This is like your neighbor driving onto your lawn, and then demanding you move out of your house. They should drive right back onto their lawn.
The problem is on the radar altimeter's Rx side, not on the Tx side.
The radalt has Rx filters to reject noise on nearby frequencies. Cell towers have Tx filters to avoid broadcasting on nearby frequencies. But no real filter has 100% rejection outside the passband. That means that a 5G cell tower will broadcast some amount of energy within the spectrum defined for radalts. Also, radalts are not able to perfectly reject all energy that is outside, but arbitrarily close to their defined spectrum.
A second factor is that power received by a radar falls as the fourth power of distance between a radar and the target (in this case, the ground). Tx power goes as 1/r^2. The target, having received that 1/r^2 power, then re-radiates it back to the radar, where it again falls as 1/r^2. That means radars have to be designed to detect very small amounts of power. Because physics.
So the combination these factors leads to a situation where the high broadcast power of a cell tower inevitably leaks some energy into spectrum near that which radalts operate in, not all of which is effectively attenuated by the radalt's Rx filter. The small amount of energy that leaks past both filters can cause interference due to the sensitivity requirements of radalts.
At a fundamental level, it was really stupid for the FCC to allocate high power transmitters in a band right next to radars. This is starting to be a pattern: See the attempted Light-Squared allocation next to GPS (for which the received signal strength on earth's surface is lower than the thermal noise floor).
Based on the number of comments confidently proclaiming that radalts are somehow using spectrum they aren't supposed to, I'm starting to think that HN is being heavily astroturfed on this subject.
> This is like your neighbor driving onto your lawn, and then demanding you move out of your house. They should drive right back onto their lawn.
No, this is like your neighbor planting some very delicate flower near the property line and then asking you not to spray Roundup all over your property because some of it drifts onto his flowers.
What is your basis for this extraordinary claim?
Depending on the strength of 5G transmitters the higher order harmonics could be above the noise floor.
How else are you supposed to make sure that equipment made today won’t interfere with equipment made tomorrow if people don’t stick within their frequency allocations?
The claim above is basically the equivalent of “car should have always operated on the roads that are allocated for them, and should never have been driving through empty fields”. If you’re driving your car through a field you don’t own, you don’t get to complain when someone builds a house there.
All radio equipment is meant to be specified to operate only within its allocated spectrum, and to handle RF noise and interference from anything outside it’s spectrum.
How a radalt works is irrelevant, the allocation determines the operating parameters. If your equipment can’t operate within those parameters then it’s faulty, simple as that.
Nobody would try to defend a building whose foundation crept over outside of the plot of land belonging to it. If you’re house can’t standup without putting parts of itself into your neighbours garden, then your house is faulty, not you’re neighbours home when they decide to repurpose their garden.
I agree that quoted suggestion lacks nuance in the history that brought us to this state, but from a technical perspective it does not seem extraordinary. More that it simply advocates a very expensive and uncooperative way to solve a problem of the commons.
“Immunity” is largely your own problem though.
That's specifically for unlicensed Part 15 devices ref. https://www.ecfr.gov/current/title-47/chapter-I/subchapter-A... ... the point of that wording is that unlicensed devices have no rights vs. licensed emissions.
It would be different if Boeing designed their altimeters to only depend on spectrum that was allocated for aviation, but then the FCC came along and carved the 5G spectrum out of the aviation spectrum after Boeing built its planes.
Note, their are airplanes built today that don't have the metal fatigue issues - material selection is a complex process that each design needs to consider. I'm not sure what the 737 is made of.
If a TV station started leaking over another channel, you can bet that'd be shut down almost immediately, regardless of the cost to the station for putting up faulty broadcasting equipment.
Boeing should be required to fix their broken electronics. Sucks to suck.
When it comes to radar, they cannot increase the TX power and have to detect a lower RX power. Also since they are analog any interference manifests itself as a direct error.
It’s like saying you should never be able to hear a whisper from your neighbours house. Obviously you can limit the amount of loud noises coming from your neighbours. You can’t silence them completely.
other radar altimeters tolerate this just fine - it is a few specific models that have problems with rejecting signals that (according to design specifications) they should easily be able to reject.
> It’s like saying you should never be able to hear a whisper from your neighbours house
if local code says that a house should have noise insulation sufficient that a 100db noise source located at 30 meters away should produce no more than X noise level inside the house, and a 100db noise source at 60 meters away (twice the original specification) is still significantly greater than X - then yes, your house fails to meet code. It's defective.
nobody is blind to the problem of radio interference. Design specs for what level of noise needed to be tolerated were standardized in the 70s. That spec hasn't been reduced, actually it has been relaxed to over twice the original guardband specification, and certain altimeters still fail. They're defective, end of story.
Replace them with the models of radar altimeters which meet the original spec, done. They exist, it's not a universal problem, it's a few specific defective models of radar altimeters. Boeing just doesn't want to ground their fleet again, and the FAA is humoring them because of regulatory capture.
Please link to this specification. Because I've read the specification (TSO-C87), and it actually says nothing about required levels of interference rejection.
Should it? Probably. But neither the FCC nor the FAA wrote it in.
Any leaking that happens is caused by the 5G transmitters in the case under consideration. You are confusing two issues. The issue here is that strong and close transmitters can not be properly blocked by some of the radar altimeters in use under the separation rules established by the FCC.
An important job of the FCC is separating TV transmitters that are close in frequency so that they do not cause problems. If you live next door to a TV transmitter you will have no hope of receiving signals on adjacent channels. That is because of the physical limitation of your receiver and has nothing to do with the transmitter leaking anything.
Is there spectrum assigned alongside the radalt to ensure no crosstalk?
The FCC allocated spectrum for radalts to use, then set aside an equivalent amount of spectrum (which is an insane amount of spectrum for a guard band, easily double what most uses get) above and below the radalts spectrum as guard bands that no one is allowed to use.
The guard bands ensure that if there is a little spill over from 5G (which there won’t be because modern radios are better than that), or if the radalts have crappy filtering on their front ends (which it seems they do). Then nothing bad will happen, because the guard bands give everyone lots of space for sloppy engineering (which Boeing seems to be abusing)
And FCC rules only specified transmission frequency of radar altimeters, and radio is analog and perfect filtering is close to impossible (especially with the kind of chirping signal radar altimeters use, where whole 200MHz of spectrum is spanned in single chirp)