Iridium Bluetooth service

The MeshSat node exposes this service next to the Meshtastic service, on the same Bluetooth LE connection. It is a binary-safe serial line to the node's RockBLOCK 9603. The client speaks the modem's AT commands through it, exactly as it would over a cable. MeshSat Android is the client we build and test against.

Contract version 1.

Characteristic	UUID	Properties	Carries
Service	b3d305a2-7310-4877-ad12-8e245e71951a
RX	b9e2d4ba-f386-4728-b77a-7df7121db7a9	write, write without response	raw bytes to the modem's UART
TX	469354dc-4c89-41ed-b939-d707c7a11f49	notify, read	raw bytes from the modem, in chunks of up to the MTU minus 3 (at most 244)
STATUS	69a4064d-78b9-46e5-a30a-1862e553245a	read, notify	two bytes: the contract version (01), then the modem's owner: 00 none, 01 phone, 02 node

Owning the modem

• Subscribing to TX notifications takes the modem when nobody owns it. STATUS then reads 01 01 and is notified on every change.
• Unsubscribing from TX, or disconnecting, gives it back (01 00).
• Bytes written to RX while the client does not own the modem are discarded, so nothing stale reaches the modem later. Wait for 01 01 before the first write.
• Modem output while nobody owns it is discarded.
• Owner 02 is reserved for routing on the node itself, which is not built yet.

The serial line

• The pipe adds and removes nothing. The link to the RockBLOCK is 19200 baud, 8N1.
• AT commands end in a single carriage return. The 9603 does not accept a line feed as the terminator.
• The modem's stored profile has echo on. A client that sends ATE0 keeps echo off until the modem next loses power.
• On node v1 the modem is powered at boot and answers about 10 seconds later. Retry the first command until it does.
• Every satellite session opened through the pipe (AT+SBDIX and friends) is billed by the airtime provider, even one with nothing to send.

Security

If the node's Bluetooth pairing mode is anything other than "no PIN", all three characteristics need an encrypted, authenticated link: the same bond as the Meshtastic service. With "no PIN" they are open, like the Meshtastic service.

Changes

Any change to this contract bumps the version byte in STATUS, and MeshSat Android is updated first. The source of truth is the firmware: src/meshsat/IridiumPipe.h in meshsat-firmware.