If you want the ability to transmit a message virtually anywhere, anytime, consider a spot connect to use with your bluetooth cell phone. It is transmit text only, no recieve capability. Along with emergency rescue if needed. We use it quite a bit to let folks know where we are when out of cell range. person recieving any message also gets gps coordinates showing your location on the ground.
It uses half duplex LEOS and is part of Globalstar, but luckily it uses LEOS rather than GEOS satelites for far better coverage. Now if it was only full duplex to allow two way transmission.