Let’s Talk About The Story Of A German Radar Vendor That Claims To Have Tracked Two F-35s With Passive Radar

An F-35A Lightning II assigned to the 4th Expeditionary Fighter Squadron breaks away after receiving fuel from a KC-10 Extender assigned to the 908th Expeditionary Air Refueling Squadron during Exercise Agile Lightning Aug. 6, 2019. The aircraft is not in "stealth mode" as it carries Radar Reflectors and external pylons. (U.S. Air Force photo by Staff Sgt. Chris Thornbury)

A German radar vendor tracked the F-35 jets in 2018 from a pony farm. But it turns out the aircraft were flying with radar reflectors and ADS-B transponder (that could have made the task easier).

An article published on Sept. 29, 2019, by C4ISRNET tells the interesting story of German radar-maker Hensoldt, that claims to have tracked two F-35A jets attending Berlin ILA airshow back in 2018.

Titled “Stealthy no more? A German radar vendor says it tracked the F-35 jet in 2018 — from a pony farm” and written by Sebastian Sprenger, the article explains how a German firm used passive radar technology to track two 5th generation stealth aircraft.

“It’s a scoop” someone says. Indeed, it sounds cool, so let’s try to understand how the German radar-maker tracked the LO (Low Observable) aircraft.

“[…] 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.”

“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.” (highlight mine).

Indeed, passive radars are often mentioned as a preeminent anti-stealth technology. As opposed to traditional radars, that use a single transmitter and receiver and study the waves that reflect off flying objects, passive radars use reflections from non-cooperative souces of illumination (such as commercial broadcast and communications signals). A passive radar system is called bistatic because it relies on signals trasmitted from a different location: by calculating the delay between the signal received directly from the transmitter and the one received after being reflected off a flying object, a passive radar system is able to determine the distance of the target. However, since only the time delay can be calculated from this technique with one transmitter and one receiver, the single conclusion that can be drawn is that the detected object is located somewhere on an ellipse whose foci are the transmitter and the receiver.

The concept of Passive Radar System. (Image credit: Nutaq).

By measuring bistatic Doppler shift of the echo and its direction of arrival, a passive radar system can calculate speed and heading of the target. Accuracy can be improved by using multiple transmitters and receivers, and their geometry.

Ok, with a bit of background theory, now let’s continue reading the article published by C4ISRNET:

“There are limitations to the [passive radar] technology. For one, it depends on the existence of radio signals, which may not be a given in remote areas of the globe. In addition, the technology is not yet accurate enough to guide missiles, though it could be used to send infrared-homing weapons close to a target.”

So, the article admits some limitations. Fair enough. Then it continues:

“Hensoldt previously said its passive-radar detection works regardless of whether the targeted aircraft has radar reflectors (so-called Luneburg lenses) installed. Those features — little knobs on the roots of the F-35 wings — can be seen in photos released by the U.S. Defense Department on the occasion of the journey to Berlin”.

RCS (Radar Cross Section) Enhancers, Radar Reflectors or Luneburg lenses are passive radar augmentation devices used to alter the aircraft radar cross section and make the LO jets visible to radars.

Here’s what I wrote about radar reflectors here at The Aviationist last year:

Stealth aircraft, such as the F-22 Raptor or the F-35 Lightning II 5th generation jets are equipped with Luneburg (or Luneberg) lenses: radar reflectors used to make the LO (Low Observable) aircraft (consciously) visible to radars. These devices are installed on the aircraft on the ground are used whenever the aircraft don’t need to evade the radars: during ferry flights when the aircraft use also the transponder in a cooperative way with the ATC (Air Traffic Control) agencies; during training or operative missions that do not require stealthiness; or, more importantly, when the aircraft operate close to the enemy whose ground or flying radars, intelligence gathering sensors.

This is what we explained explaining how the Israeli the heavy presence of Russian radars and ELINT platforms in Syria cause some concern to the Israeli F-35 Adir recently declared IOC:

[…] the Russians are currently able to identify takeoffs from Israeli bases in real-time and might use collected data to “characterize” the F-35’s signature at specific wavelengths as reportedly done with the U.S. F-22s.

In fact, tactical fighter-sized stealth aircraft are built to defeat radar operating at specific frequencies; usually high-frequency bands as C, X, Ku and S band where the radar accuracy is higher (indeed, the higher the frequency, the better is the accuracy of the radar system).

However, once the frequency wavelength exceeds a certain threshold and causes a resonant effect, LO aircraft become increasingly detectable. For instance, ATC radars, that operate at lower-frequency bands are theoretically able to detect a tactical fighter-sized stealth plane whose shape features parts that can cause resonance. Radars that operate at bands below 300 MHz (lower UHF, VHF and HF radars), such as the so-called Over The Horizon (OTH) radars, are believed to be particularly dangerous for stealth planes: although they are not much accurate (because lower frequency implies very large antenna and lower angle accuracy and angle resolution) they can spot stealth planes and be used to guide fighters equipped with IRST towards the direction the LO planes might be.

F-35s deployed abroad usually feature their typical four radar reflectors: to exaggerate their real RCS (Radar Cross Section) and negate the enemy the ability to collect any detail about their LO “signature”. As happened during the short mission to Estonia and then Bulgaria, carried out by the USAF F-35As involved in the type’s first overseas training deployment to Europe or when, on Aug. 30, 2017, four U.S. Marine Corps F-35B Lightning II joined two USAF B-1B Lancers for the JSF’s first show of force against North Korea: the F-35Bs flew with the radar reflectors, a sign they didn’t want their actual radar signature to be exposed to any intelligence gathering sensor in the area

The two radar reflectors installed on the right side of the F-35. The other two are on the other side.

Since they almost always fly with the radar reflectors, photographs of the aircraft without the four notches (two on the upper side and two on the lower side of the fuselage) are particularly interesting: for instance, some shots taken on Jan. 24, 2018 and just released by the U.S. Air Force show F-35As deployed to Kadena AB, Japan, in October as a part of the U.S. Pacific Command’s Theater Security Package program, preparing to launch without their Luneberg reflectors.

The F-35s tracked by the German passive radar had these devicess, but the C4ISRNET article goes on to say that “Hensoldt argues that passive-radar detection works in a different spectrum, making the presence (or absence) of reflectors irrelevant. In layman’s terms, passive radar tracks the entire physical shape of planes, versus being triggered by smaller, angular features on the body of a jet. […] Officials from the industry teams involved in the program increasingly converged around the idea that stealth as we know it had lost its shine — this following rumors circling the German defense scene about how Hensoldt had apparently managed to light up the American aircraft on the radar screen.”

Summing up: the German vendor tracked two F-35As using a passive radar system. However, the two aircraft were using RCS enhancers and, above all, they were cooperatively trasmitting their position by means of transponder.

This raises a basic question: can we consider the one done by Hensoldt a definite test?

If the ability to track a stealth jet (in “stealth mode” with no rcs enhancer and no transponder) is based on the above scenario only, then the answer is probably NO.

You don’t even need passive radars to track stealth jets if these use Mode-S/ADS-B transponders. F-35s can be regularly tracked online using flight tracking portals and apps or DIY kits as explained in detail in this previous post about an Israeli F-35 going “live” on the popular Flightradar24.com website.

To consider the test valid, it should have been conducted in real operational conditions: with the aircraft not carrying radar reflectors (that, generally speaking, do exist also as bistatic units) and, above all, not transmitting their position; in other words as they would operate in a contested airspace.

Moreover, let’s not forget that passive-radar detection has been considered a counter-stealth tech for decades now, but it relies on multiple radio signals that could be unavailable inside a contested airspace: the emitters could be knocked out by EW (Electronic Warfare) and kinetic air strikes well before the stealth jets engage the target area, making LO still important to carry out the operations in enemy airspace.

Therefore, it’s probably safe to say that while we can’t rule out the possibility that the Hensoldt passive radar system would be able to track F-35s stripped out of radar reflectors and not transmitting their position via ADS-B (that helps a lot reducing the need for additional trasmitters and receivers), we can’t even say the German radar is able to detect stealth jets just because it “saw” two Lightning aircraft that were doing nothing to remain undetected.

Whatever, it’s once again worth saying that Low Observability is not a cloak of invisibility and can’t be considered a silver bullet, especially if not used in a synergistic way with Electronic Attack tactics, jamming support, cyber attacks, Sensor Fusion, etc.

About David Cenciotti
David Cenciotti is a journalist based in Rome, Italy. He is the Founder and Editor of “The Aviationist”, one of the world’s most famous and read military aviation blogs. Since 1996, he has written for major worldwide magazines, including Air Forces Monthly, Combat Aircraft, and many others, covering aviation, defense, war, industry, intelligence, crime and cyberwar. He has reported from the U.S., Europe, Australia and Syria, and flown several combat planes with different air forces. He is a former 2nd Lt. of the Italian Air Force, a private pilot and a graduate in Computer Engineering. He has written five books and contributed to many more ones.