Cell Towers Can Double as Cheap Radar Systems for Ports and Harbors (2014)

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

The 5G conspiracy theorists are paying attention.

Hmm. I wonder how big a different the whole 24Ghz vs 6Ghz thing makes, when used as a radar.

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

To properly understand, how much resolution is needed for that ?

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...

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.

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:

https://news.ycombinator.com/item?id=33581696

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...

Depends on how far you want your radar to go :)

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.

this sounds over-broad. if i make a tape measure yagi with some PVC pipe that is tuned for ~100mhz, and i tune to an FM station that is "over the radio horizon"^ and aim it at a patch of sky where planes travel, if i receive the remote FM radio station at my location that means it's reflecting off something.

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