The Solo follows the mobile device (iPhone/Android). However, the mobile device must be within wifi range of the controller. So basically you have to keep the phone and the controller together.
This is a major weakness of the Solo. It's really a step back from how things work on the Iris+. With an Iris, you connect a Mavlink antenna to the phone. The phone transmits its location, speed, and heading directly to the drone and the Controller doesn't need to be part of the loop.
That means on an Iris+, you can hand the phone to a skier, snow boarder, biker and send them on their way while you keep the controller in hand, allowing you to take back control if anything goes wrong.
I've heard UNOFFICIAL rumors that 3DR is working on a solution for this, but it might only be doable with Android devices. Take that with a grain of salt though.
Also keep in mind that it takes a little experience to master Follow Me. It's a bit of an art. You need to tweak things like the length of the leash. Also, it seems to work best the faster you are moving. It's great for most action sports, but it has a tough time extrapolating your heading and speed when your just walking around - especially with the phone swinging around in your hand (because your heading changes rapidly with each swing of your arm and that all gets reported to the drone). Be patient and play with it and I think most of you will find it works quite well. One more tip, if your leash is set to 20 meters, you generally need to move at least that far before it really kicks in.
