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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.

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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.”

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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.