How Passive Radar Works | Hacker News Reader

How Passive Radar Works

passiveradar.com

200 pointssurprisetalk58 comments

I will actually concur with one of the less upvoted comments here: the most fascinating thing about this article is the website. I've seen that pattern on HN a couple of times in the past couple of months.

It's an incredibly specific vanity domain called passiveradar.com. Who would want that unless they're a radar manufacturer or an expert in the field? In both cases, they would put their name on it, but there's no attribution whatsoever.

The site contains two short articles, mostly illustrated with photos lifted from elsewhere. For example, the schematic of how the radar works in the earlier article comes from:

https://www.researchgate.net/figure/A-schematic-diagram-show...

The article is also illustrated with what appear to be two vibecoded SVG animations that don't look quite right.

So, what's going on here? I suspect it's an attempt to farm domains for resale, or for LLM spam operations down the line?

Half-broken visualizations look suspiciously like ones I get from LLMs, so who knows, maybe it's some LLM setting up their first blog?

(Not so) funny enough - I just did the same thing for https://gunsight.com complete with Claude generated interactive simulations for ballistics. We are actually building radars, and are likely going to use the domain in the future, but with zero effort I transformed an otherwise parked domain into something with a minimal amount of utility. So I do recognize the pattern.

I understand why you're doing it, but as a person who has to wade through dozens of these "minimal amount of utility" sites on almost every search, you're not making things better for the rest of us (or for the people who do hands-on ballistic testing that the LLM is just repeating without attribution).

wading through sites is still a thing? web search is still a thing? ok

Web search is a thing that is increasingly filled with AI slop that drowns out actual content. "wading through sites" for me means "wading through sites returned in search results".

It’s reminiscent of what are known as “unbranded pharmaceutical websites,” which focus on educating patients about conditions and symptoms without promoting a specific brand-name drug, which means the regulatory bar is far lower and you don’t need to include any safety info etc.

An example would be a website creating awareness around a disease for which there is (or was at the time) only one or two treatments for, like ED, crohns, a specific type of cancer, etc. in fine print at the bottom will say “Pfizer” or “j&j” but no drugs are mentioned, just a call to “ask your doctor about possible treatments.”

From the second article:

  There are two options for purchasing pre-made kits that use Blah2:  
  Retina.fm: An open-source project [...] (Full disclosure: I run this project)

It's by this guy: https://github.com/jehanazad

They make/sell Blah2: https://github.com/offworldlabs/blah2-arm

They could use passiveradar.com to advertise their product I guess, but they don't have to. I see no shenanigans here. I'm glad to be better informed about passive radar.

> By listening to how broadcasts like FM radio and digital TV bounce off objects, it's possible to determine their positions and velocities.

Wasn't some Github repo ITAR'd, couple of years ago, due to having python code for some SDR doing this?

Edit: Found it. 3years ago https://www.reddit.com/r/RTLSDR/comments/yu9rei/krakenrf_pul...

There's a note on the .fm site saying once it's assembled it can't be exported from the US, and shipping is US-only although I'm not sure if that's due to ITAR or just because international shipping is a headache.

There is a lot of talk of military uses here, but this technique is also used for environmental monitoring.

GNSS interferometric reflectometry (GNSS-IR) uses navigation satellites as the transmitter for a bi-static radar. The measurement device is any GNSS receiver (even your phone).

The technique can estimate environmental parameters like sea level, soil moisture, snow depth, and vegetation water content from systematic changes in the the multi-path around the antenna.

There is an open source Python package for this technique: https://github.com/kristinemlarson/gnssrefl

An introductory paper "The Accidental Tide Gauge": https://www.kristinelarson.net/wp-content/uploads/2015/10/La...

Here's a 10 year old study from Norway's navy.[1] It's a marginal technology for navies. It's more useful for defending urban areas from air strikes, because urban areas have lots of emitters, too many to attack.

There's a lot of current interest in what Iran is using for air defense. Search "Iran air defense passive radar". Some people speculate passive infrared. Some speculate passive radar. Nobody who's posting really knows, of course.

Ukraine uses many passive systems, including audio. Sort of like Shot Spotter, but for drones.

[1] https://apps.dtic.mil/sti/pdfs/ADA470685.pdf

I hear that Iran is using long-range quantum magnetometry, that stuff can even find a downed pilot in the middle of nowhere.

I read a lot about passive radars trying to leech off of opportunistic waves, and lots about actual troops preferring to play hide-and-seem with anti-radiation weapons just to use active machines.

A config that strikes me as obvious but doesn’t seem to be popular would be just bistatic where you fire your own transmitter far away from yourself?

There’s got to be a reason, but it seems like best of both worlds.

Probably more complicated to setup in a hostile environment because you'd need multiple transmitters, which also need to remain stationary, or at least you need to accurately know when they move.

Knowing where the transmitters are is vital. So wonder if you build in a positioning system to them. Each transmitter transmits a signal, but also rebroadcasts the signals it receives from the other transmitters on separate bands (these can be at lower power). If you can pick up a few transmitters, is that enough to build a model of where they are relative to each other, and then where they are relative to you?

If each transmitter picks up the rebroadcasts if its own signals, then with some assumptions about the rebroadcast lag (or measurements of it added to the signal!), that's enough to know the range to each other transmitter, right? So maybe they do that and then just broadcast the ranges (tagged on to their main signal), then any remote receiver can work it all out from there.

> that's enough to know the range to each other transmitter, right?

Only in a flat environment without too much atmospheric distortions. As soon as you get multipath effects from eg waves bouncing off buildings and mountains then the computational complexity goes through the roof. Also I don't think you should underestimate how much the signal degrades in a "target path" vs the "direct path". The article mentions -60 dB and I think that is fairly optimistic. The transmitter power needs to be HUGE to make it work, so it would be much easier to have stationary transmitters. Normal radars manage to do this because they are highly directional, but multistatic radars need to look in all directions at once and need to up the power as a result.

Much more covert, and civilian infrastructure also less likely to be blown up.

It seems to me this makes broadcast towers targets.

We've already got a really extreme version of this.

https://en.wikipedia.org/wiki/GNSS_reflectometry

One reason is Low Probability of Intercept radars (and transmitters / datalinks) do exist, and are very difficult (but not impossible) to identify and locate.

Multistatic radars (of which bistatic are just the case with N=2) are like the nuclear fusion of radar systems: everybody agrees it would be neat to have them, but they're always 20-30 years in the future. In practice it is extremely difficult to maintain the precise timing synchronization required for radar systems. Especially when used in moving vehicles or in sparsely populated areas the expected error goes WAY up to the point of unusability.

The survivability gains are also overhyped since 1. the enemy can just blow up the transmitters leaving you with a bunch of useless receivers and 2. most air defense doctrines already treat radars as something that should be distributed widely, so you can lose a few without the whole system collapsing.

The article goes into this only briefly, but modern radar systems don't just send out any random pulse but they very specifically tailor the waveform going out in order to do cool signal processing tricks like pulse compression. There is also the matter of frequency. The lower the frequency, the bigger the antenna you would need to get a proper direction reading out of it. Fire control radars typically operate in the X-band, around 10 GHz. Most civilian radio transmitters are around 100 MHz, so you'd need impractically large antennas and even then the bandwidth limitations would severely limit spatial resolution. One saving grace here is that stealth airplanes are typically most highly optimized against X-band radars from the direction they're going to bomb (forward), so you might have a better chance with a normal system, but then you still might not have a precise enough target to actually shoot at.

So while the multistatic system does offer some advantages, in practice it's just cheaper and (importantly for military use) requires less fiddly bits in the field to just use normal monostatic radars. Civilian use also doesn't benefit greatly from being multistatic. It's a bit like Tesla turbines or hyperloops: cool idea and it even "works" in a way, but the normal way of doing things is just way better when budgets and engineering realities come into play.

Source: I was a radar engineering officer in the Dutch navy about a decade back.

I am searching for (part-time) business partners who want to pursue a grant (or find a customer) to develop and build a passive radar system.

We know of such grants and customers, we need motivated people to help us get the grant.

It is mostly a computational software problem that needs a cheap supercomputer, we believe we are experts at that [1].

We already have two test area's where we are not restricted by laws: the Ukraine battlefield (brimming wit jammers and radar) and a radio silence area LOFAR receiver next to a military low fly zone near a large nature reserve and sea.

We hope to find people through Hacker News who can help us get the funding. Maybe even apply at YC.

Please contact me through my HN profile.

See my other comment below on how passive radar could become a game changer (that got downvoted just because I mentioned it here).

Some nice graphics related to passive radar:

[1] Cheap Wafer Scale Integration Supercomputer https://www.youtube.com/watch?v=vbqKClBwFwI

[2] How The U.S. Will Track EVERY Vehicle from Space: SAR GMTI/AMTI https://www.youtube.com/watch?v=-GTpBMPjjFc

[3] The Insane Engineering of Starlink V3 https://www.youtube.com/watch?v=U6veU66z2TQ

There is no rocket science here. Every radar has at least one midrange FPGA inside. Every small radar company at least two FPGA devs like me to minimize bus factor.

Regarding passive radar it is nice system in theory. In practical setup it’s not mobile and location bound. Every location has different RF radiation environment. Since transmission is not controlled the reception (and detection) can’t be optimized for anything.

Oh but there is a lot of physics science here [1,2] but indeed no rocket science [3]. FPGA's are just bad design by companies, no science.

I showed your comment to the 20 year old drone detection experts in Ukraine and they laughed at your dismissal, I imagine these guys know a lot more about FPGAs (slide 12 and further of [1]) then you, their lives depend on it every day.

Actual passive radar as we do in the Ukraine kill zones is very difficult science because all radio sources move all the time and the entire environment is full of reflections of moving leaves, nets, etc. Plus multiple moving antennas and jammers.

But then the rewards are big too. At the moment 80% of Russian casualties are from fiber optic FPV or cheaper radio controlled FPV drones only. So if we get even a little passive radar working (mainly by better over the network nanosecond time calibration [1] and orders of magnitude cheaper software defined radio receivers scattered densely on the ground plus cheap decoy transmitters and jammers) we save hundreds of thousands of lives, even tip the balance in winning this war.

Key is the orders of magnitude more complex calculations we do, hence the need for cheap supercomputers (cheaper than NVDIA). The first supercomputer I sold in 1986 to a Phd working on radar at Holland Signaal, the military brach of Philips.

[1] White Rabbit - High precision clock distribution in modern astroparticle experiments https://indico.kit.edu/event/22/contributions/927/attachment...

[2] New timing system for the LOFAR2.0 telescope https://videos.cern.ch/record/3015600

[3] Rocket Science is no rocket science if you get my thrust https://theconversation.com/rocket-science-isnt-rocket-scien...

p.s. Btw, rocket science [3] is the wrong term because that is actually really not very hard science at all, never was. "Elon Musk set back space travel 50 years" - Alan Kay. (He means rocket science is the wrong science for space travel, you need much better propulsion than chemical rockets because of the Tsiolkovsky rocket equation.

Update! Several people contacted us at the Fiberhood cooperative already and offered to help. Please join us, send us an email. morphle73 at g mail com.

We now have a design for an active radar and are adding a passive radar feature, 4x 10 Gbps ethernet and fast GUI software to the radar.

We need a loan from several people to build the prototype and we will make it available online. Please email us if you want to loan us 100 dollars or 100 euros. If you want to be kept informed or want to help, send us an email (in my profile).

We will sell a 2 KM version for around 8000 Euro and a 10 KM version for around 16000 and maybe a 60 KM version.

It will be open sourced and crowdfunded by two sources. A couple of scientists and students will give a lecture series on the operation and the theory behind them with if we get enough emails.

If we get enough donations we will donate the working prototype to soldiers of the Ukrainian Army together with a car, battery and backup diesel generator.

You can sign up as board member for our Fiberhood cooperative, you get to vote on all the decisions. You will have to sign a pdf document on the chamber of commerce website. The other activities of the cooperative are supporting Ukraine, rebuilding Ukraine, building radars, building batteries, building solar panels.

Thanks to everyone who participated in the effort!

Several people asked for an easier way to send us money for a loan, donation or purchasing the radar. The cheapest way we found is Western Union, for example $3.20 for sending 100 Euro in cash. That is cheaper, faster and safer than mailing it in an envelope or with digital payment systems.

We are building and improving the cheapest 20KW range radar systems, donated and delivered directly to Ukranian soldiers. https://protectukraine.nl

We hand-built these systems for anyone for higher quality and lower cost.

This is fascinating! I'm very tempted to build a "early warning door bell". Being the only house on our road, it'd be fun to try and set up a few of these to detect when a car takes the exit from the bigger road ours connects to, and give a heads up that visitors are coming!

There will be a few false positives from farmers coming to work the fields by our road, or people looking to hike in the nearby forests, etc. but it could be pretty fun nonetheless!

I agree, it will be a fun hobby. I predict we'll all have one on our house within the decade. We call them burglar alarms or porch pirate camera's

[1] Best Porch Pirate Moments of 2025 (All Clips) https://www.youtube.com/watch?v=avUWmpfoK10

[2] When Porch Pirates Get Caught In The Act https://www.youtube.com/watch?v=zR8Imw7EN78

If one visualizes the electromagnetic field in the environment, including all objects in it and any changes, one would naturally come across many applications in sensing the associated changes in the field. One classic example is the eavesdropping case at Moscow US embassy in the former Soviet Union.

What's this website? Who put it up?

Those have always been good questions to ask, but especially these days.

The image screams Australia, the image title confirms ...

https://www.silentiumdefence.com.au/our-solutions/space-doma...

but whether they have any connection or are just getting image jacked ... couldn't say.

A friend of mine did his PhD thesis on a passive radar that uses the GPS signal. I always found it such a brilliant idea, because the gps signal is everywhere and always available.

I wonder if the many Starlink satellites can be used for this? True, the signals are low and are steered, but the nature of steering creates many side lobes that will be useable in this manner. It would be a complex computational task with satellites in motion as well as ground stations transmitting on offset frequencies. I suspect various research/military labs are playing with this?

Such radar would be a game changer.

Indeed, full spectrum Western dominance over air space in the global South could come to an end. If that goes, the chains of hegemony loosen significantly.

Not really. Starlink can be turned on and off over certain territories.

Starlink can blacklist an area, but the frequent Tx/Rx emissions persist. Smuggled in terminals can be blacked once found. I am not sure how local areas discover/ID to Starlink/request blacklistimg works. I know starlink has upset many government internet rate extortion schemes in many countries in many Southern areas

Indeed, a global game changer for politics, defense, environment, climate, a step towards world governance.

Imagine a large number of crowdsourced software defined radio receivers with FPGA's and extremely accurate femto-second timing calibration connected to the internet spaced apart geographically and a distributed supercomputer to do all the realtime calculations. No single country or army would have this detector but everyone could use it for defense. Now imagine the resolution would be accurate enough for detecting planes, drones, birds, people, ground vehicles, ships, fish, insects, wind, tree leaves. We are close, we just need a cheap planetary deployment (like we had with SETI@home) and write better software. I imagine in 10 years we all have a detector, like we all have a smartphone or a router with a firewall. A passive radar in every building for spotting drones above our house and garden.

I did some searching and seems like an active area of research: https://ieeexplore.ieee.org/document/8768105/ and https://ieeexplore.ieee.org/document/9905046/

With their movement you might get some strange SAR like effects. Computationally complex but could add another dimension over a static tower.

I want this, but I do not have the experience with radio signals to build this myself without more guidance. Is there a DIY proof of concept I could lean on? How much more challenging will this be if you are in an area with overlapping FM signals from 2 transmitters sending the same signal?

Would suggest to take a look at Jackson’s book on classical electrodynamics to get some intuition.

That's very evil to recommend a graduate electrodynamics book to learn more about passive radar. I would suggest taking a look at Platos Republic to get some intuition on why that is. </s>

If you want to make an apple pie from scratch, first you must invent the universe…

A famous quote from Carl Sagan in the marvelous Cosmos documentary where he explains atoms by slicing an apple pie until you can not slice no more because you are down to a single elementairy particle the atom.

Carl also references Plato's Republic when visiting the actual cave where Plato lived.

Carl also references books classical mechanics but not the book the parent comment mentions but earlier ones like Al-Baghdadi, Cristian Huygens, Galileo, Newton.

This is a very good point to start: https://www.rtl-sdr.com/tag/passive-radar/

If you have more than one receiver, the main issue is time sincronisation between the receivers.

Using the two transmitters will complicate things a lot

For this kind of thing you need to step up to the kraken.

You can start with the radar receiver data from this 7 part DIY radar course [1] from a guy who works at a company who sell the needed chips.

Halfway in the video he mentions the MIT (or Stanford) course of a professor who recently died. That course is online and has a lot of documentation (much better than this course.

Build Your Own Drone Tracking Radar: Part 1 https://www.youtube.com/watch?v=igrN_wd_g74&t=292s

You can contact me if you want more guidance. For passive radar you can just start with recorded data. I have access to huge amount of recordings on the Ukraine battlefield. Buying or building cheap antennas and radios is not needed for passive radar software development.

Radar was developed during the last two years of WOII, mainly in Boston at MIT and hundreds of companies each building a radar, most were deployed months later to win the war. Development after the war shifted to Silicon Valley, founding Silicon valley and the later chip and software startups decades later.

Are there any clever tricks for the data processing involved here, given that the delay is a shift in the time domain and the Doppler effect is a shift in the frequency domain? Maybe involving fractional Fourier transforms, or wavelets?

Lots of clever tricks, all ITAR controlled.

Uninformed FUD, not a single dsp "trick" related to passive radar is ITAR controlled. The equivalence the OP mentioned is literally described in every undergraduate dsp textbook.

ITAR is only a US law.

you’re free to share tricks you think the Gp should be aware of.

This technique has been used in the search for the crashed MH370 flight - https://youtu.be/Y5K9HBiJpuk?t=2847.

a website for passive radars, cool xD

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