[HN Gopher] Build a Passive Radar with Software-Defined Radio
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Build a Passive Radar with Software-Defined Radio
Author : samizdis
Score : 108 points
Date : 2022-10-25 16:43 UTC (6 hours ago)
(HTM) web link (spectrum.ieee.org)
(TXT) w3m dump (spectrum.ieee.org)
| aimor wrote:
| Where, online, are the SDR communities?
| anfractuosity wrote:
| 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.
| retrac wrote:
| 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.
| anfractuosity wrote:
| 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.
| H8crilA wrote:
| 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.
| CamperBob2 wrote:
| 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.
| IrishJourno wrote:
| This is exactly the SDR I was trying out :)
| CamperBob2 wrote:
| Ah, good point, I commented without having RTFA. It's a
| really cool piece of hardware, no question about that.
| IrishJourno wrote:
| 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.
| H8crilA wrote:
| 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!
| IrishJourno wrote:
| 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.
| nonrandomstring wrote:
| > 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?
| H8crilA wrote:
| 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).
| nonrandomstring wrote:
| > 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.
| the_third_wave wrote:
| 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.
| inasio wrote:
| A scenario where all cellphones have this capability sounds a lot
| like the scene at the end of the Dark Knight.
| Thaxll wrote:
| Is it actually legal to build radars?
| H8crilA wrote:
| 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 :)
| ilc wrote:
| 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.
| thenthenthen wrote:
| This is definitely not true for every legislation.
| IrishJourno wrote:
| 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.
| secondcoming wrote:
| 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.
| coretx wrote:
| Treaty of Rome. Maybe North-Korea did not sign it but that
| is nitpicking.
| hdjjhhvvhga wrote:
| Can you give an example of a legislation where receiving a
| signal by multiple receivers is illegal?
| [deleted]
| [deleted]
| CamperBob2 wrote:
| 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...
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(page generated 2022-10-25 23:00 UTC)