These are visible not just by anyone who can receive, but pretty much everything on APRS ends up online. Most areas have an igate that forwards messages from the radio to the Internet. https://www.aprsdirect.com/center/40.55354,-104.97842/zoom/1... is my favorite site for seeing what is going on in an area and where digipeaters and igates are. https://aprs.fi also works.
Some areas have one that works in reverse, taking what is seen on the internet and relevant to the active stations in the area and broadcasting it out over radio.
When I started using SMSGTE I had great success getting a SMS from my radio to a phone, but only 1 in 10 replies from SMS made it back to my radio. I put up a gateway for my area and so we have some decent bi-directional coverage.
Hold up, does that mean you are allowed to transmit on SMS/GSM/?? frequencies with a "end user/private operator" type license? Or do you have to fiddle with your phone to get it to listen to an alternative band? (As is probably obvious I have no experience with radio)
Not exactly, you transmit on a “normal” VHF frequency (which you must be a licensed amateur to broadcast on) and the data gets picked up by a “digipeter” (radio to internet repeater) which then gets picked up and relayed to SMS by a gateway (a server looking for messages to send). This infrastructure exists and only requires a license, radio, and APRS interface (probably your iPhone/ Android device).
Some areas have one that works in reverse, taking what is seen on the internet and relevant to the active stations in the area and broadcasting it out over radio.
When I started using SMSGTE I had great success getting a SMS from my radio to a phone, but only 1 in 10 replies from SMS made it back to my radio. I put up a gateway for my area and so we have some decent bi-directional coverage.