In the GPS specification (p124) there is a facility for the satellite to send a “special message”. The content is “at the discretion of Operating Command” but is anyone aware of any statement of how this is actually used, or has someone reverse-engineered it? I’ve seen data present, but before I put in effort to more systematically collect it, I would like to know whether someone has already found out what it means.
Update 2023-09-26: I've been recording special messages for three weeks, and here's what I've received, with the time being when it was first received (UTC).
- There's no obvious structure to the messages
- I receive about 200 messages per day
- Even though the characters are 8-bit, the messages sent appear to respect the restricted character set in the specification
- All satellites (that I can see) send the same message
- The message changes at midnight (maybe UTC, maybe GPS time)
- The time between message updates varies; from what I've seen between 1 and 7 days