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.
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.
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.
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?
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...
That sounds suspicious.
Gait recognition is extremely difficult to do with any accuracy even with high resolution video in semi-controlled environments. Doing with with opportunistic 5G signals sounds far fetched.
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.
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.
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.
I love these engineering "hacks". Similarly, a friend of mine wrote a paper on how to use the GPS signal as radar source (so you only need a receiver) [0].
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.
What this article highlights for me is the unintended consequence of filling our space with electromagnetic waves. As someone who got hooked by the software defined radio (SDR) bug I was amazed with all the "stuff" that is going on between 70kHz and 6GHz[1]. And curious people thing "Hmm, what else can I do with this resource?" and the whole "seeing through walls" thing and using WiFi hotspots to geolocate in urban areas Etc have been falling out of that abundance of signals in the air.
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.
I don't know about you but I don't come here to read AI slop.
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.
35 comments
[ 2.8 ms ] story [ 50.4 ms ] threadI 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-...
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
[1] https://www.presstv.ir/Detail/2024/11/18/737423/guardians-of...
[2] https://www.yiminzhang.com/pdf/radar13_passive.pdf
https://en.wikipedia.org/wiki/Tamara_passive_sensor
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
See 802.11bf:
* https://en.wikipedia.org/wiki/WiFi_Sensing
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...
Me? I just want a car to be able to detect me so they don't run me over.
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.
[0] https://udrc.eng.ed.ac.uk/sites/udrc.eng.ed.ac.uk/files/atta...
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.
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.