Does this mean Air Force 1 and the gold-plated Trumpbo Jumbo are grounded?
Trump spectrum sale leaves airlines with $4.5B bill for altimeter do-over
Airlines operating in the US may have to upgrade their aircraft radio altimeters again at a cost of billions of dollars, to avoid potential interference with cell networks following the Trump administration's decision last year to auction off additional spectrum to bidders. On Wednesday, the US Federal Aviation Administration …
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Thursday 8th January 2026 15:39 GMT Electronics'R'Us
Older designs
A lot of avionics have a pedigree from between ten and twenty years ago and there are good reasons.
An update to anything safety critical (and radalts can come under that heading) is a time consuming process as the new kit has to be thoroughly qualified and there is a rule: Don't be the first with anything. We only want to use a proven basis, which is sensible in safety critical design.
When those earlier systems were designed, the band limit filtering was easily sufficient for the operational profile required. As noted by at least one other, brick wall filters are very difficult to achieve (and we prefer to not have any software if possible).
Circuit fundamentals haven't really changed over the years although there are some newer modules that can help with the situation but I suspect part of the solution will be to detect interference and remove it. There are a number of ways to do that depending on what the interference type actually is.
So it is not that we don't use filters - we do. It is just that we need to update them to account for a new RF threat that did not previously exist.
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Thursday 8th January 2026 23:26 GMT Malcolm Weir
Re: Older designs
Yet apparently all the bad old systems were fixed two years ago (the last time the airlines had a whine about this). Oh, well...
Since the law only requires between 100MHz and 180MHz of spectrum to be auctioned from the 3.98Ghz to 4.2GHz band, the solution is trivial for those who've mastered applied counting: sell 3.98GHz to 4.16GHz, leaving at least 40MHz as a buffer (if you sell 180MHz) or more (if you sell less). And since the RA's were fixed two years ago, that should be ample.
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Thursday 8th January 2026 15:06 GMT yet another bruce
Lazy Altimeter Design
FCC equipment rules require devices not to cause harmful interference to authorized radio services and to accept any interference received from other legal radio emissions. There is no reason that a well-engineered radar altimeter operating in the 4.2-4.4 Ghz band should be compromised by 5G cell phone signals in the 3.7-4.2 Ghz band.
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Thursday 8th January 2026 15:59 GMT Jon Bar
Re: Lazy Altimeter Design
Yes. Why don't we design equipment to be resistant to vulnerabilities that don't even exist yet just in case they may arise later. /s
Buffer zones are there for a reason, and it's very easy to design something that meets the design specs but also radiates (far) outside the intended frequency range. It's a lot harder to design against something radiating outside tolerances.
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Friday 9th January 2026 18:33 GMT isdnip
Re: Lazy Altimeter Design
The proposed rules are quite hard for devices to meet. The interference doesn't come from cell phones, which are low power. It's from the base stations, which are allowed many kilowatts of EIRP. The oldest RAs were crap and the 3.7-3.98 GHz base stations were plenty far away. But 4.16 GHz is dangerously close and will likely need a whole new RA design.
Of course this is all vital to US national security, because the extra spectrum is needed to Win The 6G Race. If we lose it, then, say the advocates, we will need to learn to talk Chinese, and McDonalds will be replacing the Big Mac with subgum chicken chow mein. What's a few billion for airplane upgrades to save America from losing to China? Plus knocking some small CATV providers off the air, since the6 GHz band being taken away is used for channel feeds.
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Thursday 8th January 2026 16:02 GMT RMclan
As an undergraduate apprentice at BAe Hatfield in 1989 I spent 6 weeks in Flight Development and one of my projects was to write a program in Fortran to help calibrate a new radio altimeter for the BAe 146 at low altitudes.
The Flight Development aircraft kitted out with multiple data recording systems would make low altitude passes (between 50ft and 250ft) along the centreline of the runway recording the measured output from the radio altimeter system every tenth of a second.
At the same time a 50fps camera would track the aircraft from a datum point on the airfield recording the pan angle of the camera and a high precision time record onto the images.
We then manually measured the distance on enlarged photos between 2 targets on the side of the aircraft and the distance of the targets from the top and bottom of the photograph. Using this data along with the pan angle and the known position of the datum point compared to the runway centreline, my program used trig to calculate the height of the aircraft above the runway in a particular photo. This data along with the time stamp was then used to compare the altitude from the photograph to the recorded altitude from the radio altimeter.
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Friday 9th January 2026 05:09 GMT FuzzyTheBear
YAM
Yet another mess of the Trump administration. They do things without the knowledge or common sense required to make decisions. T'is but scratching the surface of the huge problems he creates daily for Americans. Everything he touches will need an overhaul once the USA gets rid of him. At the end of the month we're looking at another government shutdown , i mean .. everything he touches becomes a mess. Sorry for the pollitical angle to my comment but things must be said. Trump should have to pay for this from his own pocket. In fact politicians should foot the bill of any mess they create. This one's just 4.5 billions .. Pocket change for the man.
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Friday 9th January 2026 08:58 GMT Anonymous Coward
How long before he decides that auctioning off the 2.4 - -3.4 ghz range is a nice little earner, after all it's only being used for radar height detection and they can limit themselves to the new smaller 3.4-4.4ghz range.
Of course this will meant that radar altimiters have to be replaced (yet again), maybe there will be another law added that only US made radar altimiters can be used in US airspace (for national security of course).
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Friday 9th January 2026 15:30 GMT david1024
Fix the specs and stop crying
Overall electromagnetic requirement changes are over-estimated. Most likely they need to test the design, and when it passes, recertify the example parts at the proper maintenance window. Noone is spending ro that inflated #.
The estimate basis is if they need to redesign and replace all of the altimeters. Which is not likely at all. Just more 'scary orange man' hand-wringing in a world that just doesn't exist. Just stop--and get back to tech as tech.
Biting the hand that feeds IT
