Over the past few days, headlines have popped up all around the world indicating that a German tech firm had managed to track a pair of American F-35s for nearly a hundred miles when they visited Berlin last year for an airshow. The company, called Hensold, used a new form of radar that reads and interprets the way ambient signals deflect off of stealth aircraft, effectively using the “hole” created by the F-35 in the area’s existing radio signals to determine its location.
Much like China’s previous claims that they too managed to detect American F-22s encroaching in their airspace, however, this form of detection isn’t the same as actually being able to engage these stealth platforms. Both the F-35 and F-22 were built to make it extremely difficult to acquire a weapons-grade lock on the aircraft, not necessarily to make them completely invisible to detection. The idea has always been to delay detection using a combination of overlapping technologies and intentionally chosen flight paths so the F-35 or other stealth platform can effectively engage air defenses before they can shoot back. Despite the technological misnomers spouted by officials ranging all the way up to the American president, the F-35 has never been invisible, it’s always just been sneaky.
Because radar comes in a number of different frequency bands, stealth fighters rely on trying to limit their detectability on higher-frequency bands specifically, because they’re the best for a weapons lock. Feasibly speaking then, it would still be possible to detect and even track many stealth aircraft using radar that operates at lower frequencies — the trade-off is that these lower frequency radar bands tend not to provide a sufficient track to produce weapons lock from air-to-air or surface-to-air missiles. Germany’s new radar, dubbed TwInvis, doesn’t work in this same way, but suffers from the same lack of target-grade data.
TwInvis instead detects the web of signals in nearby airspace created by cell phone towers, radio broadcasts and the like, and uses that information to identify gaps in the air that are created by radar-wicking aircraft like the F-35. The result could potentially be used as an early warning system to notify nearby air defenses of encroaching stealth aircraft, but there’s still little those air defenses could do. Feasibly speaking, they could use this or low-band radar to determine the general location of an aircraft, and then deploy infrared seeking missiles to hunt for the fighter’s engines once airborne, but it would still amount to little more than spotting the F-35 in the sky and trying to eyeball the launch.
It’s also important to note that Hensold had a number of advantages in this test that they wouldn’t in a combat scenario. Not only were they aware that the F-35s were coming, the fighters were actually running their ADS-B transponders during the flight. In other words, school kids with access to the internet could have detected the encroaching F-35s on open source flight tracking software, making it especially easy for the Hensold team to correlate the data their system provided with where they knew for a fact the fighters were.
It’s also important to note that the TwInvis system works thanks to a high density of radio traffic surrounding a city like Berlin. In many combat zones and over the open water where F-35s or similar stealth aircraft like China’s J-20 would be making their approach, there’s a notable reduction in radio traffic — likely not even enough to spot a large stealth platform, let alone a small fighter.
This isn’t to say that there are to tactical uses for the TwInvis radar system — it could potentially be used in conjunction with other technologies to identify and target incoming stealth platforms, but it’s still quite early to suggest that stealth is already slated for retirement. Spotting an F-35 already wasn’t all that tough to begin with, it’s shooting one down that’s difficult.