[HN Gopher] Homemade polarimetric synthetic aperture radar drone
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Homemade polarimetric synthetic aperture radar drone
Author : picture
Score : 561 points
Date : 2025-02-17 01:22 UTC (21 hours ago)
(HTM) web link (hforsten.com)
(TXT) w3m dump (hforsten.com)
| throwup238 wrote:
| This is why I read HackerNews.
|
| Every month there's a new article that improves my suburban
| missile defense system* and helps guarantee that my neighbors
| will never let their dogs poop on my lawn ever again.
|
| Terrain mapping my neighborhood so that my rockets can navigate
| to the right offender has always been a challenge. Now I can just
| use drones!
|
| * It's a defense system with missiles, not a defense system
| _against_ missiles. That would be silly.
| nullc wrote:
| I find that rogue poopers are most effectively addressed with
| loitering munitions. It's well known that immediate feedback is
| critical in animal conditioning.
| Nevermark wrote:
| How about a small laser with enough power for a 100ms burst
| in the non-visual spectrum that vaporizes 1mm radius, 0.5mm
| depth of protein, through 1-5cm depth of hair?
|
| No sense sacrificing any hardware. Zero resource attrition
| and imperceptible evidence of action is the name of the game.
|
| Match that with a spot of sound out of human hearing range
| and you can clear the enemy of its own accord far beyond your
| legal and overt action spheres.
|
| If a close neighbor's unit stays clear but expresses its
| dismay regarding warning sounds too loudly, well, this works
| for that too.
|
| We are talking about nuisance bears and moose, right?
| nullc wrote:
| I lost a little chunk of flesh to a pulsed laser, didn't
| even know I was injured immediately-- the wound didn't hurt
| until later.
|
| Clearly the solution is laser ignition of napalm-- don't
| want to accidentally catch the drone on fire, so the laser
| can be timed to fire once the stream is clear of it.
| throwup238 wrote:
| _> laser ignition of napalm_
|
| Now we're talking! Unfortunately I couldn't figure out
| how to deploy something as viscous as napalm through my
| sprinkler system. I've been reduced to dropping it from a
| helicopter, the old school way.
| nullc wrote:
| Four words: Diesel engine fuel injection.
| Grosvenor wrote:
| These might help.
|
| Tactical and Strategic Missile Guidance, Seventh Edition
| https://www.amazon.com/Tactical-Strategic-Missile-Guidance-S...
|
| Tactical missile warheads https://www.amazon.com/Tactical-
| Warheads-Progress-Astronauti...
|
| Fundamentals of Astrodynamics
| https://www.amazon.com/Fundamentals-Astrodynamics-Second-Dov...
| daemonologist wrote:
| Woah, prices have gone up for that Missile Guidance book.
| Used copies were going for like $15 a couple of years ago.
| secondcoming wrote:
| I own the Tactical and Strategic Missile Guidance book. I
| only bought it because I have a passing interest in this
| stuff and really couldn't believe this sort of book is
| publically available.
| philipkglass wrote:
| Every terrifying technology that people worry can be
| propagated by AI or terrorist manuals is already sitting on
| a university library shelf. Most of it already available
| via Sci-Hub or Library Genesis. I have documents on nuclear
| weapons that would have been top secret 50 years ago.
| iugtmkbdfil834 wrote:
| I occassionally mention to people that we can effectively
| have a supercomputer from 90s in our home if we so
| choose, but it that is lost on them since they use it for
| browsing at best. Tech and knowledge is only as scary as
| the individual wielding it. Human is the problem.
| flyinghamster wrote:
| I actually wanted to reply to the downstream comment, but
| it didn't give me a Reply link for some reason.
|
| In any case, I've absolutely noticed how even a Raspberry
| Pi Zero, taken back 40 years, would be a supercomputer
| beyond anything that money could buy back then.
|
| A GPU that we'd consider obsolescent today would be truly
| insane in 1985.
| gessha wrote:
| There's that and there's also anti-drone jamming. What a world
| we live in.
| samplatt wrote:
| A lot of anti-drone jamming is severing the wireless
| communication between a drone and its operator.
|
| In most of the write-up, these drones were on pre-planned
| autonomous routes, and would thus not be affected. Unless you
| also had anti-radar installations :-)
|
| Or hell, some kinda highly-illegal and mostly-unfeasible
| microwave gun or multi-watt CO2 laser that you could point at
| a drone and bring it down.
| temp0826 wrote:
| Maybe old school (but sort of James Bond-esque?) but I
| really dig these shotgun shells that shoot nets
|
| https://www.lesslethal.com/products/12-gauge/als12skymi-5-d
| e...
| pjc50 wrote:
| Ukraine and Russia are apparently building drones with fiber-
| optic tethers to defeat this. Like the old wire-guided
| missiles.
|
| https://www.businessinsider.com/ukraines-drone-makers-
| produc...
| iugtmkbdfil834 wrote:
| A new cat and mouse game.
| mmusson wrote:
| The missile knows where it is at all times. It knows this
| because it knows where it isn't...
| transistor-man wrote:
| Stellar work! This is such a wonderful writeup!
| geokon wrote:
| This is actually the first time I've seen the SAR algorithm
| explained. Every SAR project I've seen glosses over this part.
|
| Wonder if anyone else has seen more resources on this? Going to
| try to digest the linked paper..
| https://topex.ucsd.edu/rs/sar_summary.pdf
|
| And in the end it seems you can bypass a lot of the position
| problems with autofocusing! First time seeing this part too :))
| celegans25 wrote:
| Henrik has a blog post about a SAR radar he made from a couple
| years ago that goes into a bit more detail about how it works
| https://hforsten.com/synthetic-aperture-radar-imaging.html
| mNovak wrote:
| This site [1] has pretty approachable content generally, though
| it looks like the SAR page is pretty short.
|
| I find it easiest to understand in the context of a
| conventional phased array, but that might not help if you don't
| already know how those work.
|
| [1] https://www.radartutorial.eu/20.airborne/ab07.en.html
| geokon wrote:
| Yeah this kind of kindergarten explanation I've seen.. This
| is making a bunch of simplification that aren't helpful
|
| With a phased array you're beam forming and sweeping an area
| of space. Your signal returns are from the beam or side
| lobes. You can passively beam form on Rx as well.
|
| But with SAR you're not beam forming. You're illuminating
| everything - the whole ground below you. And you get a return
| from everywhere all at once. Two equidistant reflectors will
| return signals simulatenously. If your flight path is between
| these two points, and the distance is always equal, how can
| you differentiate them?
|
| You're digitally beam forming on the Rx somehow but I think
| there is more to it
| namibj wrote:
| You can't differentiate them in that case. You'd have to
| fly orthogonal across the surface for~ maximum effect.
| derstander wrote:
| > But with SAR you're not beam forming. You're illuminating
| everything - the whole ground below you. And you get a
| return from everywhere all at once. Two equidistant
| reflectors will return signals simulatenously. If your
| flight path is between these two points, and the distance
| is always equal, how can you differentiate them?
|
| There are a couple conceptual ways to think about SAR. One
| is, in fact, as beamforming. Each position of the radar
| along the synthetic aperture is one element in an enormous
| array that's the length of the synthetic aperture itself:
| that's your receive array.
|
| Regarding your question about scatterers that are
| equidistant along the entire synthetic aperture length:
| typically, SAR systems don't use isotropic antennas. And
| they're generally side-looking. So you would see the
| scatterer to one side of the radar, but not the equidistant
| scatterer on the other side.
|
| If you had an isotropic antenna that saw to each side of
| the synthetic aperture, then the resulting image would be a
| coherent combination of both sides. Relevant search terms
| would be iso-range and iso-Doppler lines. Scatterers along
| the same iso-range and iso-Doppler lines over the length of
| the synthetic aperture are not distinguishable.
|
| As to your question earlier in the chain, my preferred SAR
| book is Carrara et al. Spotlight Synthetic Aperture Radar:
| Signal Processing Algorithms. Given the title, it is of
| course geared toward spotlight (where you steer the beam to
| a particular point) rather than strip map or swath (where
| your beam is pointed at a fixed angle and dragged as you
| move along). It has decent coverage of the more
| computationally efficient Fourier-based image formation
| algorithms but does not really treat algorithms like the
| back projection that Henrik uses (I also think back
| projection is easier to grasp conceptually, particularly
| for those without a lot of background in Fourier
| transforms). But my book preference might just be because
| that's what I first learned with.
| mNovak wrote:
| >> Your signal returns are from the beam or side lobes
|
| You're skipping a step -- where does that beam come from?
| For simplicity lets think about a scene illuminated
| uniformly (i.e. from a single element) so that we don't get
| hung up on the transmit beam. I think we agree you could
| still sweep a receiving phased array beam across that
| scene. Lets further assume it's digital beamforming, so
| you're storing a copy of the signal incident _at every
| element of the array_. Not a 'beam' yet, just a bunch of
| individual signals.
|
| >> you get a return from everywhere all at once
|
| Yes! Think about each of those elements of the phased array
| -- they're also receiving signals from everywhere all at
| once.
|
| It only becomes localized into a beam when you combine all
| the elements with specific phase weights. That process of
| combining element returns to form the beam is
| mathematically identical to what you do in SAR as well --
| combine all your individual 'element' (individual snapshot
| in space) responses with some phase weights to get the
| return in one direction. Repeat the math in all directions
| to form one dimension of the image (second dimension is the
| radar time-of-flight bit, which is unrelated to the
| beamforming).
|
| Maybe not you specifically, but I think people don't
| understand the 'synthetic aperture' part. Specifically,
| that you can ignore the time between snapshots (because the
| transmitter and receiver are synchronized) and act like all
| the snapshots the platform took across the line of flight
| happened simultaneously. What you're left with is the
| element responses to a big phased array, and you can
| 'beamform' using those responses.
| stevesimmons wrote:
| Thrilled to see this because I did my PhD in radar imaging,
| specifically autofocusing ISAR images.
|
| My thesis describes how the math of this all works, in chapters
| 3 and 4 of the PDF here: https://github.com/stevesimmons/phd-
| thesis-radar-imaging
|
| Here:
|
| SAR = Synthetic Aperture Radar, where the radar flies along a
| straight line and the apparent rotation of the ground produces
| a Doppler shift that can be used to get high cross-range
| resolution.
|
| ISAR = Inverse Synthetic Aperture Radar, where the radar is
| still (e.g. on the ground) and the target (e.g. a plane) is
| flying, and their relative motion produces a rotation of the
| target, which equally produces a Doppler shift that can be used
| to get high cross-range resolution.
| farhanhubble wrote:
| As a DIYer myself I am awed by how much Henrik has managed to
| flesh out in terms of complexity: signal processing, hardware,
| GPU acceleration, and algorithmic optimizations. I wish the HN
| community had some award for such incredible feats.
| pjc50 wrote:
| It's a huge amount of work. It looks like at least a PhD's
| worth. Huge achievement for a hobbyist.
| DoingIsLearning wrote:
| This is absolutely amazing work but I would not call him a
| hobbyist.
|
| Again not diminishing the work but adjusting the
| expectations, this output is not from a curious guy in a
| shed. He is a trained Electrical Engineer working as a Senior
| Consultant/Scientist in RF design for a Finnish Engineering
| Consultancy.
|
| This guy clearly has a breadth and depth of knowledge from
| his professional career which he brings with him into his
| out-of-office projects.
| Animats wrote:
| Yes, clearly this guy is an experienced RF designer.
|
| What's impressive is that he did the RF part, the SAR data
| reduction part, and the drone control part. Those are
| different skill sets.
| lokimedes wrote:
| He is researching this stuff for s living as well.
| taylorportman wrote:
| If you need a 100 meter antenna - could one be approximated using
| multiple drones in a coordinated 100 meter parabolic trajectory?
| mNovak wrote:
| Yes, if you can coordinate and hold their position to ~1/10th
| of the wavelength (gets challenging above a few GHz), and
| synchronize their transmitters/receivers with ps accuracy
| (feasible if there's no GPS interference). And somehow you're
| exchanging the raw signal components amongst all the platforms.
|
| The parabolic distribution is not at all needed in this case,
| you would just adjust the phase at each unit based on the
| distribution you have.
|
| It is a thing that's considered, mostly for very low
| frequencies (1-100MHz) where all the above challenges are
| vastly simplified, and also large antennas (potentially km) are
| needed for any kind of directionality.
| namibj wrote:
| No need to hold their position that accurately, just need to
| know, ideally in real-time.
| tecleandor wrote:
| At 6Ghz, 5cm wavelength, 1/10 would be 5mm. I think that
| might be a bit under regular RTK accuracy.
|
| But at lower frequencies, maybe. Or you can do several
| passes with the same drone and process later :D
| nine_k wrote:
| If the drones use e.g. laser rangefinders to measure the
| distance to 2-3 known reflectors, and maybe to each
| other, with sub-mm accuracy, that may be enough.
|
| Deploying an antenna that's effectively 50 or 100 m wide
| by lifting 10-20 drones, after some simple ground
| preparations, could be invaluable in many scenarios,
| especially for the military, of course.
| CamperBob2 wrote:
| The same technique as he uses here for synchronizing the
| receivers would work. Common reception and triangulation
| of a strong local beacon would enable positioning at much
| better than GNSS precision, both because of the SNR
| advantage itself and because you'd have more options for
| dealing with carrier phase ambiguity.
| bloopernova wrote:
| Does UWB provide enough accuracy to be useful or would you
| need something else?
| nodra wrote:
| An upvote doesn't do this justice. Just an amazing write up.
| Thank you.
| fisherjeff wrote:
| Ha, wow I've seen plenty of SAR images from MUCH more expensive
| systems that don't look nearly as good. Incredible stuff here.
| slicktux wrote:
| What an amazing project! Great synthesis of skills to make such a
| cool project! I'm in awe!
| colonCapitalDee wrote:
| > Recently small FPV drone prices have dropped a lot. Small 5 and
| 7 inch propeller quadcopters can be bought for about 100 EUR from
| China (not including battery and RC controller). Despite their
| small size they are able to lift about 1 kg or even heavier
| payload which is plenty for a small radar.
|
| The price dropped because China is selling these things by the
| truckload to both sides in Ukraine. China is benefitting from the
| economics of scale, Russia and Ukraine are each burning through
| on the order of 100,000 of these things each month and most
| components are sourced from China
| bloopernova wrote:
| I wonder what the % breakdown is for domestic vs imported
| drones for Ukraine?
|
| I read recently that Ukraine was producing a lot of drones and
| ramping up production too, as well as using fibre-optic
| controlled drones to overcome electronic warfare/jamming.
| morphle wrote:
| You won't get any reliable numbers during wartime. I heard
| Ukrainian government recently mention they ordered a million.
|
| Hundreds of small companies (around 10 employees each) in
| Ukraine are making and assembling drone's right now
| (according to journalists who filmed them in Ukraine last
| month). I counted at least two dozen companies in the
| Netherlands, probably a few hundred all over Europe. Besides
| the frame that holds the 4 or six rotors together all other
| parts are off the shelf, mostly ordered from Shenzen. You can
| CNC mill a carbon fiber or aluminum frame from a single plate
| on an x-y flatbed cnc.
|
| I researched all this last night because I need $20K-$200k
| investment right now to start my own startup making
| unjammable autonomous kamikaze drones with 20K payload on top
| of 20km optical fiber spools unreeling. I think it is
| possible to make them lighter without fibers if you put a few
| thousand microprocessors in with a wafer scale integration
| for $500.
|
| Of course you could just make cheap drones that kill every
| warm body in sight[4] indiscriminately. To disable a tank you
| need these fiber tethered drones with bigger
| payloads[3][5][6]. You cab see two fiber strands still in the
| air in a few frames of the video.
|
| What you need is a Paul MacCready style development cycle for
| drones in the field. You need to be able to make 10 crashes
| per hour, fix the drone by replacing with off-the shelf
| components and try again. The programming should be live-
| coding like Squeak Smalltalk, no recompiling, just fix and
| run while you are using the software to edit itself [1] as
| Dan did in 1976[2] on this 5.88 mhz computer. We still code
| like this today but most programmers and managers ignore this
| in favor of terminals and simulated punch cards. Remember
| that it took humans 200.000 years to invent written language
| or agriculture, we just are not that smart.
|
| [1] https://youtu.be/NqKyHEJe9_w?t=433
|
| [2] https://www.youtube.com/watch?v=uknEhXyZgsg
|
| [3] https://www.youtube.com/watch?v=m3uAFBzzbTc
|
| [4] https://www.youtube.com/watch?v=TlO2gcs1YvM
|
| [5] https://www.youtube.com/watch?v=dx5_Hzbqv9s
|
| [6] https://www.youtube.com/watch?v=id48aVBj9Ak
| spyder wrote:
| hm... and why are the drones carrying the whole spool of
| the fiber instead of leaving the spool on the ground and
| unraveling it from there without the extra weight on the
| drone?
| tastyfreeze wrote:
| Its a difference of dragging a line and just laying it
| out as you go. A lot less resistance to just pay out the
| spool so there is no tension in the wire.
| sllabres wrote:
| What a impressive project and great results! Thanks for sharing
| here on HN.
| anilakar wrote:
| > A single-channel radar can only measure the distance to a
| target and is unable to detect the angle of the target.
|
| Probably in that specific use case yes, but we have had parabolic
| dishes and slotted waveguide apertures for a long time now...
| willtemperley wrote:
| This is incredible. Having had the misfortune of trying to get
| Sentinel 1 toolbox working, I'm now convinced it can be rewritten
| from scratch.
| tomaskafka wrote:
| So, this will be seeing Russians even during the night and behind
| the trees?
|
| (And soon, the Chinese drones on attack through European cities
| will be able to hunt civilians 24/7?)
| onemoresoop wrote:
| Warfare got its 21st century upgrade for sure. This is scary
| but hope we'll see some countermeasures too
| cruftbox wrote:
| Incredible engineering. Thanks for sharing it.
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