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F-35's radar invisibility cloak in question (c4isrnet.com)
105 points by XnoiVeX 21 days ago | hide | past | web | favorite | 145 comments

This isn't strictly just about a weakness in the F-35's radar resistance, but that the whole concept of "stealth" aircraft may be limited in the future thanks to ever increasing computation power and clever usage of existing radio waves (making effective, passive, "radar" that cannot be easily overcome). But as the article points out, this cannot yet be used for guided missiles, and is very early tech regardless.

I suspect the future won't be "super weapons" like the F-35, but instead just "mass" weapons like tens of weaponized drones. Where the mission isn't to be invisible but to simply overwhelming enemy defenses (essentially the World War I strategy, but without the massive numbers of dead soldiers).

PS - The people pointing out that the F-35 had radar reflective disks added didn't seem to have understand the article (or technology). This is using entirely different methods to detect the aircraft, not radar waves, so that's irrelevant. The F-35 could be broadcasting a transponder, and it wouldn't undercut the technology discussed.

> The people pointing out that the F-35 had radar reflective disks added didn't seem to have understand the article (or technology). This is using entirely different methods to detect the aircraft, not radar waves, so that's irrelevant.

It is relevant, and this system is absolutely using radio signals for detection and tracking; it just doesn't transmit them. The system described is a bi-static radar system that uses transmitters of opportunity (technical term "non-cooperative transmitters") to generate the signal and listens for the return from those signals hitting the target.

From what I read (not an expert in this topic), the passive radar solution didn't rely on this and would've worked without deflectors. Maybe it worked a bit better, but I don't think they have much opportunity to test it with aircrafts flying stealth.

A radar cannot function without some source of electromagnetic radiation. Passive radar[1] requires non-cooperative sources of such radiation. If they are detecting the plane by some means other than EM, then it isn't a radar.

Now, their system may be using EM blocking as part of its tracking. The principle here is that a low-observable aircraft creates a "hole" in the background because it absorbs or scatters EM radiation away from the receiver. This method works best when the background is very noisy (i.e., there are lots of transmitters operating in the area). In that case the reflectors (and anything else that increases the radar cross section of the aircraft) would actually hurt the performance.

[1] https://en.wikipedia.org/wiki/Passive_radar

>the whole concept of "stealth" aircraft may be limited in the future thanks to ever increasing computation power and clever usage of existing radio waves

I have to disagree. Manufacturers will continue to build stealthy planes and drones, simply because it makes it harder to detect them. Yes, you may be able to detect a stealthy plane at, say, 100km, but using the same techniques you would be able to track a non-stealthy plane at 200km. Stealth isn't an end-all be-all tool that makes vehicles completely invincible, as the Bosnian shootdown of the F-117 showed, but it does make it harder, and I don't know of any technologies that could render it completely useless.

All else being equal, they will still choose the more “stealthy” plane, yes, but all else rarely is equal.

If the stealthier choice adds cost, weight, or maintenance costs, decreases top speed, maneuverability, or MTBF, the better choice may be the less stealthy plane, and if being stealthy gets harder, the choice may be more often that of extra speed, extra weaponry, or more airplanes.

> I suspect the future won't be "super weapons" like the F-35, but instead just "mass" weapons like tens of weaponized drones. Where the mission isn't to be invisible but to simply overwhelming enemy defenses (essentially the World War I strategy, but without the massive numbers of dead soldiers).

I think this point of view will come to be seen as anachronistic, much as the WWI strategy was not long after WWI.

Why? There is a fundamental disparity between the complexity needed for a forward-operating offensive weapon, and the complexity needed for domestic defensive weapons. Offensive drones need to avoid defenses and navigate to a particular target. Defensive drones just need to interfere with the offensive drones.

Any offensive strategy you can think of that depends on simple numbers, can be countered with numbers at lower cost. If it's drones, I can build more defensive drones than you can build offensive drones for the same total cost.

> Any offensive strategy you can think of that depends on simple numbers, can be countered with numbers at lower cost. If it's drones, I can build more defensive drones than you can build offensive drones for the same total cost.

Mobility can complicate that picture. If I have a million attack drones, and you have a million defense drones, there's still the question of where you put your defenses. If you have a hundred cities to defend, you might realistically need a hundred million drones.

I imagine they will spot whatever you are using to transport your million drones...

Transport? You can't just fly them there?

I mean, it seems like any drone cheap enough to be expendable by the millions would probably be lacking enough in range as to need a carrier/mothership of some sort.

And of course the carrier/mothership could be a cloaked bomber that drops them over the target zone, which brings back the need for cloaking.

Only if you are attacking where the defence drones are.

An attacker only needs to pick one area and overwhelm it, the defender needs to pick ALL targets and produce enough to defend all of them.

Defence vs drones is a LOT more expensive than the attack.

But… We currently have offensive ICBMs versus automated defensive weapons designed to shoot them down, and it’s the offensive side that has the upper hand. In my limited knowledge, shooting down an ICBM is hard because, among other things, you have to precisely hit a small, very fast-moving object, whereas the ICBM only needs to get into the general vicinity of its target location before exploding. Now, you’re talking about planes rather than missiles, but wouldn’t many of the same principles apply?

I don't think this is true. If my low cost attack drones just fly in, aiming to explode on your infrastructure or personnel, your defensive drones will have to be able to catch and kill small moving targets that may or may not have evasive or defensive weapons. I think swarms of cheap, possibly disposable, attack drones will be a real future threat.

>Any offensive strategy you can think of that depends on simple numbers, can be countered with numbers at lower cost. If it's drones, I can build more defensive drones than you can build offensive drones for the same total cost.

if it's so simple, why are there presently no defensive drones?

Because there are many less expensive ways to defend against drones than using drones. It has been considered and rejected. The cost is even more asymmetrical in favor of defense than implied by using defensive drones.

I think you’ll find a certain UK airport disagrees with you significantly. This sort of asymmetrical warfare isn’t about “attack” or “defence” in the strict sense, but it’s more about a less well-funded group inflicting significant losses on a more well-funded group.

>but that the whole concept of "stealth" aircraft may be limited in the future thanks to ever increasing computation power and clever usage of existing radio waves (making effective, passive, "radar" that cannot be easily overcome)

This. One solution is to simply deploy a considerable number of ground and orbital (or even use blimps/balloons like some companies want to do for wireless internet, or for quicker deployment during a developing incident a fleet of autonomous drones) IR sensors and simply follow the exhaust, with low enough latency and a little predictive maths you could probably know where an aircraft is, and will be, for the next several seconds at any given second with an almost perfect degree of accuracy unless the aircraft is actively conducting evasive maneuvers.

Tie that together with traditional radar, x-ray, terrestrial radio broadcasts, etc and you can make a system that would be hard to evade detection from.

The larger an area you are attempting to detect aircraft in, the harder it would be, but you could protect specific facilities by deploying much smaller grids over likely approach paths with say ground based IR sensors to detect exhaust trails of anything on approach and then in a much tighter area close to the strategic target.

You could also probably deploy atmospheric microphones in blimps/balloons that report their location with GPS and use some sound detection algorithm to detect aircraft moving away from them and use a network of such microphones to dial in an approximate location, heading and speed. Not unlike commercial gunshot/shooter detection systems that already exist.

> "The people pointing out that the F-35 had radar reflective disks added didn't seem to have understand the article (or technology). This is using entirely different methods to detect the aircraft, not radar waves, so that's irrelevant."

Passive radar is just radar that takes advantage of existing radio broadcasts (such as TV and radio stations). This particular passive radar setup may well use a different frequency range than military radars, but it's still working on the same basic principle of any other radar.

It seems plausible that a plane equipped with RF reflectors would reflect a broad range of signals, and that a skeptical person could reasonably want an explanation of any claim that the reflectors are in fact non-reflecting in the relevant bands.

> I suspect the future won't be "super weapons" like the F-35, but instead just "mass" weapons like tens of weaponized drones. Where the mission isn't to be invisible but to simply overwhelming enemy defenses

This is basically the endgame that Diamond Age predicts.

Yes, exactly what I thought of!

For those not familiar, the book describes nanotech drones on the scale of dust mites, and cities protected by essentially artificial immune systems.

When a battles occur in this setting you can tell because everything is blanketed in soot, the microscopic corpses of dead drones.

Lots of cool ideas in that book, tech and otherwise.

It's my favorite Stephenson novel, and he's written so many excellent books.

That and Snow Crash are my go-tos of his.

Diamond Age is slower and for me a lot of the plot and characters are less memorable, but the world, tech and ideas are amazing.

Snow Crash is a force of nature. That manic, brilliant opening set up of the world, style and characters using the Deliverator blows my mind every time.

Hard to believe it's even the same author.

> Hard to believe it's even the same author.

I had the same thought when I read Zodiac, after first reading his later novels. It makes Snow Crash more clearly part of a progression.

This is spot on, Ace Combat 7 pretty much hit the nail spot on, the future of arial war is mass production mid-scale drones that are incredibly hard to down along with rapid moving low-altitude stealth weapons.

eh, ciws are cheaper than drones and laser weaponry will counter hard both maneuvering or loitering far from the action, all these stealth things are there for asymmetric warfare were the political cost of losing hardware is greater than the benefit of winning cheaply. the future of arial war is likely constant emp blasting over airfields and factories preventing combat to evern happen.

Maybe CIWS ammunition is cheaper than drones are, but I’m pretty sure the installations as a whole are incredibly expensive.

> the future won't be "super weapons" like the F-35, but instead just "mass" weapons like tens of weaponized drones

The difference is plausible deniability.

Stealth weapons let one plausibly deniably launch attacks. (Perhaps to the point of letting the attacked pass it off as an accident, which could let them save face when returning to the negotiating table.)

Swarm tactics have no such deniability (though attribution can be complicated if non-state actors are used as a front).

small drones can fly very low. wondering why none of those saudi patriot batteries managed to knock out any? probably because they would have difficulty distinguishing them from birds if they picked them up at all

I don't think anyone is arguing that stealth isn't useful. I think the argument here is that equating stealth with radar evasion is no longer accurate.

I should also point out that we do already use mass weapons in the form of cruise missile strikes.

Cruse missiles are really expensive (~1.5 million each) and not really used for mass bombardment. The basic reason to use aircraft/drones is to reuse expensive components like engines and complex sensors.

Time will tell, but I think the answer will depend on the situation. Sure, $1.5M is a lot, but it's able to deliver 400+kg of explosives at a distance of over 1000 km, while cruising at 800+kph at an altitude of 50m. I think any drone you design with those same parameters is going to cost about the same. Now, the re-usability is nice, but that isn't free. For instance, you need to either double the fuel capacity or cut the range in half, since you want your drone to be able to make the return trip. And now you have to worry about the reliability and maintainability of the engine and sensors. Plus, you have to think about survivability of your drone: how many sorties can you expect that drone to go on before it is shot down? Whether you want to use cruise missiles or drone swarms will depend on the increased cost to purchase and maintain drone vs. a cruise missile, and the odds that it will be shot down after dropping the bomb. In a low threat environment where you have air superiority you definitely want drones, but in a high threat environment where you need to penetrate an integrated air defense network, I'm not sure.

The US had ~580,000 bombing missions in Vietnam each of which dropped several bombs on average. Doing that with cruse missiles would quickly hit 10+ trillion dollars.

On top of that a cruse missile takes around an hour to reach a target making them of limited value vs a mobile target. They have real value vs thinks like air defenses or high value buildings, but building hundreds of thousands of them is never going to happen.

A cruise missile that hits the bridge the first time every time might still be more cost effective than saturation bombing the square mile surrounding the bridge until you get that one lucky bomb to connect, though. And that's what a lot of those bombing missions in Vietnam were - dumping bombs in the general vicinity of a target until you finally had one connect.

With precision guided weapons, you don't need half a million bombing missions.

a plane (manned or otherwise) with a cheaper precision warhead is an option too.

Yea, at 1/2 price your saving ~750k a pop. A 100 million dollar drone that costs another 100 million to keep running over 30 years breaks even at ~270 bombs. Alternatively, a 1 Billion dollar airplane that costs 500 million over 30 years to keep in the air breaks even at 2,000 bombs which seems sadly plausible.

Though again flexibility to chose targets at the last minute is extremely valuable. Cruse missiles are simply a poor choice vs tanks.

This also assumes no risk of losing the plane, of course. If each sortie drops two bombs and has a 1% chance of airframe loss, that increases the cost/bomb for the drone by 500k or the stealth bomber by 5 mil. (which is part of why you use cruise missiles for suicide missions, of course)

The point being that, in an 'ideal' world, you wouldn't need anywhere near that number of cruise missiles, assuming they are able to effect accurately-targetted strikes.

I was going to say, you have a point in the 2nd paragraph, thou the 1st paragraph is comparing 1970s technology with a different generation. These days precision bombing is the norm for effectiveness and trying to adhere to Geneva conventions of reducing collateral deaths. Then I remembered the Iraqi death highway https://en.m.wikipedia.org/wiki/Highway_of_Death

> The US had ~580,000 bombing missions in Vietnam each of which dropped several bombs on average. Doing that with cruse missiles would quickly hit 10+ trillion dollars.

I rather hope the US never does anything like this ever again. To this day people in Vietnam, Cambodia and Laos are still being injured, maimed or killed by the hundreds of millions of bomblets they dropped.

Cheaper drones are already used in combination with more sophisticated missiles to overwhelm defenses. Afaik that's what happened with the attack on the Saudi facilities.

Iranian-made cruise missiles seem super-cheap yet very effective. They also have lots of even-cheaper drones. The Houthi use them to attack oil installations and Patriot batteries.

I think the recent attack on the refinery with 18? Drones and 7 cruise missiles (3 fell short?) could be referred to as “bombardment”.

Ok so just tell the engineers to fix that old B-52.

This is what happened in naval warfare -- 100 subs beats 1 aircraft carrier.

What? Subs are not made in mass quantities and subs' advantage over carrier mostly comes from...stealth.

I'm not sure I've ever seen stealth sold as an "invisibility cloak," except by press lacking subject matter familiarity.

The pitch I've seen in settings that do have subject matter familiarity is that stealth decreases the detection range for a given power / bandwidth / noise tolerance. Could your opponent use more power? Sure, but then they're easier to missile lock and destroy. Could they use more installations? Sure, but then they have to maintain them and their communications. Could they crank up the sensitivity? Sure, but then they have to deal with false positives and increased vulnerability to jamming. That goes especially for passive radar. Don't get me wrong, it's still an arms race, but it's not as cut-and-dry as this piece would have you believe.

Also, it's a VHF technique, which is low enough frequency that I'd bet active cancellation is practical, possibly even through a software update :)

> Also, it's a VHF technique, which is low enough frequency that I'd bet active cancellation is practical, possibly even through a software update :)

A lot of this stuff is literally an arms race, along multiple dimensions. Want to active-cancel the VHF signal? You'll need a transmitter for that. How many can you miniaturize into your fighter, and how many can they fit into the installation on the ground?

Another way that it's a race. You have to detect the signal quickly enough and recognize that it's a signal, and then jam it before they can get enough return to discern you from the noise.

So there was always a matter of economics/engineering here, and one valid criticism of the F-35 is that it costs a lot for what it's doing, and any way that chips into its advantage (in a way that is actually production-ready, and not just an irreproducible hack) shifts that cost/benefit balance.

Very true, I just want to make sure we aren't taking the "stealth used to be perfect but passive radar makes it mostly useless" claims at face value. Passive radar certainly isn't useless and it does tip the table towards detection.

It only tips the table, though. By using ambient RF it is very difficult to track, but very easy to spoof and mess with.

How well would it deal with a dozen drones omnidirectionally blasting the star-spangled-banner at high amplitude over FM with a bunch of wacky reverb (and maybe a few leads/lags chosen to look like scattering)? Traditional active radar wouldn't have the slightest problem rejecting interference at that sophistication tier, but most passive radar systems would have a tougher time.

As for stealth,

> Want to active-cancel the VHF signal? You'll need a transmitter for that.

Like antennas in the skin hooked to software radios with resilient analog front ends? To my ears, that sounds like a bet that payed off.

> How many can you miniaturize into your fighter

A few, probably. That's not enough to do active cancellation at microwave wavelengths, but it might be enough to do active cancellation at VHF.

> You have to detect the signal quickly enough and recognize that it's a signal

With active radar, that's the game. With VHF passive radar, you characterize the "ambient" radiation, which in this case probably boils down to mapping the FM stations. It's public information, and that is both the greatest strength and the greatest weakness of passive radar.

The President of The United States has been making statements about the F-35 program for years calling it an invisible jet fighter. Here is an example: https://www.youtube.com/watch?v=_mrWUrMK5d4

The weakness in F-35's have been well known for a long time now.

They've been well known since that a F-117 was shot down in 1999 during the Serbia conflict. The actual event was mostly due to bad operational tactics on the American's side and a very lucky shot, but it did highlight that it was possible to detect stealth fighters using coldwar-era Soviet technology.

The problem is that the stealth is specifically geared towards XHF frequencies, which are the frequencies used in radar guided missiles.. which are the favored missiles for the USA.

However the planes lack the physical size and features necessary to completely absorb VHF radar. VHF lacks the resolution necessary for guided missiles and VHF radar stations must be very large, but it can be used to monitor air traffic over vast areas.

So if you detect a aircraft on VHF and then point your XHF radar at it and nothing shows up... Then you know you are dealing with a USA stealth fighter.

Stealth has been heavily oversold to the American public, but foreign governments are not so vulnerable to USA industrial-military propaganda.

This doesn't mean that stealth is worthless. It does dramatically reduce chances of detection, just doesn't make them invisible. They are still largely invulnerable to radar-guided missiles. The size of VHF radar arrays needed precludes their use on other aircaft.

So to make this sort of thing still requires a sophisticated network connecting large and easily found radar stations, which themselves can be targeted and disrupted in a combat environment. There is still major hurdles needed to solved before it would be possible to actually shoot down these aircraft with radar-guided missiles.

Also this technique doesn't work as well against aircraft the size of B-2 bomber.

I think you may be missing a key point of the system described in the article- it is passive, and does not transmit any signal. Instead it relies on RF already in the environment - radio, tv stations and the like. This makes the system far harder to detect and target. I recall hearing about a similar Chinese(?) system a little while ago which used the scatter from mobile phone signals.

These articles are spectacularly uninformed, jumbling together claims which have never been made, uninfomed guesses and lack of domain expertise. It is more interesting as a propaganda exercise, conceivably so Boeing/Dassault/Saab can sell their planes more competitively, or so the RADAR operator can get more government funding.

Problems with the article: - The F35 isn't flown to air shows in 'war mode'. It has a bunch of reflectors added to increase its RADAR signature. It is only naked when it might be used in anger or over test ranges, and the USAF is very careful to make sure non-JSF program RADARs are off, or have the recordings deleted.

- Passive RADAR is a real thing that works, but it only gives a general location. It would be meaningless to try to send a link track to an interceptor or SAM site from a passive RADAR system -- all you could do is call and say "look sharp chaps".

- When you want to shoot at it, your S band (ground) or X band (interceptor) RADAR will have a very hard time picking it up or maintaining lock (because its signature changes depending on aspect, and obviously the pilot will manoeuvre to minimise). Meanwhile your ground receiver is getting jammed and transmitters are getting blown up by JDAMs.

- Passive RADAR can also be jammed and deceived. How many times will you scramble interceptors for a false passive detection?

This is a big stretch.

A radar manufacturer claims they can detect an F-35 which was equipped to be detectable but the radar maker claims that the devices uses to make the plane visible to radar didn't matter because they were tuned to a different frequency.

They're just selling radars.

The thing about stealth is that a tiny aberration can turn an invisible plane into a flying barn. It doesn't matter if the devices used to make the plane visible were for different bands, it's radio, nothing is exact. They obviously made the plane visible to more than just the target design.

On another level, there are various maintenance things that are done to make the plane stealthy when in operation that wouldn't be done going to an airshow.

And finally, no one says the F35 is totally invisible to radar in all circumstances. It is especially detectable after it has passed your current position. The value of the stealth is that you don't detect it until it's on top of you or has already fired on you.

In a real air-superiority fight, F-22 and whatever other classified drones and planes would be the first wave and clear out a path for the less stealthy but more versatile F-35.

Stealth isn't a binary. It is a completely analog range. The stealthier you are the closer you can get to a radar before detection. Even the very stealthy planes are highly visible at relatively close ranges.

So they managed to pick up a stealth jet.. that was equipped with special markers to make it extra visible (for air safety outside of combat), with additional help from a strong polish broadcaster... with information knowing exactly when and where the aircraft would be?

This was addressed in the OP. Their explanation is that the markers are optimized for the specific band of civilian radar, and that they do not assist with passive radar detection.

As for knowing when and where the planes were flying, true, true. But then again, they deployed one van to try to detect the planes, in the one farm, and if the objective was to suggest that the passive technology could end up being useful in a military situation, that might be enough.

The argument is that the F-35 is invisible. If passive detection seems to have any effectiveness, even slight, in detecting F-35s, that ought to guide investment into passive detection and into upgrading the F-35’s stealth to avoid passive detection.

INAE, but it feels like there is enough here to cast the claims of complete invulnerability to detection in doubt.

The main argument for the F-35's stealth is to avoid target locking missiles. It's not to be "invisible".

That is a very interesting perspective, thank you for sharing it.

How does low radar observatbility help against IR missiles?

IR seekers have very (in air-air combat terms) short effective range, ~25km at best. Thus, a missile with an IR terminal-stage seeker needs to either be launched from close range (which would indicate the F-35 was probably detected via Mark 1 Eyeball) or the missile needs to be guided into range by radar. Guiding the missile that way involves painting the target with a fire control radar and having a passive radar seeker on the missile follow the return. Reduced radar signature obviously helps in the second case.

Edit: fixed seeker range

But in this case passive radar would help enormously? It could detect the aircraft and help to launch an IR missile, effectively rendering the stealth capabilities useless. Deployed at borders, it could provide good air defense against F35.

You obviously need strong signals for passive radar to work but those could be positioned inland and they don't need to be where the passive radar is, making it harder to disable it.

IR is only really effective in the short-range (see an aim-9 which is only effective less than 20 nm (really less than that practically speaking). When they talk about this, they mean protection against beyond visual range missiles.

>(see an aim-9 which is only effective less than 20 nm (really less than that practically speaking)

I would really love to see the actual Probability of Kill numbers for the US missile fleet. I know that's never going to happen/they're going to remain classified for a _long_ time, but the kid in me really wants to know "does it actually work?"

You tend to get the results from who we sell the missiles too. Ironically, I believe Iran has the most kills for the F-14 / AIM-54 missile, which had a whopping 100nm range. Then, I think Israel has been demonstrating the most combat effectiveness of the the F-35 in attacks in Syria...

Air-combat is "hard" to answer the "does it actually work" because it is very much like high stakes chess in the air. A lot of times you fire 1 missile to make the bogey defensive, and multiple missiles to kill. This gets even more complicated when you factor in differences in pilot skill, numbers, chaff, and electronic counter measures.

nm here I’m guessing is nautical miles? Nanometer range missiles would be tricky to wage war with.

Anyone claiming that the F-35 is invisible is either uninformed or propagandizing.

Yes, if the countries deploying the F-35 have any sense at all, they will install equipment to make the aircraft trackable on all air show and most training flights, so that potential adversaries can’t accurately assess their ability to track it in full stealth operation.

The radar people claim that the "markers" intended for microwave frequencies would make no difference to passive radar. That seems quite believable as passive radar uses things like commercial VHF radio transmitters as emitters. The frequencies involved are vastly different. The features are simply too small to cause much of a reflection at VHF.

So... Microwave and VHF are not vastly different frequencies, Microwave starts at 300MHz and VHF ends at ... 300MHz.

Yes, they each span a wide range of frequencies, but every frequency range spans a wide range of frequencies. These two are as close as you can be.



Not coincidently, VHF Radar is defined to be between 50 and 330Mhz, which actually overlaps both Microwave and VHF television.

Now as to the main question, if I set up a device to interact with radar at some frequency and attach it to a stealth airplane, what frequencies would you expect it to effectively interact with it (assuming it's tuned at all, they could just be attaching a big piece of flat aluminum)?

Answer: If my device has peak interaction at frequency F, you will see harmonics, so you will see significant interactions at 1/8 F, 1/4 F, 1/2 F, F, 2 F, 4F, 8F.

Airport radar is around 3Ghz. So if the radar reflector operates at around 3Ghz, it will also work pretty well at 1.5Ghz, 750Mhz, 375Mhz, and even 187.5 Mhz. 187.5Mhz is TV channel 6 or 7, depending on country. Half again is 93.7 is FM (which the article specifically mentions).

A half wavelength at 3 GHz is 50 mm. A half wavelength of 93.7 MHz is 1600 mm. The retroreflector here is not being used as a retroreflector for passive radar as the transmitter and receiver are not in the same spot. Why would a small chunk of stuff stuck to a bigger chunk of stuff somehow disturb the high power VHF signals used more than the big chunk of stuff it is attached to?

I guess the glib answer is that the big thing is specifically designed to avoid disrupting those signals, and the small thing is designed to disrupt them as much as possible.

A tiny radar reflector has the cross section of a huge object, because it intentionally has right angles that send signals back to their source without scattering them in other directions (as much as is possible).

Passive radar still requires the object to reflect signals, it just doesn't require the signals to be reflected back to the source of the signal. Consider a monitor with an anti-reflective coating (so lets take a non glossy monitor). It is effective at reducing the reflection of the window behind you, it uses a thin film to cause destructive interference so that the light coming in the window not reflected, and you see a greenish-purplish image of the window instead of getting a bright reflection. However, if you take a flashhlight and shine it down on the monitor from near the top of the monitor, you will easily see the glare from the flashlight as it illuminates the monitor.

I think this is a good analogy for active vs passive radar. The F-117 was built to avoid right angles, this is like the light from the window behind you. The F-117 won't send radar back in the same direction it came from, so active radars are thwarted. However, if it is being 'lit up' from somewhere else, it is not as effective since by not sending radar back to where it came from it sends it in other directions instead.

However, it also has a coating to just straight up absorb radar. The F-35 is much more reliant on such coatings, and so it should scatter very little EM radiation in those frequencies, regardless of whether the detector is in the same location as the transmitter.

I thought radar absorbing paint didn't work much below 1GHz. So it probably doesn't really help against passive radar.

Harmonics occur at integer multiples of the fundamental. If a body resonates at F, you will see harmonics at F, 2F, 3F...

You will not see harmonics at fractional (1/2 F, etc...) frequencies of the fundamental, though you will likely still see some backscatter.

Retroreflectors[0], to include the Luneburg lenses[1] mentioned in the article, function by directing radar energy parallel to its incidence direction. These reflectors will not significantly increase the plane's RCS for passive/bistatic receivers. Even if these reflectors did increase RCS omni-directionally, it is unlikely that a reflector designed to operate at 3 Ghz (10 cm) would also be designed to operate at 187.5 MHz (1.6 meters).

[0]https://en.wikipedia.org/wiki/Retroreflector [1]https://en.wikipedia.org/wiki/Luneburg_lens#Radar_reflector

More importantly, how does the intensity go with the order of the harmonics?

I don't know enough about the reflector. If it works like a boat's radar reflector, an 20 cm reflector would have a equivalent cross section as a metal sphere with an area of about 30 square meters.

I mean the goal of a radar reflector is to send radar signals back to their source, they are super effective. They don't really work like antennas generally, but the pictures of the F-35's reflectors do not look typical, or like a boat's radar reflector, and I have no idea what principle they use.

That is definitely not true.

Sensitive VHF radio receivers, tuned to transmitters that are located beyond line of site, can very easily detect VHF communications reflected off of a very small airborne target.

I can sit in my office with a spectrum analyzer and watch the signal strength of a distant digital TV broadcast station come in and out of view when an aircraft flies overhead.

Couple that technique with a DF network and far more sensitive receivers and GPS timing, and building a passive radar to defeat stealth technology become far easier than you think.

Yeah I wondered when someone would just create a super dense (networked) phased array system.

Exactly. It was tracked with radar reflectors to make it visible, and from a position they knew the jets would be close, while also tracking their transponders to correlate with the reflection data. This is the equivalent of saying they successfully completed the Daytona 500 by driving a Honda around the track a few times at 45 miles an hour with the track closed to everyone else.

Those markers may actually be irrelevant for the tracking, because they are very small and are meant for different frequencies. ADS-B also doesn't come into it, if they designed the system or demonstration correctly.

More interesting would be whether those planes actually had the full stealth coating and various other "stealth settings".

This. It's common knowledge that F-35 and F-22 both fly with markers to ping on radar. Additionally, the fact that the F-35 was squawking their ADS-B indicates that they were not flying in operational mode.

It's quite a change going from not exporting any F22s to the F35s, which feels like they'll sell them to anyone that wants some (minus the usual suspects). Especially when the F35s stealth technology is said to be more advanced.

It's hard to get your head around a plane the can climb over 1000 feet per second (F22: 62,000ft in 60 seconds).

Picking it up "with information knowing exactly when and where the aircraft would be" makes it a better test - there's no ambiguity about what they picked up.

The important question is whether or not they can pick it up when it doesn't have markers fitted as they claim they can. To quote Carl Sagan, 'extraordinary claims require extraordinary evidence'. If (and it's a huge if) they can do what they say they can then stealth might be rendered pointless, and that changes a lot.

That quote kind of goes both ways. Claiming that your plane cannot be picked up by any form of radar might be an even more extraordinary claim. In particular Lookhead makes it really hard for anyone to verify this claim. While it seems like Hensoldt was very willing to demonstrate their capabilities at the air show.

It was also actively broadcasting its location at the time.

I would be real curious how this passive radar design helps in an actual war environment.

Even if you discount all the possible issues like the F-35s not being in "stealth operational mode", having reflectors, etc.. this is not a real world test at all.

In the real world test there will be a massive amount of ECM energy hitting this passive radar... it might totally blind a sensor like this. In addition a lot of the civilian radio frequencies/transmitters will be getting knocked offline and it won't have it's energy sources.

I was thinking about this also. Practically speaking, I don't think you would be able to get much from civilian sources. It could be used to effectively boost the range of your own radar stations though. If you put a passive receiver (of the same receiving sensitivity as your radar station) at the maximum range of your radar station you would effectively increase the range of your system by 50%. But there are some caveats. The biggest is that both your emissions source and your passive receiver need line of sight to the target, making it easier for a jet to break contact at the outer range.

Would active ECM not be a source of illumination of the target that passive radar could take advantage of?

Sure it would if you also assume that the ECM transmitter is also the target. Decoys might break your assumption though. It seems logical to mount ECM on a decoy target.

Passive radar depends on the reflection of ambient RF by the target. A drone radiating for ECM purposes (or any other) might illuminate the airplane it is supposed to be protecting.

On the other hand, the algorithms behind passive radar might depend on knowing where the transmitters are. Maybe one could triangulate the drone directly, but not so easily if you have multiple drones transmitting short bursts asynchronously. And if there are enough decoys, it would be like chaff.

> Passive radar depends on the reflection of ambient RF by the target. A drone radiating for ECM purposes (or any other) might illuminate the airplane it is supposed to be protecting.

That's a very good point which I neglected to consider.

Not passive RADAR no, that is looking at distortion of a known signal. Picking up ECM is one aspect of ESM or ELINT, which through wide area triangulation can provide approximate location. But it's tricky, ECM can be a lot lower power than a search RADAR, e.g. repeating echos, distorting the return signal to have a different signature, etc. If it is a powerful jammer it's likely to be quite directional so distant receivers might not pick it up well.

If someone is shining a spotlight in your face, the spotlight may also be illuminating the room. That doesn't help you see any better.

You can shield your eyes from the direct glare. If the enemy does not know where you are (unlike conventional radar, you are not radiating), he can only spotlight you by chance.

Interesting, apparently it's because of "passive radar" which I'd never heard about before...

"Passive radar equipment computes an aerial picture by reading how civilian communications signals bounce off airborne objects."


If you want to be extra cheeky, you can use your opponent's radar emitter.


F22 and F35 themselves have passive radars as well, and can detect stuff without emitting anything. Passive radar is not much more than a phased antenna array without an emitter, you just don't have anything to light up the target with.

Once you start emitting you're not stealth anymore, and you can be shot down pretty easily. Same with carrying external ordnance. Same with worn out radar absorbing coating. Same with any physical defects on the plane's surface. That's why these planes are designed to only be stealth in the first few days of a real, actual war, and then once they get scuffed a little by e.g. bullets or debris you're supposed to treat them like regular fighters and not rely on their no longer existent stealth.

I've seen a presentation (over 10 yrs ago) where passive radar was set up as a secondary sensor system that is hard to locate because it's not transmitting. One attack scenario is with a stealth plane first attacking the radar systems, then regular planes come in after. So the passive systems would stay up, as most attacks won't target civilian infrastructure. Still, there's a hint of hiding behind civilians here I don't particularly like. But I'm just being impractical about the matter.

You can't optimize your civilian transmitters to send out signals best for radio detection and ranging, so you lose accuracy. Better than nothing.

I don't see how its hiding behind civilians? Typically a well marked road march isn't seen as comparable to using human shields, there mere sharing of infrastructure isn't enough.

Essentially it's using civilian transmitters as part of a military radar system. A stretch, given.

Midway through the article it finally mentions that this was with the "radar invisibility cloak" turned off by installing Luneburg lenses to make it radar visible. I don't doubt that there are current advances in radar technology that weaken the stealth claims of the F-35, but I don't think that this specific case demonstrates this.

The lenses are irrelevant to VHF scattering. Civilian air traffic control is S-band, quite a bit up from radio transmissions.

Goes a long with a story I heard last year concerning the presence of a German radar and electronics company at the ILA. The story goes that Lockheed didn't want to fly the F-35 as long as the most advanced radar was on premises. After the radar was moved the F-35 cleared. Also, the radar was set up further away. And the F-35 successfully located. (I wasn't there, so that is a second hand story).

EDIT: The Spiegel reports more or less the same story, occurred in Aptil 2018. The radar comes from Hensoldt, a former Airbus subsidiary.

And, as already stated in another comment, it was passive radar in the sense it took existing signals and analyzed them and their absence.

EDIT 2: Yeah, submitted article reports the same story... Lazy me...

It's worthwhile to note that while passive radar might be able to detect stealth aircraft, there's not much that can be done other than that. Passive cannot guide missiles, and active radar is the only method used to guide missiles. So even if you detect a squadron of stealth aircraft, what can you do about it?

No doubt there are already missiles in the works that will fly to a given location on internal guidance, and then attempt to home in on a nearby target. As others have pointed out, stealth is not invisibility.

Scramble your interceptors.

Wait, the passive detector was set up only "a couple kilometers" away from the jet? At that range, I'm not surprised you can detect it up with passive radar. You could also detect it with an infrared sensor, a pair of binoculars, or even just listening for the jet engine.

Sure, passive radars need a lot of antennas to cover a lot of ground, but the point is they can be made very hard to find since they are passive and that the hardware doesn't cost so much.

Definitely, but unless this offers better range than other passive methods (like infrared) this doesn't really change anything.

In the future, due to advances in radar tech, satellite and others. Skirmishes between highly advanced nations will likely emphasize the use of swarm drones to serve as a battalion and a distraction against the more technically advanced and less defended solutions (e.g. F-35).

Whenever someone looks at the F-35 and others, you should aim to build a holistic picture of what 'X' feature serving sub-optimally could entail, not just "Invisibility cloak has weaknesses under these specific conditions, therefore it is a failure" and whether the group behind optimizing the aircraft is aware of it and accounts for that when upgrading and /or sending it on missions.

This is trivial to see. There will be a F-35 demo at Fleet Week in SF this weekend.

Turn on your TV to a UHF digital channel during the demo. Watch the interference. Now, it could all be reflectors, but you get a sense how a bistatic RF detector could work.

The hard part is probably calculating where the object is and where it's going. With that information, you could then feed the data to IR missiles.

All comes down to whether the Luneburg lens reflects background noise too. If yes then this is of little use and the result is tainted.

If not well that could be a game changer

A bit of Google suggests that they are indeed frequency specific see link. So that sounds like it has real potential


According to that article, the most common use of a Luneberg lens is as a reflector, returning the RF back to its source, and I would guess that is what you would want it to do when your goal is to enhance its visibility to conventional radar. I do not think this would help passive radar, however, even if the lens functions as a retro-reflector at the frequencies used by the passive radar, as the sources and detector are typically a long way apart.

These lenses presumably have some isotropic scattering at the frequencies used by this passive radar, and the question is, how much?

Misleading. The article says things like the F-35 is "designed to be undetectable by radar."

Nothing is designed to be undetectable by "radar." "Stealth" aircraft are designed to be "invisible" (low-observable, really) in one or more radar bands. Once you get outside those EM bands, the aircraft is obviously detectable (e.g., you can see it because your eye doesn't operate in the X band).

You are not wrong. But air defense radars operate in specific bands, too. So when articles targeted at laymen use phrases like "designed to be undetectable by radar" - I think they mean that the aircraft is designed to have a small RCS for the bands/radars generally in operation. But of course you are technically right, the jet isn't literally invisible...

True. The reason I said what I did is because if you have an aircraft that's designed to defeat airborne X band, and somebody on the ground pulls out a truck-mounted radar in a different band, I still wouldn't say the stealth has been defeated, if the reason X band was chosen was to defend against other aircraft.

Don't know your background, so I apologize if this is old info for you: the band of radar you use is driven by the size of the antenna, which is why pretty much all fighter radars are X band (maybe +/- a little bit). You only have so much space in the nose of an aircraft, and that becomes a constraint for what radar frequencies you can use. If you know that, and your goal is to not get shot down by enemy fighters, you can optimize your radar signature for that threat.

To me, this article is like saying something like "bullet-proof vest defeated by falling boulder." My reaction is well...no...not really....

For some reason there is the expectation that a new weapon generation is somehow completely infallible and will be able to defeat any future generation. Except new generations of tech make the old generation obsolete. Nothing more nothing less. A non stealth aircraft will have an extremely hard time against a stealth aircraft.

I recall that in at least one case an F-35 used for an airshow demonstration had visible deterioration of the stealth material forward of its canopy. It seems that the low-observable systems used by the F-35 are very maintenance-intensive and that the jets used for shows may not be in combat-ready condition.

(or, if you prefer, maybe the jets used for demonstrations are intentionally damaged in visible ways so that there is a convenient excuse for the ease of detecting them on radar)

edit: I was thinking of the F-22, see comments below for a source. But similar considerations probably apply to the F-35.

I think you are thinking of these recent photos of an F-22: https://www.thedrive.com/the-war-zone/29218/these-images-of-...

However you're no doubt correct that radar absorbant materials is high maintenance on any aircraft.

You're right! Thanks for finding the source.

Similar considerations do not apply to the F-35. It has a completely different skin coating than the F-22.

Different it may be, but I doubt it's maintenance-free. Got a source?

Does F-35 use the same toxic coating as F-22? I believe one could be easily harmed just by touching F-22.

Where there are no existing civilian emitters, such as FM stations, it would seem possible to deploy transmitters deliberately to create a hostile RF environment to assist the passive radars.

Supposedly, passive radar can use sources as weak and diverse as individual cell phone towers, the GPS system and analogue TV signals. I doubt there is anywhere left on land you couldn't get at least 2 of those signals. Though I'm no expert on the tech and I guess the manufacturer has a big incentive to overstate it.

I've been looking for a source, but I remember a previous air show incident where a stealth aircraft was detected and the excuse was that the active stealth system was not turned on, i.e. the aircraft would have used electronic active radar cancellation in addition to stealthy body design if it had been in actual combat.

Whether any electronic system would help against passive radar, or would in fact make things worse, is another question.

I wonder if these F35s had external fuel tanks attached, for the long flight back home?

Excellent question. Embedded within the article is a video of the airplanes arriving in Berlin, and there is no sign of external stores. They might have stopped elsewhere in Europe and removed them there, but a caption on the video describes it as "a record-setting long-duration flight for the F-35 from Arizona" and "an 11-hour flight."

It would have been air to air refuelled, the F-35 isn't certified for any type of external tank, though it gets mooted occasionally when someone wants more endurance.

Anything made by man can be defeated with enough effort, time, and people.

Tracking any aircraft, stealth or not, isn't the issue. The issue is whether you can get a weapons grade track.

If this is true then this is bad.

I always warned that the F35 is a one trick pony. Can't fly fast, can't fly far, can't carry much load, can't dog fight, needs ton of maintenance and may or may not be invisible.

By definition, no plane can be invisible. It is a questions of frequencies and angles you look at it. This plane was not build to bomb Syria or Iran. If the German radar can see it, they Russians and Chinese can detect it for sure.

> Can't fly fast, can't fly far, can't carry much load, can't dog fight, needs ton of maintenance and may or may not be invisible.

that is exactly all the "can't"s the F35 ancestor - Yak-141 [1] - was designed with. All the stealth and quasi-fighter related capabilities of F35 are just bolt-ons onto that design in an attempt to mitigate the "can't"s and to stretch if even just a bit the very limited otherwise capabilities of that design. The only "can" in that design, its raison d'être, was the VTOL with swiveling nozzle (which can't even vector in flight). While that swiveling nozzle is a great thing for a country with no capabilities to build real aircraft carriers, it is a thing born in the pre-stealth age, and thus naturally compromised the stealth (especially in IR) of F35 as there is no way of making it "flat rectangular", like for example on F-22 and B-2, at sane (even by DOD standard) costs.

[1] https://www.quora.com/Why-did-Lockheed-Martin-fund-Yak-141/a...

Lies. I've been reassured by the highest authority in the land that this jet is literally invisible.

I rant about the F-35 a lot, but seriously: this thing is a trillion-and-a-half dollar shitheap, and this is just the latest proof.

What's really damning about this news is that it calls the entire value proposition of the plane into question. By which I mean, every time someone like me points out that the F-35 loses dogfights to planes from two generations ago, the apologists always come in and say that doesn't matter because the stealth and ECM technology means it will never be in dogfights, only BVR engagements.

But if you think about, that statement is essentially a gamble. It's a wager that our stealth and ECM technology will always, for the entire 40+ year projected lifespan of the plane, be ahead of competitors. Is that a safe bet? I don't think it is. This news shows it, and that's only what's publicly admissible. Imagine what Russia and China have that they're not talking about.

This thing is a piece of junk based on multiple failed ideas (V/STOL, stealth and ECM obsoleting actual combat performance, on-the-fly development, etc.) and it needs to be scrapped yesterday and started from scratch.

I have my doubts about all these 5+ Gen fighters myself. As you said, the planes will fly for 40 years, so it is certain that stealth will be lost sooner or later. And still everybody is developing them. Just looking at the economic side of it, in a war against an enemy with a strong air defense (against ever other enemy stealth doesn't make a difference anyway) exact how many of these planes can you afford to loose without ruining your air force and your economy with it?

The F-35 angled at not just its combat capabilities but at a NATO wide domination of combat aircraft, including operations for the full operational life for all operations of these planes. Almost worked out, and my still do.

It strikes me that having an entire fleet of high stealth aircraft is only going to be important for... maybe the first 48 hours of any conflict. I would suspect that the very first targets in any air war would be radar installations and command and control centers. Once those are down, and air superiority is achieved, stealth doesn't even matter; you could fly B-52 bomb trucks around.

Even for that opening stage of the battle, I would suspect that cruise missiles and drones would make more sense.

You're right, but it's unlikely that any modern near-peer conflict will last much more than 48 hours. The war will probably be decided by then, with only mop-up operations left after that. At that point, if the losing country is nuclear it'll go ahead and use their weapons, and planes, stealthy or not, are going to be mostly useless.

One issue with using drones in a high threat environment is that it needs to be able to communicate with a controller. Not only can that communication be potentially jammed, it can also be used to triangulate the coordinates of the drone (and controller) which limits your ability to surprise the enemy. Then you need to consider both bandwidth and latency. Latency matters because that lag between the drone operator getting data and sending orders greatly degrades your reaction time, which can be critical in situations like air-to-air engagements. Bandwidth matters because if you have a swarm of these over enemy territory all using satellite communications you won't be able to stream high quality video feeds back to your operators. That matters a lot if you are searching for targets. I think you'd need the drones to have very high autonomy, which is tricky both technically and ethically.

You are basically describing some if the potential concepts. And in addition to all the technical points you raised in scenario like that you would need a lot of drones, not all of which would come back. Which again drives costs up.

Would kind of make sense, IMHO, to have a superior, super stealthy dog fighter that can serve as a drone "mothership". Supplemented by droves of cheaper, non-stealthy specialized planes for ground attack, recon, bombing and whatever nasty fun things a future air force might dream of. Otherwise I think the next world war starts with zhe equivalent of a X-Wing and ends with the use wooden bi-planes.

>> in a war against an enemy with a strong air defense (against ever other enemy stealth doesn't make a difference anyway) exact how many of these planes can you afford to loose without ruining your air force and your economy with it?

> It strikes me that having an entire fleet of high stealth aircraft is only going to be important for... maybe the first 48 hours of any conflict.

It goes against the generalist, jack-of-all trades dogma; but I think the air force should have cheaper non/less-stealthy designs of equal or greater performance for almost every role occupied by a stealth plane.

The air forces if the world are run by air combat pilots, and until that changes there will be manned aircraft and it will be a priority that killing them is difficult. So drones are a long way off in that sense, at least until we have a drone-vs-manned war where the drones win.

It should also be noted that in stealth '5th gen' drone vs '4th gen' drone simulation, the stealth drones win convincingly, not because they are 'invisible' but because they can shoot first, before the other drone can get a lock.

Personally I think we're more likely to see '5th gen' drone wingmen. Finally each SQNLDR will have their own squadron, albeit robotic. With short range comms are much harder to jam, but it's uncharted territory to see how the leader will guide the engagement.

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