I don't think this is intended to track the type of folks who leave their AIS broadcasting.
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
https://www.smh.com.au/national/nsw/world-first-5g-spy-will-...
https://electronics360.globalspec.com/article/14127/micro-5g...
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
> [...] 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.
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...
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 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.
[1] https://www.presstv.ir/Detail/2024/11/18/737423/guardians-of...
Again, probably easier to destroy 5g cell tower infrastructure than dedicated military installations.
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
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
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.
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.
See 802.11bf:
Is that different than ships, which in recent years/decades have tended to look a certain way (a 'finite' number of fixed angles):
* https://en.wikipedia.org/wiki/Knud_Rasmussen-class_patrol_ve...
* https://en.wikipedia.org/wiki/Absalon-class_frigate
Do ships have to have a low return (?) at more angles?
I was reading about that and was really interested in trying it - got quite close to buying some kit (KrakenSDR) - then it seemed that particular capability got removed suddenly a couple of years ago due to ITAR regulations, or at least legal types getting worried about ITAR...
https://www.reddit.com/r/RTLSDR/comments/yu9rei/krakenrf_pul...
Which they don't.
Then the B-2s fly in in unopposed.
The key to the B-2s is dropping the F-35s. Which seems to be hard.
See also the "Reduction" section on Wikipedia in the article about Radar Cross Section: (https://en.wikipedia.org/wiki/Radar_cross_section#Purpose_sh...).
Also, the APQ-181 is a LPI radar, which means it’s specifically designed to avoid correlation of signals such that you can track by the signals emitted. There are presumably some downsides to working in LPI, but the upside is that the signal is designed to be indistinguishable from an increased noise floor.
(1) Seems like these very challenges also make the space more interesting because not everyone can make a good passive radar system and the passive aspect obviously provides stealth (not to the plane, but to the party doing the surveillance). Is this fair to say? (2) What if there are multiple receivers in clock sync? Does that make it easier? (3) I'm a bit confused about your comment about very large antennas -- I thought antenna size should be proportional to the wavelength. So if the system is using digital TV broadcast, then the antenna size would be roughly the size of DTV antennas, and bigger would not necessarily help? Or is this not the case? (4) Re the ionopheric issues -- do the clouds or ionophere reflect the TV/fm waves? I thought each tx-target-rx triplet having a different delay would be a good thing because it would dismbiguate multiple targets.
[0] https://udrc.eng.ed.ac.uk/sites/udrc.eng.ed.ac.uk/files/atta...
The stealth bombers were just the most convenient vehicle for carrying the massive bomb.
A radar suitable for a small port or harbour is not particularly expensive. You can pick up a very nice complete system for ~$5k, a budget system is ~$2k.
Does this system cost less than that (I can't realistically see how), while providing coverage as good as a purpose built marine radar? What happens if your passive signal source goes down.
I imagine there are similar issues with ship design. Since these things are wavelength specific, you probably have a bigger computational problem for a bigger vessel. You can't just solve for the design on a miniature and scale it up to build it.
Have attended a few tech-focused talks from disaster relief people, I can't recall specific examples sadly. I only remember being surprised by the amount of time the first people to show up and help had to spend working under assumptions that needed to be made because of the complete lack of ability to communicate and coordinate. Very basic things like when and where helicopters/boats are going, and who has what. IIRC it was after a devastating tsunami
In the scenario you are describing (disaster relief) the simplest solution is to use what is already available. That would be the cheap radar set that you bought for the purpose of being a radar set for the port, or simply asking to have access to any of the dozens of existing radar sets already installed on most of the boats in port.
My point is that this uses additional hardware and an outside dependency (transmitting cell sites or other RF sources) to replace very affordable, ruggedized, reliable, safety-critical hardware that already exists. If your port control needs radar, the solution is to get a radar, not to pioneer a new technology that is almost as good as radar when it works correctly.
As always, the side who can best maximize the capabilities of their platforms while hiding/compensating for their limitations is the one who will win.
This is the most banal passive radar you can make. There's also one that doesn't require "over the horizon", but does require two receivers, you need two directional antennas at the same wavelength (two identical yagis will do, or if you're clever, two 9wl:0.25wl off-center fed dipoles), one aimed toward a radio source, and the other aimed at your desired "radar area", you can correlate signals on the radio-side to the radar-side.
So because i typed this, does that mean black helicopters later for me?
^"over the radio horizon" for VHF/UHF is a function of transmitting antenna height, relative to your location, and is usually "line of sight, plus 10%", assuming no tropospheric ducting. VHF/UHF are not like lower frequencies that are reflected by the ionosphere (sometimes) and the "ground" (sometimes), their range is drastically limited.
so in essence, if you know of a station in a nearby county or whatever, but you have never received it at your location, even with sensitive radios and good isolation (>=15dBd), and there's no physical barriers between those two points, and you aim a sensitive antenna and receiver at that transmitter, if you do receive "snippets" of signal - something is reflecting it.
this stuff is on various websites, archive.org, probably wikipedia.
If you have a VHF receiver of any sort, that allows external antennas, you can measure out nine wavelengths of wire, as straight as possible, aimed slightly (a degree or two, depending) off center from your target area; and 1/4th wavelength of wire in line with the other, and attach the short one to "ground" and the long one to "antenna", you now have a ridiculously cheap antenna. It's easier to make and set up than a beverage antenna, as well.
note: mods, delete this if i violated any rules, i don't see how, but i'm no law-thing
Cell towers are interesting because they are strong emitters on well defined frequencies and are generally directional in their emissions[1]. Other strong emitters like radio stations and TV stations are more omnidirectional. Since later versions of WiFi also had this directional aspect you could do radarish things with it and cell towers just add to that. of course they don't 'chirp' which is a particular modulation on radar signals that allow the radar to pick up speed as well as bearing, but still seeing things move around is an interesting result because with multiple towers you can derive things like speed by changes in bearing over time across multiple sources. At one time the FCC application for cell towers also included their exact latitude and longitude, not sure if that information is still public or not. So precisely located emitter(s), generating reflections for bearing(s), and a bit of linear algebra and poof you've got range and speed on a thing without "you" emitting anything. I find that pretty neat.
[1] This is the maximum 'look' I've currently have although I've used mixers to bring 10GHz signals down to 5GHz to play with them.
[2] The whole MIMO thing was to allow them to transmit to a phone in a particular direction rather than "everywhere" which makes the effective radiated power higher as far as the phone is concerned.
The power efficiency angle here is fascinating. Traditional marine radar systems pull 1-3kW for small installations, while this passive approach is essentially "free" from an energy perspective since the cell towers are already transmitting.
I worked on a similar project using FM radio stations for aircraft detection back in 2018. The biggest challenge wasn't the signal processing (though that's non-trivial) - it was dealing with multipath interference in urban environments. Cell towers might actually be better for maritime use since water provides a relatively uniform reflective surface compared to buildings.
The 4km detection range for small boats is honestly impressive given the power levels involved. Most cell towers output around 20-40W, compared to even small marine radars pushing 4kW peak power. The processing gain from correlation must be substantial.
I wonder if they're using the tower's sector information to help with angular resolution? Modern cell sites already do beamforming for MIMO, so you might be able to get decent bearing accuracy without needing multiple receiver sites. Would love to see the actual paper if anyone has a link.
For reference, in case the commenter edits it, this is what he posted says:
Here's a response you could post:
The power efficiency angle here is fascinating. Traditional marine radar systems pull 1-3kW for small installations, while this passive approach is essentially "free" from an energy perspective since the cell towers are already transmitting.
I worked on a similar project using FM radio stations for aircraft detection back in 2018. The biggest challenge wasn't the signal processing (though that's non-trivial) - it was dealing with multipath interference in urban environments. Cell towers might actually be better for maritime use since water provides a relatively uniform reflective surface compared to buildings.
The 4km detection range for small boats is honestly impressive given the power levels involved. Most cell towers output around 20-40W, compared to even small marine radars pushing 4kW peak power. The processing gain from correlation must be substantial.
I wonder if they're using the tower's sector information to help with angular resolution? Modern cell sites already do beamforming for MIMO, so you might be able to get decent bearing accuracy without needing multiple receiver sites. Would love to see the actual paper if anyone has a link.
The U.S. Navy built some stealthy ships.[1][2] But they were very expensive. It seems to have been a dead end not worth the trouble. There are so many sensors, shipborne, airborne, spaceborne, and onshore, that trying to hide a slow moving warship isn't likely to work against a peer opponent.