The GPS ICD (specifically Appendix II of the document IS-GPS-200) specifies something called Special Messages transmitted in the GPS LNAV message. The field, which is transmitted as page 17 of subframe 4, is described as "specific contents at the discretion of the Operating Command" and that "it shall accommodate the transmission of 22 eight-bit ASCII characters". As ASCII tends to be used for encoding text, this sounds like a global one-way short message service.
To add to the mystery of this message, there doesn't appear to be any further information about this message online. It is sometimes mentioned as being part of the LNAV message with some quote of the ICD text. I have not been able to find anything on the contents. Is it actually in use? What are the actual data contained within? Given that the LNAV data is globally broadcast, it's surprising that this information is not readily available.
Over a few years, I've been on-and-off learning about GPS by attempting to build my own GPS receiver. This spring I've got the point where I can track a satellite in real-time and collect data from it. On March 15 (around 20:45 UTC). I got several minutes worth of good data from PRN 32. Among the data was one instance of page 17 of subframe 4. I decoded the content of the special message as:
N9Y2L L'O:CJL O2AY"WMG
The parity bits of all words in the subframe were correct, and the decoded characters match the valid character set defined in the ICD. Overall I'm very confident that I have decoded the contents correctly. The field is thus obviously in some use. but it's a bit of a dissapointment that the meaning of the contents is not immediately obvious.
Now, it's not yet clear to me if all satellites transmit the same message. In fact, it's not even clear if a single satellite transmits the same contents on each message repetition. I'll be looking into these in the future as I get more data :-)
