RFC : Add ground station mode

[stronnag]

This RFC summaries chat discussion on the above.

Some users may wish to connect a GCS (Ground Control Station) to an inav-radar node in order to view cooperating aircraft. Currently this is somewhat possible (e.g. https://github.com/stronnag/mwptools/issues/90) but is sub-optimal from both sides:

• The (assumed) stationary ground station is required to masquerade as a FC, sending positional, flight status and GPS data. The GCS may have to "fake" this.
• Masquerading as an operational FC takes up a LoRA slot, which is wasteful and inefficient.

This proposal seeks to address these issues whilst maintaining a degree of integrity to the connection.

• The GCS shall operate as an FC as far as MSP direction flags are concerned.
• The GCS shall respond to the following MSP request:
  • MSP_FC_VARIANT with the "GCS" (e.g. 0x47, 0x43, 0x53, 0x0)
• The inav-radar mode may issue the following MSP, to which the GCS may respond:
  • MSP_NAME: The GCS should provide an identifying name within MSP limits (16 octets).
  • MSP_FC_VERSION: The GCS should provide an identifying version (3 octets, e.g. 0x06, 0x06, 0x06).
  • The inav-radar node shall NOT issue the following MSP requests to a client of variant "GCS"
  • MSP_STATUS
  • MSP_BOXIDS
  • MSP_RAW_GPS
• Once a node is identified as "GCS", the node will send unsolicited MSP2_COMMON_SET_RADAR_POS messages.

[snaewe]

+1 Maybe it would also be possible, to use the USB-Serial port as the connection to the PC/GCS ?

[OlivierC-FR]

Looks good to me ! Current code should comply with that, with the limitation that MSP_NAME is truncated to 6 chars, because OLED. MSP_STATUS and _BOXIDS are not used, and I added a check to not MSP_RAW_GPS a GCS.

About the serial over the USB port, it's on the to-do list. It was in there at some point, a long time ago, aswell bluetooth, but I removed it all because that's what I do, remove things :)

[stronnag]

Olivier, sure that MSP_STATUS is not used for GCS? In 2.0, the check is still curr.host != HOST_NONE rather than curr.host == HOST_INAV (src/main.cpp#940), unless I'm not seeing some prior if(){}.

So it's still asking for ANALOG and STATUS, neither of which are relevant to a GCS.

[OlivierC-FR]

Olivier, sure that MSP_STATUS is not used for GCS? In 2.0, the check is still curr.host != HOST_NONE rather than curr.host == HOST_INAV (src/main.cpp#940), unless I'm not seeing some prior if(){}.

So it's still asking for ANALOG and STATUS, neither of which are relevant to a GCS.

you're 100% right, forgot this one, I force-pushed the fix, the check is done at 944, in the next IF