAA suddenly running into this problem now? Hasn't 5G been coming for a long time? What did they expect would happen instead?
- labcomputer 5y ago"5G" doesn't refer to one specific set of bands. Radar has been operating this way for decades, and only last year did the FAA decide to allocate the nearby spectrum to high power transmitters.
- dataflow 5y agoYeah so my question is (how) did they not see this problem last year? What made it so unpredictable until Feb 2022?
- mulmen 5y agoThey did know. Both the FAA and the FCC knew.
- dataflow 5y agoYeah so we circle back to my original question: What did they expect would happen instead? Why didn't they do anything? The problem wasn't going to just disappear randomly right?
- tjohns 5y agoTwo different government agencies with different incentives. The FCC really wants to proceed with the 5G rollout. The FAA really wants to make sure aircraft don't crash. They do not agree with each other on this, so they each proceeded in the only way they administratively could... The FCC continued with the 5G rollout, the the FAA recently banned use of approaches requiring a radio altimeter until each individual model can get recertified or upgraded - and the public will just have to tolerate additional travel delays for the next couple years as a result.
- mulmen 5y agoFAA issued NOTAMs. They don’t have the authority to stop the FCC from doing anything. FCC was warned by industry groups but dismissed the warnings. We’re watching the process play out.
- Wiseacre 5y agoHere's an article from 2020 [0]. I'm sure plenty of this prior research was classified. 0 - https://www.defensenews.com/2020/12/21/the-military-is-scrambling-to-understand-the-aviation-crash-risk-from-a-new-5g-sale/ https://www.defensenews.com/2020/12/21/the-military-is-scram...
- p_l 5y agoThe decision to sell frequencies as close as 220 MHz away from lower end of radar altimeter band came only in 2019, and aviation world already been working on possible interference at the time - which led to much higher guard bands everywhere else and/or extra requirements on use of 3.8-4.0GHz allocations, especially in terms of usage near airports. The issue is that the FCC sale of frequency spectrum up to 3.98GHz doesn't have such limitations, so essentially in 2019 aviation got an unique problem of extra heavy interference in USA