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Comment by H8crilA | Hacker News Reader

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.

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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.

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 :)

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

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.

> 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).

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.

> 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.

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