minor fyi. Had to look up how to get lat/long on an iphone without internet access. It’s in the default compass app.

Where? Mine has zero options now - just displays the compass, with the level in the middle.

Seems it's not available everywhere, for some reason.

> Note: Your coordinates and elevation may not be available in certain countries or regions.

https://support.apple.com/en-gb/guide/iphone/iph1ac0b663/ios

some reason = GNSS: https://medium.com/@ilyakorogodin/where-is-a-navigation-chip...

Article opens iPhones of several generations since iPhone 3G to discover how they do GNSS positioning.

Thanks, this article pointed me in the right direction. Coordinates were missing for me because locations services were disabled for the Compass app.

Google Maps and Apple Maps show it when you set a marker. You don‘t need internet.

I would be less worried about bytes and more about the general TCP handshake/acks making it through. You can easily add height, heart rate, and all other kinds of information before you come close to the network overhead. You could also gamble UDP+overhead, but I assume you want some kind of assurance that your SOS made it through.

You also need to be able to get a session within the beam. If you're stuck in the middle of a desert you'll be fine, but if you're within range of a minor town you may not be able to get any signal.

Not an expert, but my understanding is that SMS text messages don't rely on TCP or UDP. Instead, they use their own protocol called SMPP with an overhead of less than 100B.

This hypothetical case, on the other hand, only require two bytes of the reply, with letters "OK".