What are the conditions to get a lock with the Javelin Missile System?
There are multiple ways that you can achieve a solid lock with the Javelin Missile System. Take a look at a similar postings about it here.
SOFLAM:
The recon class has the ability to to use the SOFLAM which is a laser designator which, when used properly, can increase the targeting of the Javelin to roughly 1500 Meters as mentioned by Tony
Check out this previous post by Angel.King.47 here that mentions:
Do look at this post: http://forums.steampowered.com/forums/showthread.php?t=2238182
When used in conjunction with the SOFLAM, a Javelin shooting at a "Painted" target will do 3x the normal damage
Laser Designator:
Tanks and Helicopters can use the unlock of the Laser designator, which again increases the range of the Javelin to roughly 1500 meters.
Otherwise:
Tony is correct and through my personal experiences the range of the Javelin alone is between 0-300 meters. My recommendation would be to party up or communicate with other players to achieve maximum range and damage while using your Javelin.Note: most recon players with put down a Soflam on request (through my experiences). I mean who doesn't love more XP?
Stare at a vehicle long enough to get a lock; this has a fairly limited range. I've read this has a max range of 250-300 meters. From experience this seems about right but I have no source for this. This also requires vision of the vehicle.
According to this a javelin has a max range of 1500 meters. The only way for someone to lock on at this distance is with assistance of a laser designator, this can be a SOFLAM, tank, etc. In this way, there is no need to be able to see the target.