[HELP] Getting Targets on Perspective

[#MacTavish]

hi all, i created a script that works for shooting other vehicles with heatseeking rockets by Hunter Helicopter. But i can shoot other vehicles if they are on my screen, so i can shoot other vehicles even they are behind of hunter helicopter and this is unrealistic. I want to make it realistic, so, i want to shoot other vehicles if they are only on Helicopters' perspective even when i don't look at them, here is the script, please help ; 

function shootProjectile()
    local vehicle = getPedOccupiedVehicle(localPlayer)
	if vehicle and getElementModel(vehicle) == 425 then
		local x, y, z = getElementPosition(getLocalPlayer())
		local vehicles = getElementsWithinRange(x, y, z, 300, "vehicle")
    	local target = vehicles[1] ~= vehicle and vehicles[1] or vehicles[2]
		
		if target and isElementOnScreen(target) then
			createProjectile(vehicle, 20, x, y, z, 1, target)
		end
	end
end
bindKey( "lshift", "down", shootProjectile)