More coverage of RF sensing, including laptops/phones with radios+NPU to sense their human:
2025, "Espargos: ESP32-based WiFi sensing array", 30 comments, https://news.ycombinator.com/item?id=43079023
2024, "How Wi-Fi sensing became usable to track people's movements", https://www.technologyreview.com/2024/02/27/1088154/wifi-sen...
2023, "What Is mmWave Radar?: Everything You Need to Know About FMCW", 30 comments, https://news.ycombinator.com/item?id=35312351
2022, "mmWave radar, you won't see it coming", 180 comments, https://news.ycombinator.com/item?id=30172647
2021, "The next big Wi-Fi standard is for sensing, not communication", 200 comments, https://news.ycombinator.com/item?id=29901587
Right. The longer range versions of multistatic radar are used to detect stealth aircraft.[1][2] All that careful stealth geometry to minimize direct reflections doesn't help much when the emitters and receivers are in different locations.
[1] https://www.presstv.ir/Detail/2024/11/18/737423/guardians-of...
Well, you don't even need a radar. Tamara sensor could detect B-2, when it had it's onboard radar on.
No but the highly classified radar absorbing compounds that stealth aircraft are wrapped in definitely help :)
(context: I used to be involved in the design of military radar systems for the Dutch navy)
The radar absorbing compounds of stealth aircraft are highly optimized for specific wavelengths (usually X-band) and fall off heavily outside that frequency band. Similarly, the radar cross section of stealthy aircraft is highly optimized for specific purposes (usually evading GBAD in the forward direction) and rapidly falls off in other scenarios. Most "stealth" aircraft are actually fairly visible from other directions.
That said, multistatic radar with transmitters-of-opportunity like cell towers and civil radio stations has always been in strong competition with fusion power as "the tech that is forever 10 years in the future". The transmitters are often not very powerful compared to dedicated radar systems and worse, they transmit energy in the horizontal plane rather than upwards where the planes are. The frequencies involved are much lower, which inherently leads to less radial accuracy unless you use VERY large antennas. Unlike a dedicated radar system the signals they send out are typically not shaped optimally for radar purposes, so signal processing like pulse compression becomes much harder. Because the signals are inherently not as predictable as normal radar signals you need MUCH more computing power. Finally, atmospheric conditions become fiendishly tricky for long range, because signal delays between each transmitter-target-receiver triple will be different. This means resolution goes way down if there's too many clouds or ionospheric interference, often to the point of uselessness.
Many of those problems are mostly terrible when trying to detect aircraft at long range though, and largely go away for short range surface use like in port. I'm still not entirely sure why for a port, which is stationary and requires tons of infrastructure investment anyway, this system would be preferable to a normal civilian type radar system. You can get a conventional one for at most a few tens of thousands, while this system apparently requires a trailer full of RF signal processing equipment. That is likely to cost at least in the order of magnitude more, while probably being less accurate.
How does it help if you're passing between transmitter and receiver?
(either directly, or by bouncing a radar signal off the ionosphere and receiving it again)
You should still show up as a shadow?
direct is practically useless, because that's point-to-point. lighting up the ionosphere that way seems like the hardest case scenario, requiring a very powerful transmitter, somehow ignored by sensitive high-resolution scanning over a large area of sky. and you'd be disrupted by other occluding objects like water vapor
there is precedent https://en.m.wikipedia.org/wiki/Over-the-horizon_radar but it seems like a limiting factor is suitable frequencies and resolution
More on Wi-Fi RF sensing:
2014, "We Can Hear You with Wi-Fi!", https://dl.acm.org/doi/abs/10.1145/2639108.2639112
2015, "Keystroke Recognition Using WiFi Signals", https://dl.acm.org/doi/abs/10.1145/2789168.2790109
2022, "Human Biometric Signals Monitoring based on WiFi Channel State Information using Deep Learning", https://arxiv.org/abs/2203.03980
5G signals can be used to track pedestrians on the street, not just ships in the port.
I spoke with a startup that is using 5G cell towers as radar. They said it is high-enough resolution to perform gait recognition.
There's a whole host of radar research using OFDM/ Wifi (I wrote a paper on the topic a while back where i implemented it with some software defined radios).
The best paper on the topic is Martin Brauns[1]. It's insanely comprehensive and easy to digest.
[1] https://publikationen.bibliothek.kit.edu/1000038892/2987095
Doesn't the thesis assume you are the one sending out the OFDM signal, while the OP is about a passive radar thing? Maybe I got one of those mixed up.
There are proposals for the 6G standard to support Integrated Sensing and Communication(ISAC)[0]. So the hardware might natively be able to support gait recognition. The use cases given are UAV detection and localization. It sort of seems like this could bring Vernor Vinge's localizer mesh to reality, privacy implications be damned [0]https://www.ericsson.com/en/blog/2024/6/integrated-sensing-a...
I seem to recall reading (on HN, no less) that advanced passive radar technology is classified as munitions, by the US Government and is under export controls?
Yes, they are on the BIS Commerce Control List. It doesn't need to be particularly advanced to be export controlled.
5A001.g Passive Coherent Location (PCL) systems or equipment, “specially designed” for detecting and tracking moving objects by measuring reflections of ambient radio frequency emissions, supplied by non-radar transmitters. Technical Note: For the purposes of 5A001.g, non-radar transmitters may include commercial radio, television or cellular telecommunications base stations.
https://www.bis.doc.gov/index.php/documents/regulations-docs...
You are probably thinking of this thread:
Hmm. I wonder how big a different the whole 24Ghz vs 6Ghz thing makes, when used as a radar.
Depends on how far you want your radar to go :)
To properly understand, how much resolution is needed for that ?
The 5G conspiracy theorists are paying attention.
Depending on node density of a 5G network (think street lamp cells), it is not outside of the realm of possibility that you're going to be able to obtain radar derived point clouds from cellular networks doing double duty as phased array radar networks. Greater density = greater observability and surveillance capabilities through SDR (limited by hardware frequency band operating tolerances).
https://electronics360.globalspec.com/article/14127/micro-5g...
The gap between the people demanding these systems and those who design it it is so large, it’s vulnerable to corruption in infinite ways, let’s be honest.
It underscores how important cybersecurity is in mobile, IoT and Wi-Fi systems. A few critical exploits chained together is all it takes for physical surveillance or bio-sensing[1].
A 2007 NSA hacking toolkit catalog leaked by Snowden[2] shows what state-of-the-art was 18 years ago. Just imagine what a remote attacker can do with today's commercial hardware.
[1]https://www.mdpi.com/1424-8220/24/7/2111
[2]https://www.eff.org/document/20131230-appelbaum-nsa-ant-cata...
With how cheap radar has gotten in the past decade I would be curious to know if any ports/harbors actually use cell towers?
What about helping intercept missiles and drones? Asking for a friend.
Sure, but not as well as a dedicated radar system and at much higher cost. In TFA they spotted small speed boats at 4 km, and needed a trailer full of RF equipment. Drones and missiles would be detected even later, since the antennas of cell towers are designed not to radiate any energy upwards (there's usually no cell phones high in the air, so that energy would just be pure waste).
You might be interested in a similar system in Ukraine that uses a huge amount of acoustic sensors (basically just weatherproofed microphones) to detect the very loud engines of Shahed drones as they fly by, and then directs air defense crews based on that approximate location data.
how dos this acoustic system work, and how does it distinguish drones from other noise? Does it use any form of AI?
Drone propellers make a distinctive noise, which can be isolated quite well from background noise with FFT analysis. I doubt they'd choose to use AI for that, as classical methods work perfectly fine and need much less processing power.
Also flood forecasting
https://www.smh.com.au/national/nsw/world-first-5g-spy-will-...
Flood sensing with 5G?
> [...] New South Wales State Emergency Service (NSW SES) and the NSW Government, University of Technology Sydney (UTS) researchers working with industry partner TPG Telecom [...]
> “We want to tell people exactly how high [the flood] is. We’re now down to accuracy of 0.1 metres.”
> [...] “Currently, residents will receive the warning that the water is going to come, and they’ve got to get their cattle to higher ground. But how high is high?” she said.
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No? It's significantly smarter and easier to use AIS.
AIS, like ADSB, is secondary surveillance - not radar. It's a mechanism for cooperative targets with functioning electronics to identify themselves and provide operational information. However, it does not detect uncooperative entities or those not equipped with the electric transponders. For example, AIS won't show you an enemy's invading fleet, and ADSB won't show incoming missiles. Those needs are fulfilled by primary surveillance radar, like the passive solution from this article.
If you're honestly worried about being bombed then you need to buy radar.
With your logic all I have to do is take the additional step of disabling your cellular infrastructure before I steam up to your port.
This is not a tactical solution. It can only be for convenience or cost savings. In that realm, AIS is the obvious answer.
AIS is not mandatory for all vessels, and in any case it can fail both on the vessels themselves and in the control center. Just for normal safety purposes you would want to have a secondary system to be able to continue operating busy ports.
Ukraine war shows improvised capability from cots hardware can have a meaningful impact. Probably easier to get 5g cell tower infrastructure than dedicated military radars.
You're telling me Ukraine can't get dedicated radar? A non weapons package that any Western nation would sell to them without reservation?
Again, probably easier to destroy 5g cell tower infrastructure than dedicated military installations.
Of course they can, though not as easily as you seem to think I suspect. Radar technology is very secret. Regardless, it’s a matter of numbers. I’m sure the Ukranians would love to field unlimited predator drones if they could. In reality, they field DJI and other commercial drones en masse because of availability. Only relatively recently have they got their own cheap mass produced drones online. These drones are also easier to destroy than a predator, so by your logic, why would they have invested in this? Wunderwaffe never wins wars, logistics do.
It can also be used for defense in depth. Each additional sensing system which must be disabled before an attack is an additional barrier.
From the first paragraph: "Without radar installations, it can be hard for port employees to detect small ships like those employed by pirates or by the terrorists who attacked the USS Cole in 2000"
I don't think this is intended to track the type of folks who leave their AIS broadcasting.
I wouldn't go so far as to call this RF "pollution", but it is a reminder that the EM spectrum is getting a lot busier.
Me? I just want a car to be able to detect me so they don't run me over.