HN Reader

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

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.

Is this solving a real problem?

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 spoke with a startup that is using 5G cell towers as radar. They said it is high-enough resolution to perform gait recognition.

5G signals can be used to track pedestrians on the street, not just ships in the port.

Also flood forecasting

https://www.smh.com.au/national/nsw/world-first-5g-spy-will-...

What about helping intercept missiles and drones? Asking for a friend.

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.

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.

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].

[0] https://udrc.eng.ed.ac.uk/sites/udrc.eng.ed.ac.uk/files/atta...

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.

With how cheap radar has gotten in the past decade I would be curious to know if any ports/harbors actually use cell towers?

No? It's significantly smarter and easier to use AIS.

Why add “cheap” qualifier? Everything must be about money?