How to use the guided missile unlock for tanks?

Solution 1:

Guided shell is a perk that requires teamwork to use effectively (or at all). You need a teammate using a laser designation device (SOFLAM, CITV station (last tank unlock), laser painter (on jets and choppers)) before you will be able to lock on to anything in the air or out of line of sight. You can lock onto land vehicles in line of sight.

Until that point, it is just a lovely piece of useless machinery.

Solution 2:

In your first round, some one on your team must have been laser designating your opponents. That is the only way the guided missile unlock can target air vehicles.

Air targets must be laser designated before you can target them with the guided shell perk.