I’m reminded of Aeroperú Flight 603.

The pilots suspected their altitude indicator was wrong, so they asked air traffic control to confirm their altitude. They did and it agreed with their own readings so they ignored the subsequent indications that they were going to crash into the ocean, which is unfortunately what happened.

Neither the pilots nor air traffic control apparently knew that both readings were supplied by the same source: the plane’s transponder.

I often think of this when I hear people say that multiple sources indicate something strange. These systems are often intertwined and networked in unintuitive ways and just because something appears on multiple panes of glass does not necessarily mean it’s been independently confirmed.

Something similar happened with the Hubble telescope's infamous blurry mirror

The manufacturer tried using two instruments to measure the shape of the mirror. One instrument said that the mirror had unacceptable error. The other said it was perfect. The latter instrument was designed to be more precise, so they went with that and ignored the measurements from the other null corrector which disagreed.

The problem was, the more precise null corrector was the same one that was used to help construct the mirror originally. All it could verify was that the mirror was built to the same specs as the null corrector. No one realized that the "better" null corrector itself was off until it was too late.

That's amazing. So ATC didn't have the capability to measure the altitude of any plane themselves?

They can via primary radar, but the default displays secondary radar information (the transponder interrogation) as it tends to be significantly more precise. Surveillance radar has very coarse altitude resolution.

Isn't it a given that if a plane is requesting a read that they want to check their measurement against a different one, though?

ATC probably thought they were giving a different measurement. Especially in older planes, systems aren't particularly well integrated, and most of the time you ask them what they're seeing because you can't check your own Mode C altitude in the cockpit and want to compare it to your barometric altimeter (and yes, they generally do use the same static source, but it isn't atypical for them to read differently).

Thinking about it more, I've never tried asking ATC for an altitude check specifically from primary radar. It may well be that the system isn't set up for it, even if it's technically computable to some degree. When I'm talking coarse, I mean an altitude spread at range that may be measured in miles.

That sounds kind of worrying. It sounds like if a pilot has a malfunctioning altitude meter they can only check it visually or have a completely independent GPS on hand. I don't see what else the crew or the ATC could have done in that instance, given their equipment. I also don't understand why a radar of all things can't give a more precise measurement of altitude. Even a person with binoculars could probably be more precise than +/- 1 mile.

For what it's worth, I'm not particularly worried. My understanding is that the accident was caused by a combination of poor maintenance practices, inadequate preflight procedures, and possibly poor ground procedures by the pilots that led to them taking off with an inoperative pitot-static system. Modern aircraft can also get altitude data from GPS (as you noted), as well as inertial navigation systems and radio altimeters, and generally have redundant pitot-static systems (at least an alternate static air source) for these kinds of contingencies.

Surveillance radars have narrow beam widths in azimuth, but broad in altitude to allow them to quickly survey around them and get good enough awareness of where everyone is. There's always a trade-off between resolution and volume--some military tactical and approach radars have very precise azimuth and altitude capability, at the cost of scan volume/time. Mode C (and now, arguably, ADS-B) was the solution to this problem, allowing ATC radar to have both good position and altitude precision with an acceptable refresh rate, at a non-prohibitive cost.

You'd be surprised--unless your hypothetical binocular observer was really handy with stadiametric ranging, I imagine they'd be nowhere near a mile. They might be able to get a decent azimuth and elevation on a single aircraft, but that's a much different problem than determining position near-simultaneously for a hundred.

A barometric altimeter and altitude reporting transponder are self contained units with separate determination of altitude based on pressure. The source in common they share is the static (pressure) system.

In the case of this flight, maintenance tape covered the static ports, rendering all instruments that use the static pressure system, significantly in error. Although there's more than one port, and thus redundancy, the maintenance operation covered all of the ports.

Sensor fusion glitches are high on my personal list of “what could this be explained by”.

There are all the passive system “failure” modes, between optical (and infrared) focusing, sensor noise, radar synthesis, multi system combat data links passing in extra information the system is designed to integrate into targeting/tracking/tasking the sensors. There’s a lot that can go wrong… add active failures like the GP comment mentioned about the astronomical sensor saturation and lasers, and the picture is even messier, a military drone/vehicle could be using active countermeasures like dazzling lasers and phased array RF emitters to try and mess with the pilots, the sensors, and the data links…

At the end of the day I just want the stigma around investigating the root cause of this stuff to go away, it’s like the Hound of the Baskervilles, stuff can genuinely have all the trappings of being supernatural or aliens or whatever… and still turn out to have a mundane explanation when properly investigated. I want the systems engineers and the scientists and the researchers and anyone else who sees stuff like this to log the “bug report” and for people to get to the bottom of things, because as long as we live in a world where weapons like fighter jets and guided missile destroyers are needed and/or in use, I’d like to know the systems and people aiming them at targets are making as few mistakes as humanly possible… which won’t happen if people don’t rigorously investigate and dig out the root causes of stuff like these UAP incidents.

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Or an undisclosed radar/laser capability. Plasma can reflect radar and it emits light - a focused phased array could create local plasma somewhere that other craft pick up.

Ofc, once you#ve done that you may not want third parties to know. Something as banal as 'we made air glow a few miles away' can expose capabilities of some vital systems that you don't want third parties to know. Edit: I mean, with that information, someone could do some napkin math on what can still be seen and what can't (by e.g. a naval radar suite, focusing on a spot in the air using a phased array).

Anecdotal obviously but I’m in the industry (I’m assuming you are as well from the nature of your comment) and I work with many current and former pilots and I think this characterization is a little unfair. Many military pilots do come from engineering backgrounds since a degree is generally a requirement for pilot training, and an engineering degree is often viewed as a good qualifier for becoming a pilot. Military pilot training is also highly technical in nature and involves rigorous classroom learning on the underlying physics and systems of tactical aircraft. But aside from this, my experience is that technically competent (not necessarily a given) pilots can have a better practical understanding of these sensor systems than the engineers that designed them because the designers very rarely get a comprehensive picture about what it’s actually like to use them in combat scenarios, simulated or otherwise. The problem in my mind with simply writing off the TICTAC and GIMBLE incidents as a bad reading of noise on the scope is that, from multiple pilot testimonies, dozens of tracks were observed via radar, ir/eo, and visually (like just the eyeballs). I’m not saying that these are craft that are violating the known laws of physics, I personally have other theories, but I’m not sure it’s just noisy sensors either.

You’d be hard pressed to be a naval aviator without a degree. Even if you’re prior enlisted, you’re going to go back to school to get some kind of 4 year degree before getting a commission.

Maybe a long time ago you could attend OCS as prior enlisted, but I think we’re way past those days: All service branches have their pick of officers with degrees to send for pilot or navigator/RIO training.

Fair--we've both seen different parts of the elephant. I agree that military flight training is extremely rigorous, but I'd argue that it's so broad it has to gloss over a lot of technical detail in order to focus on developing the skills and instincts that matter in combat.

Based on the videos and reports I've seen, most or all of these cases involve sensor operation outside of typical A/A and A/G workflows, where the type of detailed technical knowledge that the system designers have could be more useful than years of practical experience. I've heard (and told) enough sea stories to be skeptical of eyewitness testimony (not that I think the aircrew who reported seeing this stuff are lying, but I've been humbled often enough by my own fallible recollections), and none of the publicly available footage I've seen of these events strikes me as anything more than a confluence of sensor limitations and (very understandable) human factors.

There are a number of reasons I'm glad the government is running these kinds of reports to ground, but I think wildly credulous reporting (particularly the "I'm not saying it was aliens, but it was aliens" interviews by a handful of aviators, but even presenting advanced adversary technology as the sane middle ground) is blowing this way out of proportion. We're already taking it way more seriously than it deserves, and there are plenty of other alligators closer to the boat.