Has anyone used this service?

I was thinking of buying a Garmin Fenix 3 and noticed it is compatible with the LiveTrack service but can not find much information on how the service works.
I think it needs to link to a phone to work but I can not find out if it uses GPS alone or needs a cell connection. Obviously if it is GPS only it would be ideal. I doubt it will be that useful though.

Needs a phone connection so as good as any other app out there really.