Iridium IMT

Internet Modem Transceiver on a RockBLOCK 9704. The newer Iridium bearer: far larger messages than SBD, over the JSPR protocol rather than AT commands.

Maximum message	100 KB, against 340 bytes on SBD
Serial	230400 baud, JSPR

The thing that will catch you out

The 9704 has no onboard message caching. Mobile-terminated messages must be polled frequently or they are lost when the satellite link drops. The bridge polls for you; this matters if you are writing anything that talks to the modem directly.

Provisioning

Topics are provisioned on the Iridium network rather than configured locally, and a newly activated modem needs ten to thirty minutes with sky view before its topics appear. Until then the modem is healthy and has nothing to send to.

The bridge queries the modem for its provisioned topic list and reports it, so you can tell "not provisioned yet" from "provisioned and failing".

When to use it over SBD

Use IMT when the messages are too big for 340 bytes and the hardware supports it. Use SBD when you need the widest device compatibility. Both are billed by your own provider account.