Build a passive radar with software-defined radio | Hacker News Reader

Build a passive radar with software-defined radio

spectrum.ieee.org

221 pointssamizdis53 comments

I worked on a downstream consumer of a passive radar system years ago and was amazed at its capabilities. We had it set up on a tennis court outside of Farnborough UK during a bi-annual air show and it was able to track all the activities of the various planes going through their routines. And I recall seeing commercial aircraft out in the English channel, 50 miles or so away. I don't know what became of the system, but there's an old marketing brochure online here: http://www.mobileradar.org/Documents/Silent_Sentry.pdf

Over-The-Horizon Radar (OTH) : https://en.m.wikipedia.org/wiki/Over-the-horizon_radar

This is cool but it's not quite a radar yet. He saw an increase in signal strenghth when a plane overflew his position and reflected some of the signal.

Modern SAM systems are said to have the capability to track targets passively, for example by using reflected DVB-T transmissions. This is very important because combat aircraft have radar warning receivers, and it's best to not tell the enemy you're engaging him so that he can use countermeasures or perform evasive manoeuvres. Also SEAD/DEAD (suppression/destruction of enemy air defenses) is near impossible unless you can first locate the enemy air defense radars, which becomes a lot harder if you cannot simply home in on strong RF emitters.

Some very nice results have been reported with a new multichannel coherent SDR: https://www.crowdsupply.com/krakenrf/krakensdr

Haven't heard a lot of talk about this one yet, but the video at https://youtu.be/GZAbPsT3oRM?t=12 looks really promising.

This is exactly the SDR I was trying out :)

Ah, good point, I commented without having RTFA. It's a really cool piece of hardware, no question about that.

It really is, has a super solid build quality too. Stumble around crowdsupply if you get a chance, tons of other projects just like this. My problem is stopping myself from funding more of them.

Looks great, ordered one. Couldn’t find any guide as to which frequency band to select from https://www.arrowantennas.com/arrowii/krsdr.html can anyone help? Is it best to choose based on local Digital TV bands? And is there any resonance changes in the array setup to factor in?

If you are doing passive radar, then a UHF/VHF TV antenna should cover you for whatever digital channel you choose

Sorry if I didn't make this clearer in the article due to space constraints, but the TV surveillance antenna gives me a fair degree of directionality, enough to distinguish planes on different LGA and JFK approach/departures paths as they move through different parts of the sky, although once you get within 90 degrees of the reference signal all is lost. You also get a velocity and a range, albeit a bistatic one, similar to the first U.K. Chain Home radar stations.

Did you maybe write down more details somewhere else? Also, consider sending your article to the https://www.rtl-sdr.com/ blog, it is really cool work!

Thanks! I have notes scattered around, but they're not coherent (unlike the RTLs! <rimshot>) and I doubt I'll have time to publish them as once I've written an article I generally have to move on the next thing, which is the downside of being allowed to play with nice toys.

> for example by using reflected DVB-T transmissions

Now imagine if you had a constellation of thousands of orbital satellites with precisely known positions...say Starlink. Will ground stations even be a thing as passive anslysis matures?

There are many, many ISTAR satellites that work in visual, infrared or SAR for various intelligence agencies and militaries. But that's not necessarily good enough to home in a SAM, they don't have enough TX power (we're talking about hundreds of kilowatts or even a megawatt of TX power in pulse), and they are too high/far (radar reflection energy decreases with the fourth power of distance). The satellites are still terribly useful though, just look at what's happening in Ukraine.

The next major war will probably see a destruction of most or all of those satellites, and consequently a multi-century pollution of the low earth orbit with extremely fast deadly debris. Multiple countries already have anti-sat weapons, and they're even getting deployed to "3-rd party" countries. For example American ballistic missile defence installations in Europe (Aegis-ashore).

> But that's not necessarily good enough to home in a SAM, they don't have enough TX power

Fair enough. I guess if you need to hit back you got to give yourself (or your trackers) away.

Scary what your saying about anti-sat though. That really would put the kibosh on space exploration.

It only has to get the SAM within such a range that the target can no longer avoid it. Once within this range the SAM can home in using active radar which will light up the threat display in the target but leave them no space/time to react.

A scenario where all cellphones have this capability sounds a lot like the scene at the end of the Dark Knight.

Imaging with Your Wi-Fi Hotspot : https://physics.aps.org/articles/v10/50

Imaging Radar : https://en.m.wikipedia.org/wiki/Imaging_radar

'WiFi radar/imaging' : <https://html.duckduckgo.com/html/?q='WiFi radar/imaging'>

Would it be possible to construct a network of passive radar receivers (assuming some time-sync between them, like the KiwiSDR's GPS receiver), and trilaterate the actual positions of planes?

From there, the next step is to hide the ones that seem to line up with ADS-B data, and show only what's left, and that would be all the "black" flights...

Some ADS-B receivers do that with GPS sync.

If a position has poor accuracy and is shown as "mlat" it' multilaterated based on when multiple receivers received the ADS-B packet. So while the plane is not transmitting it's location, the transmission can be used to identify and locating it.

An actual passive radar network with multiple receive sites is reportedly a commerical product from multiple vendors, but it's not easy nor cheap.

Reminds me of stories of US stealth aircraft being detected by using reflected cellular signals. e.g. https://www.extremetech.com/extreme/49498-cell-phone-tech-co... (2001) It seems like the same technique.

Reminds me of SNW 1x04[1]

      "You just turned a compass into a radar."
      - Captain Christopher Pike

[1] https://www.youtube.com/watch?v=-NBxUqAHYTo&t=0m22s

Out of curiosity might it be possible to exploit signals generated by Over-the-horizon radar in a passive manner, for detection of planes etc?

Or does reception of such signals have to be done near to where the transmitter is located.

That's not too far off how radar was accidentally discovered, over and over. There is no one clear unambiguous first claim to radar, as far as I can tell. The general story is that plain ol' mediumwave/shortwave was bouncing off planes and ships as they passed, and operators couldn't figure out where the little signal spikes were coming from.

Actual over-the-horizon radar systems have used transmitter and receivers at a significant distance from each other -- the Soviet Duga radar system in the 70s had its transmitter and receiver separated by ~50 kilometres or so. The receivers (and in active systems, also the transmitter) need to be in very precise time sync, and being physically close enough to run a direct cable makes that a lot easier.

But theoretically, assuming the system is sensitive enough and time-synced enough and you have enough computational oomph available, an arbitrary number of receivers, at any distance, can be used to synthesize an arbitrarily large aperture. It's actually a similar matter to the synthetic aperture radio telescopes used in space astronomy, where telescopes with an effective aperture ~100 million kilometres wide have been created, using space-based radio telescopes linked with ground-based radio observatories.

Such systems may not have the sensitivity of a 100 million kilometre wide telescope mirror (no matter to catch the photons since the "telescope" is mostly empty space) but it does have the equivalent angular resolution of such a 100 million km wide telescope mirror. It's mind-boggling when you consider the consequences of this as computation power improves. It will soon be possible to do this at such high speeds that it will allow frequencies into the far-infrared spectrum, not just microwave, for example.

Thanks a lot for your reply, that's really interesting re. the mediumwave/shortwave spikes. I also hadn't realised that the receiver of the Duga radar was so far away from the transmitter.

I assume now you could probably use GPS/atomic clocks to keep them in sync rather than a cable.

The rx/tx separation in over-the-horizon radars (OTHR) serves to increase isolation between the transmitter and receiver. The received signal is tiny, as it has not only bounced off the target but done a double bounce off the ionosphere. Doppler shifts are also very small, so one needs to minimise the clutter at zero Hertz, and a direct signal will appear as zero Hz clutter.

You can get an idea of the precision of the timing required by considering how far a radio wave travels in a unit of time corresponding to the uncertainty in your time source and comparing that to the repetition period of the waveform. OTHRs typically have large ranges and use long period waveforms to minimise range ambiguity, so the timing requirements probably aren't too bad.

Summary: A passive OTHR should be doable, especially as a cheapish SDR+computer can digitise and process the entire HF band. Front end sensitivity might be a concern? Low phase noise is critical. Phase noise is where many SDRs fall down?

It's always been in the back of my mind that a distributed passive radar would be a cool project. Something like the existing network of hobbyist ADS-B receivers but pumping out timestamped HF samples which could be used for all manner of purposes. GPS could probably achieve a precision of single digit nanoseconds, which might be small enough to consider the receiver outputs as coherent, allowing synthesis to be done.

If a node's network connection couldn't support continuous transmission it would still work to do burst mode, whereby each node might capture the first second of each minute (for example) then have a minute to transmit that data over the network.

There's definitely been some interest in using amateur HF activity for radar, e.g. https://www.rtl-sdr.com/nils-critiques-the-mh370-wspr-aircra...

I was working on something related for work many years ago... commercial gps receivers aren't sufficient. We kept time with GPS disciplined rubidium clocks, but that was only good enough for about 10 seconds of coherent operation before the phase wandered off too far @1GHz. (Not HF exactly, but...)

It would be cool to try again with caesium or hydrogen maser ...

KiwiSDR's are GPS synced and have timestamps. There's a TDoA direction finding implementation for it.

So people have used them for locating transmitters, but the accuracy is not amazing.

Still very impressive capability available for free to everyone.

Nice. I wasn't aware of KiwiSDR. It's pretty well what I was thinking about, if the samples can be streamed.

The docs mention that the clock is GPS disciplined, but is it the case that the samples are timestamped in a way that can be traced back to an absolute time. (If not, it would be a matter of programming?)

Soviet Duga Radar : https://en.m.wikipedia.org/wiki/Duga_radar

Soviet Duga-2 Radar Tower : https://abandonedplacesmap.com/huge-secret-soviet-duga-2-rad...

Where, online, are the SDR communities?

A lot of the discussions and chat I follow are on Various Discord servers and forums like https://www.rtl-sdr.com/, also look into various defcon/bsides groups there's usually a handful there too.

I joined the crowdsupply kraken funding and have been impressed with the hardware so far, but I only knew of them from their previous hardware. It's still a pretty niche topic relative to other things.

A random sampling:

r/rtlsdr

##rtlsdr on libera.chat

The SDR++ discord

The RTL-SDR discord

The Great Scott Gadgets discord

The Signal Identification wiki's discord

The Signals Everywhere discord and many other places and niches.

Is it actually legal to build radars?

It is legal to receive and process almost any electromagnetic radiation, yes. Transmission is licensed for nearly anything slightly below infrared, mostly because it would become a mess if everyone transmitted whatever they felt like. It is also restricted above UV, but for other reasons :)

It's not legal in the UK

as a UK person living in the US who recently passed the US (technician) Ham radio exam, it astonishes me how draconian the UK laws are about RF.

it's even illegal to listen to UK air traffic control radio? what? how can they even be serious

> it's even illegal to listen to UK air traffic control radio

How exactly would they know that you're listening?

Generally, they don't, and no-one cares. Airband scanners are freely available to buy online and in shops.

It really manifests itself when you look online to see "listen to ATC" live streams and YouTube videos. There aren't any from the UK.

https://www.liveatc.net/faq/

You can receive anything... sending is another issue.

This is discussing passive radar, where you use another emitter, like a FM station or 20 to be transmitters, and your "radar" just sits and listens for the bounces off other objects.

This is definitely not true for every legislation.

Yes, the United States has a you-can-recieve-anything philosophy (with some limits for old analog cellphone frequencies, and restrictions on using, e.g. a police scanner in a car) that isn't matched in many other countries, which is the legal basis of how Ireland and the U.K. require you to buy a license to watch over-the-air television.

The TV licence has been expanded to include any live broadcast, so watching sport on Amazon requires a TV licence.

I also think that in the UK it’s illegal to listen to ATC.

How did the UK ever get to be this way?

Centuries of legal scar tissue accumulated through continuous, unbroken political order.

Treaty of Rome. Maybe North-Korea did not sign it but that is nitpicking.

Can you give an example of a legislation where receiving a signal by multiple receivers is illegal?

Yes -- the UK. You need to sit an exam and get a radio operator's license to receive anything civil aviation related, for example.

Arguably everyone in the US who owns or constructs an SDR is a felon, by virtue of violating the Electronic Communications Privacy Act of 1986 [1,2]. Certainly everyone who sells one.

AFAIK this misguided law has never been repealed, despite being rendered obsolete by the demise of the old 800 MHz AMPS standard. No 800 MHz-capable SDRs I'm aware of make even the most casual attempt at blocking coverage of that range.

So, yes, it's legal to build a radar -- certainly a passive one -- but given that most SDRs cover the ECPA-prohibited frequency range, it would be hard to build a radar with off-the-shelf equipment without breaking other laws.

1: https://en.wikipedia.org/wiki/Electronic_Communications_Priv...

2: https://forums.radioreference.com/threads/cellular-blocked-s...

The ECPA makes it illegal to eavesdrop on all phone calls, it’s the 1992 amendment to the Federal Communications Act of 1934 that allows the FCC to restrict the part 15 authorization of anything that can receive cellular calls:

https://www.govinfo.gov/content/pkg/STATUTE-106/pdf/STATUTE-...

  SEC. 403. INTERCEPTION OF CELLULAR TELECOMMUNICATIONS.
  […]
 “(d)(1) Within 180 days after the date of enactment of this subsection, the Commission shall prescribe and make effective regulations denying equipment authorization (under part 15 of title 47, Code of Federal Regulations, or any other part of that title) for any scanning receiver that is capable of—
  "(A) receiving transmissions in the frequencies allocated to the domestic cellular radio telecommunications service,
  "(B) readily being altered by the user to receive trans- missions in such frequencies, or
  “(C) being equipped with decoders that convert digital cellular transmissions to analog voice audio.

Which is why wideband SDR's like hackRF are licenced as laboratory equipment. So the regulatory regime is the one for spectrum analyzers and signal generators.

I have always wondered if people in the old days used the FM demodulation option to listen in on phone calls.

That was what was so stupid about the mandatory cellular-blocking legislation. You didn't need test equipment or even a scanner to spy on AMPS phones... you just needed an old TV with a UHF tuner. The upper end of the UHF TV band overlapped the 800 MHz AMPS band nicely, and NTSC TV audio was FM, just like cellular (albeit WBFM rather than NBFM.)

It's probably lucky that no one ever brought a case to court (or testified in Congress) regarding 'civilian' use of a spectrum analyzer for cellular eavesdropping. Otherwise we'd be unable to buy anything even remotely resembling today's SDRs. They'd be driven out of the market like pirate radio transmitters were. The law should be brought up to date, but who wants to open that can of Pandora's worms?

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