In the first week of October 2025, Munich Airport shut down twice in roughly twenty-four hours. The first closure stranded around 3,000 passengers, the second about 6,500. Police mounted a large operation on both nights and found nobody. A week earlier Copenhagen had suspended operations for some four hours, with at least 109 cancellations and 51 diversions, after what Danish police described as several large drones. The officer leading the investigation, Jens Jespersen, would say only that it appeared to be a capable actor.

That pattern has now repeated across a dozen European countries. Germany’s air navigation service DFS logged 192 drone-related disturbances in 2025, up from 141 the year before. Very few of the incidents have produced a suspect, and in several cases the police could not confirm afterwards that a drone had been there at all. Europe is closing its airports over objects it frequently cannot prove existed.

Underneath the politics sits a sensor problem, and one of the stranger answers to it is a radar that does not transmit. It has no transmitter at all. It listens to the radio and television signals already soaking the sky, waits for them to bounce off something, and works out where that something is. The idea is older than the Cold War, the hardware is commercially available, and the gap between what the brochures claim and what the peer-reviewed literature has measured is about two orders of magnitude.

A radar with no transmitter

A conventional radar is a transmitter and a receiver bolted together. It shouts, it listens for the echo, and it knows exactly when it shouted, so the round-trip time gives range directly.

A passive radar throws away the first half of that. It picks a transmitter it does not own and cannot control — an FM broadcast mast, a DAB block, a television multiplex, a mobile base station — and it builds everything else around that borrowed signal. One receiver channel, the reference channel, points at the transmitter and samples exactly what is going out. A second set of channels, the surveillance channels, look at the sky. Cross-correlate the two and any delayed, Doppler-shifted copy of the reference waveform is an echo off something airborne.

What falls out of that correlation is not range in the familiar sense. It is bistatic range: the extra distance the signal travelled by going via the target rather than straight from transmitter to receiver. A single bistatic range measurement puts the target somewhere on an ellipsoid with the transmitter at one focus and the receiver at the other. Add the bistatic Doppler shift and a bearing from the antenna array and you have a poor fix. Add a second transmitter, and a third, and the target is where the ellipsoids intersect. Fraunhofer FKIE put it flatly in a NATO paper: fusing multiple bistatic pairs is what makes accurate Cartesian localisation possible.

The vocabulary around this is unusually precise, and worth getting right. Hugh Griffiths, who holds the Thales/Royal Academy of Engineering Chair of RF Sensors at University College London, separates a hitchhiker, which borrows another radar’s transmitter, from true passive bistatic radar, which borrows a transmitter that was never meant for radar at all. He also notes a pleasing geometric consequence: in a bistatic system the contours of constant detection range are not circles but Ovals of Cassini, defined by the product of the transmitter-to-target and target-to-receiver distances.

Why build a radar that cannot see very far

The answer has nothing to do with range and everything to do with survival. Anything that radiates can be found by what it radiates, and then killed.

Ukraine has made that argument in hardware. In October 2026 Defence Blog reported a Ukrainian fixed-wing strike drone, the SHOT-L1, built specifically to hunt Russian electronic warfare systems by their emissions: it scans for a signal, takes a bearing, and steers itself onto the source, going for the strongest emitter if several are active. The reported numbers are 80 km range, a 4 kg payload and a unit price around 9,300 US dollars, against Russian jammers such as the R-330Zh Zhitel that Ukrainian estimates put in the tens of millions. That report is single-sourced and the manufacturer is not named, so treat the specifics with care — but the principle is not in dispute. A transmitter is a beacon.

A receiver is not. That is the whole case for passive radar, and it is a physics argument rather than a marketing one.

There is a second, subtler advantage. In a bistatic geometry the target can sit near the line between transmitter and receiver, where the scattering is dominated by forward scatter rather than by backscatter — and forward scatter behaves very differently.

Read that carefully, because it is the sentence that gets misquoted most often. If the forward-scattered field is set by a target-shaped hole, then it is set by the target’s silhouette and the wavelength. Malaysian researchers studying forward-scatter radar put the consequence in print: the forward-scattering radar cross-section mainly depends on the target’s physical cross section and the wavelength, and is independent of surface shape and of any radar-absorbing coating. Shaping and radar-absorbent material are the two pillars of stealth, and in the forward-scatter region neither of them does anything.

Which sounds like the end of stealth, until you read Griffiths’ next slide.

Near the baseline, bistatic range, bearing and Doppler all tend towards zero. You get a tripwire, not a track. Something crossed the fence; the fence cannot tell you what, where exactly, or where it went. That is a genuinely useful thing to own, and it is not an air-defence sensor.

The band matters more than the stealth argument

Here is where most writing about passive radar goes wrong, including some written by people who should know better. The stealth argument and the small-drone argument get welded together, and they do not belong together.

Long wavelengths help against stealth aircraft because a combat aircraft has metre-scale features that resonate at VHF, and because its shaping was optimised against centimetric radar. A 30-centimetre quadcopter is a different problem entirely. At FM wavelengths of around three metres it is far too small to resonate, and its echo collapses accordingly. The Warsaw University of Technology team that measured this for NATO were explicit about where each illuminator is useful: medium size drones (>30 cm) can be detected by DVB-T signal, and microdrones (>6 cm) can be detected by WIFI and upper LTE signals.

In other words the most powerful illuminator in the sky, FM, is the worst one for the targets everybody is currently worried about. The headline passive-radar results of over 100 km are FM results against subsonic aeronave. For a DJI-class quadcopter the useful bands are the shorter ones: DVB-T in the UHF television band, DAB, GSM and LTE. Those transmitters radiate far less power, and the range shrinks to match.

What has actually been measured

The strongest result in the open literature is also the newest. In 2025 Bruno Demissie and Christian Steffes of Fraunhofer FKIE published a passive radar built on LTE450 — the 450 MHz mobile network that Germany and several other European countries are rolling out for critical infrastructure precisely because it is designed to survive a power blackout. Their illuminator was an LTE450 base station at a centre frequency of 467.37 MHz; their target a DJI Matrice M210, roughly 88 by 88 by 39 centimetres and just under five kilograms.

The single-channel result: with a false-alarm probability of one in ten thousand, the drone could be detected out to a bistatic range of about 1,600 metres. The multichannel result, using two base stations 9.6 and 10.8 km from the receiver against a drone at 30 metres above ground doing about ten metres per second, gave detection probabilities between 75.0 and 96.4 per cent. Coherent integration took 0.8 seconds per scan, during which the processor chewed through 26,666,640 samples per channel.

Why LTE450 rather than television? Because it closes the obvious hole in the whole concept.

That sentence is the reason this technology is being revisited now. A drone trailing a fibre-optic spool emits nothing. Every counter-drone system built around detecting the control link — and most of them are — is blind to it. A sensor that needs the target to transmit has run out of targets.

The other published numbers are humbler, and more honest about what a short-wavelength illuminator buys you. An earlier Fraunhofer FKIE experiment using a GSM base station at 945.8 MHz, 800 metres from the receiver, tracked a DJI Matrice 210 V2 flying an x-shaped course between 60 and 320 metres away. Across the whole run the bistatic range error was 163.3 metres RMS and the azimuth error 17.95 degrees; across the cleanest fifty-second window those fell to 16.3 metres and 4.83 degrees. The difference between those two sets of figures is clutter, and clutter is what actually defeats drone detection in the real world.

The Warsaw University of Technology team, working with RS Technologies, used a genuinely powerful illuminator: the DVB-T transmitter at Trzeciewiec, 45 km from the receiver, radiating 100 kW across 562, 594 and 634 MHz. Against a Parrot AR.Drone 2.0 they saw the target at a bistatic range of 200 metres. Using WiFi as the illuminator they managed 50 metres, and noted the limit was the drone’s operator rather than the sensor.

NATO has been running its own trials. In September 2025 a team from the Science and Technology Organization gathered in Germany to demonstrate passive radar against drones, framing it in the bluntest possible terms: on battlefields today, being able to detect drones is often the difference between life and death.

NATO’s Science and Technology Organization demonstrating passive radar against drones in Germany, September 2025.

Two hundred and fifty kilometres in the brochure, 1.6 in the paper

Set the measurements above against what the industry says it can do, and the gap is startling.

Hensoldt unveiled TwInvis at the ILA Berlin air show on 25 April 2018. The company’s release said the system evaluates up to 16 FM transmitters plus five DAB and DAB+ frequencies alongside DVB-T and DVB-T2, and quoted the then chief executive Thomas Müller saying that new digital receivers made it possible for a single TwInvis system to monitor up to 200 aircraft in 3D within a radius of 250 kilometres. A NATO measurement campaign on the Polish Baltic coast in 2019, run with the Fuerza Aérea Polaca, produced company claims of ranges up to 300 km against targets from light aircraft to missiles.

Those numbers are not fraudulent. They are simply about a different target. A 200-seat airliner and a five-kilogram quadcopter differ in radar cross-section by something like five orders of magnitude, and the brochure never promised otherwise — Hensoldt’s own product page for TwInvis does not mention drones at all. The problem is what happens when a 250 km figure gets copied into an article about airport closures.

The same caution applies to the story everybody repeats about TwInvis and the F-35 Lightning II. It comes from a single piece of reporting, by Sebastian Sprenger for C4ISRNET on 30 September 2019: Hensoldt said it had tracked two F-35s for 150 km after the 2018 Berlin show, from a horse farm near Schönefeld. Read the article to the end and the caveats pile up. Hensoldt correlated its passive readings against the jets’ own ADS-B transponder signals. Lockheed Martin’s spokesman told the reporter that the F-35 carries radar reflectors when it is not flying a stealth-dependent mission — air shows very much included — so that controllers can see it, and the reflectors were visible in official photographs from the trip. The F-35 Joint Program Office declined to comment. Nobody has independently confirmed the tracking in the seven years since.

Hensoldt’s own film on TwInvis. Manufacturer material, and useful mainly for the hardware and for one unusually precise claim about what passive radar does and does not need.

Passive radar is not the same thing as a passive sensor

This distinction is routinely mangled in defence coverage, and it changes the answer to almost every practical question.

A passive radar detects reflections of somebody else’s transmissions. Electronic support measures — ESM — detect the target’s own transmissions. The Czech VERA-NG and the Ukrainian Kolchuga are ESM systems: their own manufacturer calls VERA-NG a passive surveillance ESM tracker working on time-difference-of-arrival from a central station and three outstations. No third-party illuminator is involved anywhere. They are superb at finding an aircraft that is radiating, and completely blind to one that is not.

A third category muddies things further. NATO’s most recent drone-detection work, shown in March 2026, is about building a database of the radio frequencies drones emit so that detection systems can tell one model from another. That is ESM too, not passive radar — and it is exactly the capability a fibre-optic drone defeats.

A different problem, often confused with this one: NATO’s work on identifying drones from the radio frequencies they emit. That is electronic support, not passive radar, and a drone on a fibre-optic tether emits nothing for it to hear.

And a system can be continuously staring at the whole sky without being passive at all. Aveillant’s holographic radar, later bought by Thales, floods the volume and watches all of it at once; its chief executive David Crisp described it as having no beam, lighting up the entire sky in all directions with a separate staring array watching everything, at all heights, all the time. It is an impressive piece of engineering and it is an active radar, with everything that implies for being found.

The catch: somebody else owns your transmitter

Griffiths states the central weakness in one line of his lecture notes: in many cases the transmit source is not under the control of the passive radar. Broadcasting policy is set by broadcasters and regulators, and it moves.

Switzerland is the sharpest illustration, and it is on MiGFlug’s doorstep. In 2019 Switzerland became the first European country to switch off digital terrestrial television entirely, a cost saving agreed between SRG and the Federal Council after the No Billag referendum, affecting an estimated 64,000 households out of 2.7 million because almost everybody watched by cable or IPTV anyway. It was a sensible decision on its own terms. It also means that the band which actually works against a 30-centimetre quadcopter has essentially no national illuminator network over Swiss territory. That consequence is our inference rather than a published finding, but the arithmetic is not complicated.

War is blunter. On 1 March 2022 a Russian missile struck the Kyiv TV Tower, disrupting Ukrainian broadcasting and killing five people including the journalist Yevhenii Sakun. Broadcast masts are tall, fixed, unhardened and already on somebody’s target list. Nobody needs to switch them off to deny a passive radar its illuminator.

Then there are the engineering problems, which are not small. Broadcast waveforms run at a 100 per cent duty cycle, so the direct signal from the transmitter arrives continuously and enormously louder than any echo; Griffiths notes that significant processing has to be used to suppress it before weak target echoes can be detected at all. Clutter is worse still: the review literature describes small UAVs as highly susceptible to being masked by strong clutter, with current suppression algorithms struggling to remove it. And accuracy remains modest — even Hensoldt’s product page claims only that 70 per cent of altitude estimates fall within 1,000 metres RMS, which is perfectly adequate for air surveillance and useless for a quadcopter at thirty metres.

So where does that leave the airports

Europe has decided, at least on paper, to build its way out. The European Drone Defence Initiative, one of four readiness flagships in the Commission’s October 2025 defence roadmap, is described as a multi-layered network capable of detecting, tracking and neutralising hostile drones, with initial operational capability due at the end of 2026 and full functionality by the end of 2027. The briefing naming those dates does not specify a single sensor technology.

Passive radar will be part of whatever gets built, because its one great virtue is unarguable and the alternatives all have the same flaw: they announce themselves. But anyone reading a vendor’s 250-kilometre figure next to a story about Munich should hold two numbers in mind. Against airliners, passive radar is a mature air-surveillance technology with decades of work behind it. Against the thing that closed Munich, the best peer-reviewed result anybody has published is 1,600 metres.

That is not an argument against the technology. It is an argument for reading the footnotes.

Sources: Demissie & Steffes, IET Radar, Sonar & Navigation 19:e70092 (2025); Jovanoska et al., NATO STO-MP-MSG-SET-183 paper 11; Rzewuski, Kulpa, Pachwicewicz, Malanowski & Salski, NATO STO-MP-MSG-SET-183 paper 13; Hugh Griffiths, Passive Radar — From Inception to Maturity, Sensor Signal Processing for Defence, 2017; Raja Abdullah & Ismail, International Journal of Engineering and Technology 3(1); Tang, Ma, Qu & Mao, Drones 9(1):76; Hensoldt press releases, 25 April 2018 and 29 October 2019; C4ISRNET, 30 September 2019; ERA; Defence IQ; NATO Science and Technology Organization; Euronews; CNN; DVB Project; European Parliamentary Research Service briefing PE 777.962; Defence Blog.