Phone camera parts would be overkill. The Javelin sensor isn't nearly that high-resolution, we're talking low triple digits in "pixels". It does however refresh its readings very fast, a necessity given its speed. The old Javelin used active cryo and a filtered IR imager, the new one is passive like the IR camera in some phone attachments. It is stupidly simple in operation: CLU provides the target "signature" and the imager seeks it. After the initial ascent in top-down mode, a stronger signal on one edge of the sensor pushes the control surfaces in the opposite direction until it strikes its target. I'd give the Wikipedia page a read. It contains a surprising amount of information that informs the design and thought behind the missile. Military systems are cool for how robust yet simple they are.
