When you lose your GPS connection while flying under a metal structure it is because the GPS signal was blocked by the structure. Flying under or through bridges, docks or whatever is not a good idea. You will more than likely lose your GPS signal. Even if the RTH function kicks in while you are under a structure, the drone would attempt to fly straight up to achieve the RTH altitude and the drone would then crash into whatever you were flying under. In any case as such, the drone is doomed. Sorry you lost your Spark though.