I came across this via a link from the Extreme Shortwave Listening group and posted from the blog of nw7us.
There is an ITU paper which discusses the problems of reliable ship-to-shore comms using only satellites, and a design for moving them back to HF. The paper includes architectural design for the system, along with explanations of the type of transmission, in this case, OFDM or Orthogonal Frequency Division Multiplexing. No, me neither.
A few quotes from the paper:
The NAVDAT HF system can use a simple time-slot allocation similar to the NAVTEX system which could be coordinated by IMO.
That NAVDAT HF system can also work on single frequency network (SFN) as described in Annex 4. In this case, the transmitters are frequency synchronized and the transmit data must be the same for all transmitters.
The NAVDAT HF digital system offers a free broadcast transmission of any kind of message from shore‑to‑ships with possibility of encryption.
That last one is telling “a free broadcast transmission”. That reads to me that one of the reasons to return to HF is the cost of renting satellite time, whereas HF is “free to air” with no ongoing costs other than maintenance.
Its not a large paper, but worth the read. The link to the pdf file is here: NAVDAT