That requires a subscription plan? Ouch.
The basic subscription plan is quite reasonable. I pay about $12 month, which gives me I think 10 regular messages, with a minor charge if I go over that. And of course SOS ability. It is fairly easy to upgrade the plan, or drop back down to the basic plan as your needs change.
I think we need a tiny JS8call transceiver (js8call.com). That is a weak signal ham radio mode similar to FT8, but you can type any messages you like, instead of having a fixed QSO format. Tiny FT8 tranceiver is here:
https://hackaday.com/2021/09/25/the-simplest-ft8-transceiver-youll-ever-build/Basically it is an Altoids sized tranceiver with around $50 of parts, that can send and receive text messages over transcontinental distances, but at very low speed (like several minutes for a 50 character message). It would use a wire antenna a few feet long or preferably longer. For normal use you need a ham license, but in a real emergency saving people takes priority. I have been playing with this idea for a while, but haven't tried to really pursue it.
You could make the tranceiver even smaller and cheaper, by getting rid of the display and just using a phone as a UI and power source. Of course that means you have to bring a phone.
While this sounds like a fun and interesting project, I don't see it as a viable alternative to an InReach.
1. First, for an emergency device, when you need help, you usually need it right now. That means someone needs to be monitoring JS8call when you send your "help" message. To my knowledge, no rescue agency guards JS8call.
2. You could of course just send out a general "help" message, and yes, likely someone somewhere in the world will receive it. However, you are assuming this random person somewhere in the world will understand your problem, be willing to help, and know who to relay the message to. That's a lot of assumptions.
3. An InReach sends your GPS coordinates imbedded in an SOS (or a routine text message as well). To use the system you propose for emergencies, you would still need to also carry a separate, dedicated GPS (or smart phone). Then you would need to hand type in those coordinates, with the potential for error. Likewise, whatever random person received your JS8call needs to copy those coordinates, and relay them to at the appropriate rescue authority (assuming the random person knows what authority to call). This will likely be done by phone. But then the rescue authority needs to transcribe those coordinates yet again. Speaking from real world SAR experience, almost NOTHING is easier to corrupt in voice communication than a GPS coordinate. (
Always get a read back of coordinates!)
4. As noted in #2 your plan increases the number of middlemen in the operation. An InREach SOS goes directly to a rescue coordination center. This outfit responds to SOS messages from all over the world, all the time (not just for InReach, but other devices as well.) When your SOS comes in, the coordinates will pop up on a map, the center folks will check their database of SAR agencies, and relay it on. It has proven to be an effective system, and has achieved a good track record.
5. Then there is the issue of building said device rugged enough to be reliable in field conditions. My InReach, as well as every stand alone GPS I've used has been quite solidly built. Smartphones not so much, although there are a wide variety of protective cases available.
6. With your plan, you need to rig an antenna
"a few feet long or preferably longer". Maybe not a big deal for a healthy person, but that could be problematic if you are badly injured. With an InReach, you just remove the safety, and push the button. It will then send an SOS (with GPS coordinates). If you should then pass out from your injuries, the InReach will continue to send an SOS until the batteries die.
Note that I'm a ham (Extra class), and enjoy the hobby. While I don't have much experience with JS8call, I have actually played around with it briefly. Your idea sounds like a fun project. But I don's see myself replacing my InReach Explorer with JS8call anytime soon.