 Opening a portal without preparation will give him access to another dimension completely at random.He seems to possess some talent for finding dimensions he has been to before, yet he could not immediately find Earth again once he first got lost in the dimensional planes.Presumably, then, Portal has a kind of extrasensory "marking" ability, allowing him to record the space/time coordinates of a dimension while he is present in it, so that he could return to it again in the future if he chooses.