Starlink and stealth aircraft: what passive radar can and can’t do

News2026-10-08

This week, a simulation showing how the stealth aircraft F-117 and an ordinary fighter, the F-15, would look on radar from space has been making the rounds on X. The comments underneath claim that the era of stealth aircraft is over, and that Starlink is to blame.

But is that true?

What actually happened

The images were calculated by a physicist who posts on X as A Weird Physicist. They’re computer simulations, not images from SpaceX satellites.

The simulations also use a frequency of 35 GHz. Starlink transmits at a lower frequency, around 11 to 12 GHz, and that’s the frequency used for the rest of this article.

The author refers to Starshield, SpaceX’s program for government and defense customers. But SpaceX has never claimed that Starshield searches for stealth aircraft.

It is true, though, that Starlink can be used to detect aircraft. Its signal can be used for passive radar. And that principle is more than 80 years old.

How passive radar works

A conventional radar sends out a pulse and waits for the reflection. But that gives away its location.

Passive radar doesn’t transmit anything. It listens to a signal that’s already in the air: radio, TV, a mobile network, or Starlink. One antenna picks up the signal directly from the transmitter; another picks up that same signal after it reflects off an aircraft. The reflected signal takes a detour, so it arrives a little later.

The delay tells you how much farther the reflected signal traveled. But that alone doesn’t tell you the aircraft’s position – you need another measurement or more receivers for that.

This is nothing new. During World War II, the Germans built the Klein Heidelberg passive radar, which used signals from British radar stations.

Why stealth isn’t enough against passive radar

A stealth aircraft isn’t invisible. It’s designed to reflect radar signals away, rather than back toward the radar. That works against conventional radar because the transmitter and receiver are in the same place.

With passive radar, the receiver is somewhere else. The reflected signal can end up hitting it.

The most interesting moment is when an aircraft flies directly between the satellite and the receiver. For a moment, it blocks the signal path, and the receiver records its shadow – a tiny drop in signal strength. The shadow’s shape depends less on the aircraft’s design than a reflection does; its overall size matters more.

But the shadow is narrow. In a simplified model, at a fighter’s altitude of 10 km it’s only about 20 meters wide, and the aircraft passes through it in roughly a tenth of a second. Outside the shadow, the stealth aircraft is hard for the receiver to see again.

You may have read that Starlink offers resolution of around 1 m. That applies to the distance of a reflection, not to an aircraft’s position. And it barely works in the shadow.

Try it yourself

In the simulator, there’s a Starlink satellite at the top, a receiver below, and a dashed line between them. The orange wedge around the line is the shadow cast by an aircraft as it crosses the signal path.

Start the fighter flyby. Away from the line, the receiver can barely see it. In the shadow, it detects the aircraft, but only for about a tenth of a second.
Try the airliner. It’s larger, so the shadow is stronger, but also narrower.
Try the small drone. Its shadow is weak, but the drone flies slowly and stays in it for over a second.

The figures in m² are neither the aircraft’s size nor its conventional radar cross-section. They’re just a handy way to indicate how strong the shadow is. The simulator is a simplified model, but it’s good enough to give you a sense of the scale.

What the Chinese experiment showed

In 2024, Chinese researchers showed that Starlink signals can reveal a flying object. The South China Morning Post reported on the experiment.

They listened to a satellite passing over the Philippines and flew a DJI Phantom 4 Pro drone through its signal path. The authors say it has a radar cross-section similar to that of an F-22 fighter. They detected the drone in the signal and reportedly even identified the rotation of its propellers.

Detecting that something passed through isn’t the same as knowing exactly where it is or where it’s going. Nobody has publicly demonstrated reliable tracking of a stealth fighter this way yet.

Starshield isn’t Starlink

Starshield is built on the same technology as Starlink, but it’s a separate program for government and defense customers. According to Reuters, SpaceX is building a network of hundreds of satellites for the US National Reconnaissance Office (NRO), under a $1.8 billion contract. What exactly the system can do is not known.

Czechia is a leader in passive surveillance

The Pardubice-based company ERA builds on development work that has been underway in Pardubice since the 1960s. Its predecessors were called Kopáč, Ramona, and Tamara. Today, ERA makes VERA-NG.

VERA-NG works differently from the radar discussed in this article. It doesn’t listen for reflections of signals from other sources; it listens for signals transmitted by the aircraft itself, such as its radar or transponder. If the aircraft switches everything off, VERA can’t hear it.

PLESS, another ERA system, works on the same principle. The Czech Ministry of Defence approved the purchase of three units for about $100 million. Czechia has also ordered 24 F-35A stealth fighters.

A homebrew passive radar

You can try out the basics of passive radar at home, too.

You can receive Starlink signals with an RTL-SDR V3, a satellite LNB, and a bias tee to power the LNB. Czech radio amateur Derek OK9SGC has described the process. If you’re new to RTL-SDR, take a look at this beginner’s guide to RTL-SDR.

For an actual passive radar, you need two receivers sharing a clock, two directional antennas, and the open-source software blah2. The cheapest option is to use two RTL-SDR dongles. DVB-T television or FM radio is the most commonly used signal source.

For verification, it’s useful to receive ADS-B at the same time and compare the aircraft’s self-reported position with the radar’s calculation. My MeteoPlaneRadar uses ADS-B, for example.

You can also detect drones with RTL-SDR. I wrote about detecting drones with an inexpensive SDR device in another article.

So, are stealth aircraft finished?

Passive radar is an old, working technology, and it has an advantage against stealth aircraft: it doesn’t transmit anything itself and can detect signals that stealth aircraft reflect away.

But this isn’t the end of stealth aircraft. The shadow is narrow and fleeting, and a single receiver can’t provide an accurate position.

My take: it’s a good demonstration that stealth doesn’t mean invisibility. An aircraft is simply detected later, and passive radar could reduce that head start in the future.

Sources: Czech news site Novinky.cz: Stealth fighters have a problem · The simulation on X · South China Morning Post: The Chinese experiment · Reuters: SpaceX is building a spy satellite network for the NRO · Study: Signal Structure of the Starlink Ku-Band Downlink · ERA: PLESS product sheet · RTL-SDR.com: Receiving Starlink beacons · blah2: Open-source passive radar

Article prepared by Claude Opus 5.5

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