TERCOM is not some super secret god-like tech. Working principles behind it have been well known for a long time. The name itself tells you half the story. One needs detailed and current data of the flight route area, target area and of the target itself. Then you need a good set of sensors feeding the actual images into the missile's computer where the new set of images is compared with the stored set of images. A very rudimentary version of tercom could probably be done in some nerd's garage with a thousand bucks budget. It couldn't guide a missile but would probably be able to tell a remotely controlled model car how to navigate itself through a obstacle free apartment. With more time to practice and experiment, tons more money, and good experts i would say it's a given a decent TERCOM guidance is not used by just the western powers.