[HN Gopher] Cell Towers Can Double as Cheap Radar Systems for Po...
___________________________________________________________________
Cell Towers Can Double as Cheap Radar Systems for Ports and Harbors
(2014)
Author : transpute
Score : 131 points
Date : 2025-06-29 21:48 UTC (1 days ago)
(HTM) web link (spectrum.ieee.org)
(TXT) w3m dump (spectrum.ieee.org)
| timewizard wrote:
| No? It's significantly smarter and easier to use AIS.
| zomiaen wrote:
| 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.
| jchulce wrote:
| 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.
| timewizard wrote:
| 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.
| timschmidt wrote:
| It can also be used for defense in depth. Each additional
| sensing system which must be disabled before an attack is
| an additional barrier.
| denkmoon wrote:
| 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.
| timewizard wrote:
| 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.
| denkmoon wrote:
| 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.
| WJW wrote:
| 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.
| ImPostingOnHN wrote:
| I spoke with a startup that is using 5G cell towers as radar.
| They said it is high-enough resolution to perform gait
| recognition.
| userbinator wrote:
| The 5G conspiracy theorists are paying attention.
| toomuchtodo wrote:
| 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.
| ..
| bee_rider wrote:
| Hmm. I wonder how big a different the whole 24Ghz vs 6Ghz thing
| makes, when used as a radar.
| esseph wrote:
| Depends on how far you want your radar to go :)
| polalavik wrote:
| 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
| stefan_ wrote:
| 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.
| makeitdouble wrote:
| To properly understand, how much resolution is needed for that
| ?
| supportengineer wrote:
| 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?
| syedkarim wrote:
| 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...
| genewitch wrote:
| 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
| charcircuit wrote:
| You are probably thinking of this thread:
|
| https://news.ycombinator.com/item?id=33581696
| gene-h wrote:
| 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...
| brk wrote:
| 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.
| nelox wrote:
| Also flood forecasting
|
| https://www.smh.com.au/national/nsw/world-first-5g-spy-will-...
| toomuchtodo wrote:
| https://archive.today/krT4z
| westurner wrote:
| 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._
| ofalkaed wrote:
| With how cheap radar has gotten in the past decade I would be
| curious to know if any ports/harbors actually use cell towers?
| transpute wrote:
| 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
| Animats wrote:
| 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...
|
| [2] https://www.yiminzhang.com/pdf/radar13_passive.pdf
| 4gotunameagain wrote:
| No but the highly classified radar absorbing compounds that
| stealth aircraft are wrapped in definitely help :)
| kuschku wrote:
| 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?
| ruined wrote:
| 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
| WJW wrote:
| (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.
| throw0101b wrote:
| > _(context: I used to be involved in the design of
| military radar systems for the Dutch navy)_ [...] _Most
| "stealth" aircraft are actually fairly visible from other
| directions._
|
| 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?
| WJW wrote:
| Ships like that are typically optimized to look small
| from "low" angles, ie from the perspective of other
| surface combatants and sea skimming anti ship missiles.
| The large flat surfaces are not so much used to reduce
| RCS by themselves, but mostly to reduce instances of
| "corner reflectors" like hatches and exposed cranes the
| like, which can have a RCS many times larger than their
| physical size due to their shape.
|
| See also the "Reduction" section on Wikipedia in the
| article about Radar Cross Section: (https://en.wikipedia.
| org/wiki/Radar_cross_section#Purpose_sh...).
| throw0101b wrote:
| Would airborne radar be better able to find ships with
| these designs (at least relatively speaking)?
| saltcured wrote:
| As I recall, the faceted look of the early stealth
| aircraft was said to be a practical matter. It reduced
| the complexity of modeling the reflections during the
| design process. So with additional computational
| complexity, they could go back to smooth surfaces in
| later designs.
|
| 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.
| sorenjan wrote:
| I've seen SDRs being used to track civilian airplanes
| using TV transmitters. Using two antennas/receivers, one
| pointed at the transmitter as reference and one towards a
| big air traffic plane, they might get a couple km range.
| While the concept is really interesting, it doesn't seem
| very practical to try to see smaller fighter jets or even
| stealth planes beyond visual range. And TV transmitters
| are probably among the most powerful transmitters in
| common use.
| logifail wrote:
| > I've seen SDRs being used to track civilian airplanes
| using TV transmitters
|
| 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...
| smath wrote:
| The code from kraken was removed -- I think because it
| was open source? I think its still ok to write your own
| code (Discalimer: I havent done this so please verify on
| your own obvisouly)
| smath wrote:
| Very interesting, thanks for sharing. I'm curious about
| the following:
|
| (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.
| Grayskull wrote:
| Well, you don't even need a radar. Tamara sensor could detect
| B-2, when it had it's onboard radar on.
|
| https://en.wikipedia.org/wiki/Tamara_passive_sensor
| rcxdude wrote:
| stealth does in general go out the window when you turn on
| your radar. It's much like dressing in black and then
| running around with a flashlight at night. (and yes, there
| are equivalents to the various forms of night vision here,
| with associated tradeoffs)
| engineer_22 wrote:
| Nah, there is much secret about radar
| ta20240528 wrote:
| It turns out for the Iran drama, that radar's like the
| Tamara have to survive the F-35s first, then the F-15s...
|
| 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.
| chipsa wrote:
| Doubt: the APQ-181 radar on the B-2 is a Ku band radar,
| about 15 GHz. Tamara is about 1 GHz. This is entirely
| incompatible frequency ranges.
|
| 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.
| wat10000 wrote:
| I'm pretty sure that "don't operate your radar in enemy
| airspace" is right below "don't email your flight plan to
| the enemy" on the list of tips for stealth pilots who want
| to survive a mission.
| psunavy03 wrote:
| . . . because we all saw how effective Iranian air defenses
| were at countering stealth aircraft recently.
| prasadjoglekar wrote:
| To be fair, Iranian air defences were awful at countering
| non stealthy Israeli aircraft as well.
|
| The stealth bombers were just the most convenient vehicle
| for carrying the massive bomb.
| psunavy03 wrote:
| Open-source reporting has Israeli F-35s kicking the door
| down on the Iranian IADS. Strong data point as to their
| effectiveness when properly employed as compared to some
| of the woo-woo airshow fanboying over things like
| bistatic radar.
|
| 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.
| knetl wrote:
| 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
| throw0101b wrote:
| > _2021, "The next big Wi-Fi standard is for sensing, not
| communication", 200 comments,
| https://news.ycombinator.com/item?id=29901587_
|
| See 802.11bf:
|
| * https://en.wikipedia.org/wiki/WiFi_Sensing
| soundpuppy wrote:
| 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.
| knetl wrote:
| 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...
| blendo wrote:
| 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.
| janpmz wrote:
| 5G signals can be used to track pedestrians on the street, not
| just ships in the port.
| ReptileMan wrote:
| What about helping intercept missiles and drones? Asking for a
| friend.
| WJW wrote:
| 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.
| 400thecat wrote:
| how dos this acoustic system work, and how does it
| distinguish drones from other noise? Does it use any form of
| AI?
| WJW wrote:
| 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.
| dostick wrote:
| Why add "cheap" qualifier? Everything must be about money?
| mbonnet wrote:
| it must be when everything costs money.
| ziofill wrote:
| 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...
| aidenn0 wrote:
| That's black-magic there. Direct reception of GPS is
| challenging enough with how little power reaches ground level.
| dghlsakjg wrote:
| 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.
| monster_truck wrote:
| Worst case disaster relief scenarios during the first weeks-
| months, before all of the gear shows up. Historically there
| have been some pretty big wins from using what's around to do
| stuff it's otherwise pretty bad at.
|
| 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
| dghlsakjg wrote:
| Sure, but in that circumstance this seems like an even worse
| solution. This requires hardware as well as uptime on
| services that you have no control over.
|
| 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.
| ChuckMcM wrote:
| 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.
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(page generated 2025-06-30 23:01 UTC)