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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).

2023-09-05T00:15:36 43+26SV-G"-S6+DSK3H85F
2023-09-10T00:08:06 NBBK.KQO-C75R1FUY6BJZ6
2023-09-11T00:05:36 0DKNSOM"X"MI:R.5UBZTZI
2023-09-17T00:08:06 -NM"UJDX°ME+HS-7CQ+/S/
2023-09-24T00:08:06 YKXW/Q'°+RIDN'O7Z723TF
2023-09-25T00:05:36 MUA9"LG.'0II-'080"5NIH
2023-09-26T00:03:06 /Q"RNM-C62ELZH1QHD0IDC

Some observations:

  • 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

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The only other reference I've ever seen is in the RTCM spec and this just describes it as a null terminated 90 character ASCII field, with parity. The example is "HELLO". I have not see real world data, but then I haven't looked.

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    Thanks for looking into this. I've started to collect data, though the message is the same for all satellites and changes only every few days, so I don't have much. Here's what I've recorded so far: 43+26SV-G"-S6+DSK3H85F, NBBK.KQO-C75R1FUY6BJZ6, 0DKNSOM"X"MI:R.5UBZTZI, -NM"UJDX°ME+HS-7CQ+/S/. Sep 22, 2023 at 13:25
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    I had seen a mention in a chat long ago that these messages might be used to update decryption keys for the secure precision GPS channel used by the military. No idea if that is right.
    – colintd
    Sep 22, 2023 at 13:44
  • It’s not implausible. The lack of any pattern is consistent with something cryptography-related. Perhaps they retrofitted key rotation on the W-code, but there’s basically nothing public about it to verify. Even if I could find the W-code with a high gain antenna, I doubt the generator key could be discovered to check. If the special message was used as key rotation, however, I’d expect a consistent frequency of update but I haven’t found any pattern there. Sep 23, 2023 at 16:04

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