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Comment by ChuckMcM | Hacker News Reader

Hmm, not your typical RADAR. This is basically using the RF emissions of the plane, and their detection at multiple RF detection stations with synchronized clocks, to determine the difference in arrival time. The radio emissions all travel at the same speed through the air, the difference in arrival time can be used with some simple trig to figure out where the RF emitter is. There are a number of Time Difference of Arrival or TDOA systems out there. It is why I expect aircraft stop using their radios when they are about to engage their targets.

That said, they are "easy" to jam in the sense that when you have two radio signals on the same frequency, they will interfere with direction finding. And there is a lot of noise in the RF spectrum out there, especially during thunderstorms.

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More to the point, detecting a stealth aircraft is very different from shooting at a stealth aircraft.

This radar would likely not have the location accuracy of where exactly the B-2 was to accurately engage it with a missile. Your accuracy would be limited by the rate of RF pings the B-2 is putting off, and then the margin-of-error of these RF waves and receiver.

This is one difference between the B-2 and stealth fighters like the F-22 & F-35: The latter are not necessarily designed to be invisible, only impossible to reliably hit. Their shapes and radar-absorbent paint deflect, diffuse, or absorb the high-frequency bands used in the terminal guidance of missiles. So they are hard to target. But they can be picked up at range by long-distance, long-wavelength VHF and UHF radars. These frequencies, used in early-warning radars, have too low-optical resolution* however to be any good at aiming guns or missiles. The B-2's "flying wing" shape is able to not-interfere with these wavelengths though, and hence hides from them. In that sense, this is somewhat interesting, if neutered for the reasons you mention.

*You can only localize the detection to a few sq hundred meters, even kms.

And also, searching the top speed of a B-2 at 628mph, if detected at 150 miles out only gives them 14 minutes. Which is both a lot, or a little. Depending on how prepared for action someone is.

Or a lot less than that, if the B-2 isn't using gravity munitions.

Well since there are so few B2 and they are essentially irreplaceable, I imagine standard wartime doctrine for the adversary would be to ignore target protection as a concern and go straight for the plane even at the risk of losing several of yours by flying over territory you don't control.

In reality, what would make a difference is coordinating communications and decision making.

If you can localize down to the range of a km? Can't you just shoot down optically? A giant dark plane blotting out the stars...

Yup, it seems to me that the optimal missile would use mid-course guidance before doing IR / visual light homing in the terminal phase.

All the stealth craft also included some IR avoidance as a matter of course, IR seeking missiles existed at the time these were designed. It's unclear if a more modern missile would still be sensitive enough to lock on and avoid chasing flares.

The techniques they used to decrease the IR visibility are pretty neat. The engine nozzles are over the wings and there are a lot of airflow tricks used to cool it down faster so from below there might not be enough plume to track from a surface launched missile coming, from below at least.

https://en.wikipedia.org/wiki/Northrop_Grumman_B-2_Spirit#In...

Conventional IR countermeasures (spatial, spectral etc.) really are becoming a thing of the past.

When you’re talking about terminal IR guidance on any modern missile, generally the IRCCM will be so good that the probability of defeat with a flare cocktail is below an acceptable risk threshold. DIRCM is the current answer to this problem and—to tie this in with the parent comment—renders terminal IR guidance much less effective than MMW or other RF guidance.

https://en.m.wikipedia.org/wiki/Directional_Infrared_Counter...

Flares don't do much against anything besides mid and short wave infrared.

It's impossible to hide from long wave IR sensors, which detect the heat signature of the plane itself, and not its exhaust.

Recently in DC they detected a plane that did not respond to warnings. The optical system then also detected the plane and panic ensued. Problem was the radar system was showing a phantom and the optical system saw a nearby plane and thought it was confirming the phantom.

Tldr; it's complicated

https://www.thedrive.com/the-war-zone/31222/mysterious-airsp...

As we saw with both Iran and Russian AA teams shooting down passenger jets...

Don’t forget USA navy vessels shooting down passenger plane.

Hopefully the US has learned something in the past 32 years since that incident. It seems that even our allegedly sophisticated adversaries haven’t in the present time.

At least the Russian team was merely operating a Buk in a clandestine op in the middle of a field in Ukraine (and the US ship was technically in Iranian territorial waters in 1988 during an actual war), I’m curious what Iran’s excuse was shooting it down in their own property?... maybe we’ll learn more about it eventually. Bellingcat was able to provide the actual team member names and wiretaps of the Russian soldiers.

An airplane at altitude doesn't blot out stars. Optical sensors are nearly useless, especially against a black aircraft at night.

Not to sound skeptical, but I've seen some stuff Ratheon was putting out at least 10-15 years ago and I think it had some pretty impressive optical sensors. Are we including the various infrared frequencies?

Optical and infrared sensors have improved a lot and can be a useful supplement but due to limited field of view and interference from weather you generally can't count on them for primary search. Unless the target is really time sensitive, B-2 mission planners try to fly at night in bad weather when those sensors are seriously degraded.

I doubt it. If people are able to photograph dark birds at night, I see no reason why a missile with much bigger optics and more efficient (wider band and monochrome) sensors wouldn't be able to find a plane.

They're also not quiet.

there have been developments using the sound for terminal guidance to augment that.

I'm sure any modern IR missile can shoot down any current stealth plane if it knows the location to within a few kms.

There are FLIR images of thr F22, nothing extraordinary about it in the IR spectrum.

A lot of Russian planes have Electro Optical systems for targeting. Usually they are IR based locking onto hot engines and such like a typical Fox 2 (Heat seeking missile) Range of IR tracking is pretty small for head on planes and more importantly your missile needs to know where to look in the first place to acquire the target. Also stealth aircraft have reduced heat signatures making it harder to pick them out against the background. Also you have problems with determining contrast against the background due to diffraction.

Essentially the passive array in this article is in the class of "Early Warning Radar" which is used to alert Combat Air Patrol aircraft and SAM sites where to start looking for the contact.

If you wanted to use this with a networked radar guided missile you would have problems with terminal guidance. If the missile could acquire the target as it came into radar range for the radar cross section of a stealth aircraft, there is very little chance it would have the energy to track the target. It would essentially need to make a right turn midair to track on the target. All the while the missile's radar will be making the plane's RWR light up like a Christmas tree.

> they are "easy" to jam in the sense that when you have two radio signals on the same frequency, they will interfere with direction finding

Nope, it's a more sophisticated algorithm they're using. Our company held all the patents for locating GSM cell phones calling 911 using UTDOA. The U is uplink, meaning the network of receivers listens when the phone talks. The patents entail each receiver taking a sample of the signal they heard in that time slot on that frequency. They upload the sample to a processor which tries to correlate all the samples with some DSP math, sliding them around in time to see if they match. If they do correlate, the offsets of each one is your delay for doing trilateration.

So if you have multiple talkers at the same time, they can probably be subtracted out by the correlation step.

ECM can be directional (phased array antenna). If an aircraft knew the approximate location of the receiving antennas it could theoretically jam them seperately.

> That said, they are "easy" to jam in the sense that when you have two radio signals on the same frequency,

Not at all. Check out the VLBI technique from radio astronomy, it's not hard to tell that there are two discrete signals.

But that's not really the point: you can see stealth aircraft already with lower frequency radars, just not well enough to shoot them down. It's hard to imagine this passive technique being good enough to shoot them down, either.

https://en.wikipedia.org/wiki/Stealth_technology#Low-frequen...

the low frequency radar thing relates to the active radar in the missile warhead. Not accurate enough for hit to kill missiles, but plenty for modern shaped charge warheads that only need to get within 250 meters or so of the target for a kill, and completely irrelevant for warheads supplemented with optical targeting that only need to be within 5 miles clear line of sight to confirm a lock.

Here's an interesting variation that uses RTL-SDR's: https://dopplerfish.com/passive-radar/

Cool we can build our own radar!

Passive radar uses background RF, usually TV transmitters. Which is why they are the first to go everytime America decides to punish a nation for trying to use something other than the dollar to trade oil or decides to crack down on opium smuggling.

Not really enough though, which is how they lost all the F117s over Serbia in the 90s. One of which I have a framed picture of on the wall along side an original "sorry we didnt know it was invisible" flyer.

Tech has come a long way since then.

> Not really enough though, which is how they lost all the F117s over Serbia in the 90s. One of which I have a framed picture of on the wall along side an original "sorry we didnt know it was invisible" flyer. <

Only a single F117 was shot down over Serbia.

https://en.wikipedia.org/wiki/Lockheed_F-117_Nighthawk#Comba...

Perhaps choose a better source than wikipedia, like people who were there.

https://images.app.goo.gl/vUd8z2gkWEtKWKcJ8

even your source says they hit 2. They claim they downed 3. Americans denied they lost any for decades.

A propaganda poster knowingly conflating the words stealth and invisible, a distinction its intended audience wouldn't be able to make, to mock the enemy? I think I'll stick with the Wikipedia source.

And even that says: Some American sources state that a second F-117A was damaged during the same campaign, allegedly on 30 April 1999;[61] the aircraft returned to base, but it supposedly never flew again.[62][63]

Point is with AA missile is that getting them "on target" is almost as effective as a hit. target has to deploy its counter measures and return home, else next time they are destroyed.

I was addressing the dubious choice of sources delivered with arrogance and condescension. Couldn't care less about how many airplanes did Serbia take down.

so you otherwise accept the Serbians/passive radar got the entire F117 fleet mothballed?

I was addressing the dubious choice of sources delivered with arrogance and condescension. Couldn't care less about how many airplanes did Serbia take down.

Hit 2. Only 1 crashed. You wrote "lost". A plane that flies back to base isn't "lost."

Also, the US didn't deny that they lost any for decades. I remember the news footage (in the US) of the night it was shot down.

You have a peculiar axe to grind here. Are you trying to sell radar systems or something?

> ... their detection at multiple RF detection stations with synchronized clocks, to determine the difference in arrival time.

I believe this is generally called multilateration (MLAT):

* https://en.wikipedia.org/wiki/Multilateration

* https://www.skybrary.aero/index.php/Multilateration

* https://www.sciencedirect.com/topics/engineering/multilatera...

* https://www.navcanada.ca/EN/products-and-services/Pages/on-b...

>multiple RF detection stations with synchronized clocks, to determine the difference in arrival time. The radio emissions all travel at the same speed through the air, the difference in arrival time can be used with some simple trig to figure out where the RF emitter is.

That's a fancy way to say 'triangulation'.

I believe it's technically trilateration, not triangulation.

Contemporary passive radar arrays are phase sensitive so it ain't that simple either.

It would be triangulation if the system was measuring the arrival angles of the incoming signals. If the system relying solely on arrival time differences, it's likely doing something based on trilateration.

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