This is interesting:

The U.S. military has likely been quietly broadcasting codes for its global encryption network using public GPS for nearly 20 years, turning each satellite into a hidden “numbers station,” according to Steven Murdoch…

That means every device that uses GPS has been receiving hidden government information for years, and nobody outside the military knew it until now.

[…]

Murdoch discovered that this particular sentinel was transmitted by all 31 operational satellites within a window of a few hours on May 26, 2011, potentially heralding the activation of a new operational system. He confirmed that this timeline coincided with the rollout of the military’s Over-the-Air Distribution (OTAD) and the Over-the-Air Rekeying (OTAR) by cross-referencing declassified documents, including a 2015 presentation about the dates of the operation.

“There was a perfect match between the timeline and that presentation and the change points that were automatically identified from the data,” Murdoch said. “That was the smoking gun that made me think: This is what it’s for.”

These automated systems replaced the cumbersome manual distribution of cryptographic keying material, allowing military GPS receivers around the world to be rekeyed remotely through satellite broadcasts rather than through onsite procedures.