The ciphertext has a calendar

In March 2023, someone pointed a home-built GPS receiver at the sky and found this: N9Y2L L'O:CJL O2AY"WMG It was not noise. The parity checked out. It used exactly the peculiar little alphabet permitted by the GPS specification. Other satellites were transmitting the same 22 characters, and after a day or two they would all switch to a different, equally meaningless string. GPS has been doing this for at least nineteen years. That sounds like the opening of a spy story, which is why the discovery has been described as a global numbers station. There is probably some truth in the comparison. A transmitter covers the planet; the intended listeners are invisible; and anyone can receive the message while only someone holding the right secret can understand it. But the comparison is also a distraction. The best explanation is not that field agents are receiving orders between the ephemeris and the almanac. It is that hundreds of thousands of military GPS receivers are being serviced by a…