No, iridium is definitely transmit-before-receive.
The only thing you can receive without transmitting is a one-bit "message waiting" indicator, so you don't waste uplink bandwidth polling the satellite unless there's something there for you. Basically the satellite broadcasts a list of terminals that have messages waiting for them. I recall there being some kind of bloom filter, so technically it's less than one bit.
You can't get anything else downstream unless you transmit first to ask for it. Also the only way to clear the "message waiting" bit is by transmitting to the satellite, so you can't even try to encode a message by flipping that bit on and off.
No, the original Iridium paging service really is (was?) one way.
You had to manually update your paging location by dialing some number or by using a web interface when moving significantly, or link an Iridium phone to your pager, which would then automatically update the paging area (MDA) every time you made a call.
What you mean is probably Iridium SBD, but that’s a different, bidirectional service (also used for pager-like devices these days).
Yes, Iridium SBD requires transmitting before you can receive.
I did not know about Global Data Burst; I think that's what you're referring to. Holy cow it's expensive though, although I guess it has to be since you're using a lot more bandwidth*footprint. Like $6.00 per message per delivery area expensive. And they're kinda vague on how big a "delivery area" is.
No, I’m referring to the Iridium pager, which is just its own service and has been around longer than SBD, I believe. Messages were (are?) free to send on top of a monthly flat subscription rate per pager.
GDB is just a branding for their own flavor of what they're selling which is Iridium Burst, which also appear to run on the 9602/9603 SBD modules, so... maybe Iridium Burst is just spicier Iridium SBD?
SBD is bidirectional unicast (and mandatorily so; as you say, SBD can't transmit to devices before they initiate a mailbox exchange, with the exception of a "message waiting" indicator.
Iridium Burst seems to be multicast-capable, and optionally bidirectional or unidirectional, so "spicier SBD" makes sense!
The only thing you can receive without transmitting is a one-bit "message waiting" indicator, so you don't waste uplink bandwidth polling the satellite unless there's something there for you. Basically the satellite broadcasts a list of terminals that have messages waiting for them. I recall there being some kind of bloom filter, so technically it's less than one bit.
You can't get anything else downstream unless you transmit first to ask for it. Also the only way to clear the "message waiting" bit is by transmitting to the satellite, so you can't even try to encode a message by flipping that bit on and off.