The inReach is a GPS tracker, like the SPOT tracker, using the Iridium satellite network. You use it to send messages with your position via satellite. It, like the SPOT, also has an SOS mode where they automatically link up with local emergency services to come looking for you. It is a bit more expensive than the SPOT, but allows for two-way satellite SMS messages. The model I got interfaces with an Android phone and a mapping app from DeLorme for the composition and sending of two-way messages. You do not have to be connected to the phone to send pre-programmed messages or 911 messages (which is what the SPOT does). You do not have to have cell service to send and receive messages--like in the middle of many National Parks and Forests, not to mention the Maine woods. Details at the DeLorme site. It normally goes for $249, not including the service subscription.