If you read the headline and full article, you might be persuaded to believe that the end of stealth is near. https://www.c4isrnet.com/intel-geoint/sensors/2019/09/30/stealthy-no-more-a-german-radar-vendor-says-it-tracked-the-f-35-jet-in-2018-from-a-pony-farm/
Stealth doesn’t work in every frequency range on the spectrum. For example visible light is on a very different portion of the spectrum than X Band radars, which is on a very different portion of the spectrum than FM radio, which is on a very different portion of the spectrum than commercial television.
“Stealth” is designed to be stealthy against the types of radars that can provide “targetable data” to missiles or aircraft. Even infrared homing missiles need to get within line of sight to where they can target a hot engine or tail pipe to be effective, and in a sky with clouds or rain, that needs to be pretty accurate as water absorbs infrared energy that an IR seeker needs to guide a missile.
So am I alarmed that a passive radar on a field right next to where two F-35s took off reported back that they were able to use Polish FM radio as a way to “see” the two F-35s, even though the resolution wasn’t good enough to provide that “targetable data”? No, because any radar operating at that low of a frequency that close to the airport would also see the F-35s as they aren’t designed to be stealthy in that spectrum. This is neat technology, but it is not a game changer based on that article.
The infamous shoot down of the F-117 by antiquated Soviet technology used by Serbian forces had more to do with good intelligence work to pattern out flight paths, repurposing low frequency radar, and positioning the missile at a place where it had a high probability of success. Stealth had allowed the US Air Force to become complacent, and they paid for that complacency. This is why on the combat debut of the F-22 Raptor in the Syria campaign, it was escorted by the Vietnam era EA-6B Prowler Electronic Attack aircraft which can do escort jamming.
Stealth is useful, but it is like all technology in that other technology can be adapted to address it.
Secondly, passive radar suffers the same medium attenuation problems as sonar. Sound in water travels differently through areas of different temperature and salinity, and radio waves travel slightly differently through areas of different pressure and humidity. This is why weather radars can track clouds, or why AM radio reception is much better at night. So if a stealth aircraft were to exploit environmental effects, like flying over a storm during the transition period between day and night, without knowing exactly where to look a passive radar station would have a very hard time filtering through environmental noise to identify a location of a stealth aircraft, or likely even a low observable aircraft like a F/A-18E/F or F-16 flying slick.
This does not mean that passive radar is useless, far from it. Passive radar requires far less power than active radar, and they don’t advertise their position the way active radar does. So with enough passive radar stations it is quite possible to create an array of sensors that could produce “targetable data” for something with a self guiding terminal stage. Of course to defeat that network, all the attackers would need to do is shut down TV and radio stations that are geographically likely to be the frequency emitters that the passive radars are using to sense the environment. That’s as simple as bribing a maintenance guy.
I am interested in where passive radar is going, and hopefully it gets to be pretty cheap technology.