Is the F-35 fighter jet truly stealthy? A German firm claims they tracked the aircraft following the Berlin airshow in 2018, using a simple passive radar system. Handout.

Oh, the embarrassment …

 In the illustrious history of the F-35 fighter jet — an aircraft likely to cost the US$1 trillion over its operational lifetime due to an unending series of technical and design problems — a pony farm outside Berlin is allegedly the site where a company claims the stealth plane’s expensive cover was blown.

The story that follows is a snapshot in the cat-and-mouse game between combat aircraft — designed to be undetectable by radar — and sensor makers seeking to undo that advantage, C4ISR.net’s Sebastian Sprenger reported.

In the case of the F-35, the promise of invisibility to radar is so pronounced that it has colored much of the jet’s employment doctrine, lending an air of invincibility to the weapon: The enemy never saw it coming. Or so we thought. Keep in mind, a stealth fighter must not have any fuel tanks or weapons outside its coated fuselage to remain stealthy — and the aircraft does sport weapons bays.

But technology leaps only last so long. Russia and China are known to be working on technology aimed at nixing whatever leg up NATO countries have tried to build for themselves.

Now, German radar-maker Hensoldt claims to have tracked two F-35s for 150 kilometers following the 2018 Berlin Air Show in Germany in late April of that year. The company’s passive radar system, named TwInvis, is but one of an emerging generation of sensors and processors so sensitive and powerful that it promises to find previously undetectable activities in a given airspace.

What happened in Berlin was the rare chance to subject the aircraft — stealthy design features, special coating and all — to a real-life trial to see if the promise of low observability still holds true.

Stories about the F-35-vs.-TwInvis matchup had been swirling in the media since Hensoldt set up shop on the tarmac at Berlin’s Schönefeld Airport, its sensor calibrated to track all flying demonstrations by the various aircraft on the flight line. Media reports had billed the system, which comes packed into a van or SUV and boasts a collapsible antenna, as a potential game changer.

At the same time, F-35 manufacturer Lockheed Martin was still in the race to replace the German Tornado fleet, a strategically important opportunity to sell F-35s to a key European Union member state. The company set up a sizeable chalet at the air show, bringing brochures and hats depicting the aircraft together with a German flag.

The most convincing pieces of marketing for Hensoldt were meant to be two F-35s flown in from Luke Air Force Base, Arizona. The trans-Atlantic journey marked the jets’ longest nonstop flight, at 11-plus hours, officials said at the time.

But Lockheed and the US Air Force did not fly the jets during the show so that its engineers — and anyone walking by the company’s booth, for that matter — could see if the aircraft would produce a radar track on a big screen like the other aircraft.

Reporters never got a straight answer on why the F-35s stayed on the ground.

Regardless, with no flight by the F-35, companies could not try out their technologies on perhaps the most illustrious of test cases. Passive radar equipment computes an aerial picture by reading how civilian communications signals bounce off airborne objects. The technique works with any type of signal present in airspace, including radio or television broadcasts as well as emissions from mobile phone stations.

The technology can be effective against stealthy aircraft designs, which are meant to break and absorb signals from traditional radar emitters so that nothing reflects back to ground-station sensors, effectively leaving defensive-radar operators in the dark.

Because there are no emitters, passive radar is covert, meaning pilots entering a monitored area are unaware they are being tracked.

There are limitations to the technology. For one, it depends on the existence of radio signals, which may not be a given in remote areas of the globe — a big disadvantage.

Hensoldt said various radio station broadcasts in the area, especially a bunch of strong Polish FM emitters broadcasting deep into Germany, improved TwInvis calibration during the Berlin show. The border is about 70 kilometers away from Schönefeld Airport.

During a system demonstration by Hensoldt, company engineers convened around a large TwInvis screen showing the track of a Eurofighter performing a thundering aerial show.

But the prized target of opportunity, the two F-35s, remained sitting on the tarmac. Until … the event ended.

Hensoldt kept a close eye on any movement of the heavily guarded F-35s on the airfield. As exhibitors began to clear out, it looked like the chance of catching the planes during their inevitable departure back home would be lost.

But in Hensoldt’s telling, someone had the idea of setting up TwInvis outside the airport, which ended up being at a nearby horse farm.

Camped out amid equines, engineers got word from the Schönefeld tower about when the F-35s were slated to take off. Once the planes were airborne, the company says it started tracking them and collecting data, using signals from the planes’ ADS-B transponders to correlate the passive sensor readings.

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