> currently the only options are proprietary devices that cost several hundred dollars AND a subscription plan that will run you $12 a month or more
Consider a 406 MHz PLB [1][2][3]. Several hundred dollars. But no subscription fee.
They're terribly marketed, a vacuum Garmin et al happily fill. And you can't send text messages. But as an emergency beacon, they are best in class. (Garmin transmits at <2 watts. Cell phones maybe 125 mW. 406s, meanwhile, scream at the sky at 5+ watts; that's in class with a cell tower.)
On the other hand, Garmin/InReach use Iridium, which has safety-critical communication as its primary mission and provides it using a network featuring inter-satellite message routing.
COSPAS-SARSAT is usually a secondary payload of other satellites, is store-and-forward (i.e. if you're far from a ground station, it might take a while for a satellite to forward your emergency signal), and for most (but see [1]!) PLBs does not offer a return channel, i.e. you won't know whether your signal actually made it through or whether you'll need to keep moving to get line-of-sight.
> Garmin transmits at <2 watts. Cell phones maybe 125 mW. 406s, meanwhile, scream at the sky at 5+ watts; that's in class with a cell tower.
They use exactly as much transmit power as they need to. Importantly, Iridium modems get instant feedback on whether their transmission has made it through and can retransmit if required. Screaming isn't always the best way to be heard :)
> I think Iridium can handle four calls at a time.
It supports over a thousand simultaneous calls per satellite! And that’s voice calls; text messages like those used by InReach need much less resources still.
> Garmins have a ceiling. Terrain attenuates. There are more places you will be alone with a Garmin than a 406 MHz PLB.
What “ceiling”? As long as you have line-of-sight, the transmit power is enough to reach the satellite, and given polar orbits, you almost always do if you can see a bit of sky.
If you don’t have line-of-sight, COSPAS-SARSAT won’t work either, would it?
I assume the higher transmit power there is mostly to enable reaching satellites farther out (like the GPS and Galileo satellites, which are in MEO).
> 406 MHz does this […]
In some regions only, I believe, and with some beacons. It is a very important development though!
> Sorry, I meant inter-satellite. Iridium’s inter-satellite capability is highly limited and irrelevant for emergency comms.
It absolutely isn't, where are you getting this from? Both routine and emergency communications regularly traverse inter-satellite links. For a while, Iridium only had a very small number of operational ground terminals; nothing would work without inter-satellite links in that scenario.
> In ideal conditions. 5 watts vs something like 1.6 makes a huge practical difference.
Can you name a scenario where you'd get a signal through to a satellite with 5, but not 1.6 watts? This is absolutely negligible; even just rotating your transmitter ever so slightly will make a much, much bigger impact.
I have never not received an Iridium signal when there was line-of-sight to the satellite.
> There is a reason 406 MHz is the requirement for aviation.
Aviation and marine safety of life specifications care a lot about redundancy and availability. COSPAS-SARSAT is conceptually very simple and does not require ongoing subscriptions, so it makes for an excellent fallback option if other communication methods are unavailable.
For example, for passenger or cargo ships, you need at least two other bidirectional means of long-distance communication when crossing oceans. For polar routes outside of geostationary satellite coverage (i.e. Sea Area A4), the only real options (certifiable under SOLAS/GMDSS regulations) are HF radio and Iridium.
Consider a 406 MHz PLB [1][2][3]. Several hundred dollars. But no subscription fee.
They're terribly marketed, a vacuum Garmin et al happily fill. And you can't send text messages. But as an emergency beacon, they are best in class. (Garmin transmits at <2 watts. Cell phones maybe 125 mW. 406s, meanwhile, scream at the sky at 5+ watts; that's in class with a cell tower.)
[1] https://news.ycombinator.com/edit?id=37850377
[2] https://www.sarsat.noaa.gov/emergency-406-beacons/
[3] https://www.rei.com/product/161982/acr-electronics-resqlink-...