A constellation of classified defense satellites built by the commercial company SpaceX is emitting a mysterious signal that may violate international standards, NPR has learned.

Satellites associated with the Starshield satellite network appear to be transmitting to the Earth's surface on frequencies normally used for doing the exact opposite: sending commands from Earth to satellites in space. The use of those frequencies to "downlink" data runs counter to standards set by the International Telecommunication Union, a United Nations agency that seeks to coordinate the use of radio spectrum globally.

Starshield's unusual transmissions have the potential to interfere with other scientific and commercial satellites, warns Scott Tilley, an amateur satellite tracker in Canada who first spotted the signals.

"Nearby satellites could receive radio-frequency interference and could perhaps not respond properly to commands — or ignore commands — from Earth," he told NPR.

Outside experts agree there's the potential for radio interference. "I think it is definitely happening," said Kevin Gifford, a computer science professor at the University of Colorado, Boulder who specializes in radio interference from spacecraft. But he said the issue of whether the interference is truly disruptive remains unresolved.

SpaceX and the U.S. National Reconnaissance Office, which operates the satellites for the government, did not respond to NPR's request for comment.

Caught by the wrong antenna

The discovery of the signal happened purely by chance.

Tilley regularly monitors satellites from his home in British Columbia as a hobby. He was working on another project when he accidentally triggered a scan of radio frequencies that are normally quiet.

"It was just a clumsy move at the keyboard," he said. "I was resetting some stuff and then all of a sudden I'm looking at the wrong antenna, the wrong band."

The band of the radio spectrum he found himself looking at, between 2025-2110 MHz, is reserved for "uplinking" data to orbiting satellites. That means there shouldn't be any signals coming from space in that range.

But Tilley's experienced eye noticed there appeared to be a signal coming down from the sky. It was in a part of the band "that should have nothing there," he said. "I got a hold of my mouse and hit the record button and let it record for a few minutes."

Tilley then took the data and compared it to a catalog of observations made by other amateur satellite trackers. These amateurs, located around the world, use telescopes to track satellites as they move across the sky and then share their positions in a database.

"Bang, up came an unusual identification that I wasn't expecting at all," he said. "Starshield."

Starshield is a classified version of SpaceX's Starlink satellites, which provide internet service around the world. The U.S. has reportedly paid more than $1.8 billion so far for the network, though little is known about it. According to SpaceX, Starshield conducts both Earth observation and communications missions.

Since May of 2024, the National Reconnaissance Office has conducted 11 launches of Starshield satellites in what it describes as its "proliferated system."

"The NRO's proliferated system will increase timeliness of access, diversify communications pathways, and enhance resilience," the agency says of the system. "With hundreds of small satellites on orbit, data will be delivered in minutes or even seconds."

Tilley says he's detected signals from 170 of the Starshield satellites so far. All appear in the 2025-2110 MHz range, though the precise frequencies of the signals move around.

Signal's purpose in question

It's unclear what the satellite constellation is up to. Starlink, SpaceX's public satellite internet network, operates at much higher frequencies to enable the transmission of broadband data. Starshield, by contrast, is using a much lower frequency range that probably only allows for the transmission of data at rates closer to 3G cellular, Tilley says.

Tilley says he believes the decision to downlink in a band typically reserved for uplinking data could also be designed to hide Starshield's operations. The frequent shift in specific frequencies used could prevent outsiders from finding the signal.

Gifford says another possibility is that SpaceX was just taking advantage of a quiet part of the radio spectrum. Uplink transmissions from Earth to satellites are usually rare and brief, so these frequencies probably remain dark most of the time.

"SpaceX is smart and savvy," he says. It's possible they decided to just "do it and ask forgiveness later."

He notes it's unlikely the signals from Starshield have caused significant disruptions so far, otherwise other satellite operators would have complained.

Tilley told NPR he has decided to go public with his discovery because the world's satellite operators should be aware of what's happening.

"These are objects in classified orbits, which could potentially disturb other legitimate uses of space," he said.