it's according to the type of mission what the requirements are and the stock game isn't too clear on that sometimes. Some missions are maintain a station for a length of time. Some missions are sink shipping, a certain amount of tonnage. Some missions call on you to take photos or deliver a spy to shore. It's crucial to ID they type of mission requirements you must fulfill and do it.
To muddy the waters further, missions have the option to allow or not allow you to go outside the radius and then return to complete the mission. Of course the stock game doesn't tell you what's expected of you. In practice, just chase the battleship, put it on the bottom and don't worry about it.
When you ask for another mission you'll get the same one again and can do it right. Fine trade for a battleship!
