The visual effect of constantly circling the globe is great and very well done!
I didn't understand the 'must lock to fire' mechanic at all. Would not have figured it out if not for medicinestorm's comment. Once I realized how it worked though, I kind of dug the mechanic. It creates a nice tension as you track the enemy hoping to get a lock before they make it off screen. A similar result could be achieved by giving the enemies a big health bar that depletes slowly as you shoot it. That way the player has the satisfaction of being able to shoot whenever they like but they still need to track the enemies for a good bit in order to destroy them.