The phone recieves data from gps satellites, but the gps reciever on the moment is weak and prone to losing lock. OTH, the map data for both Telenav (sprint) and Google Nav both come over the data conection. Once you run off of the buffered map, you're out of luck. Fortunately, most places you would run out of map and have no data conection (i.e tunnels, the middle of nowhere) don't usually have a whole lot of route changes. I don't know if there is any way to store additional map data beyond buying nav software that stores all data on the sd card. Frankly, I'm of the opinion that Samsung made the Moment a piss poor and unreliable navigator and I don't trust it as my main nav unit. You can find PNA's new for $75 and much less used that are far more reliable. Use the Moment for traffic avoidance and poi's or if you have a wife with a Pre, use that as it works much better too.
