"C J Campbell" wrote in message
...
Rely on relative motion. If the dot isn't moving relative to your
viewpoint, you're on a collision course.

Not necessarily. The dot could be moving directly away from you.

That's still a collision course. It will just take a lot longer (and a lot
more fuel) for you to eventually collide). You'll either catch up, or the
other aircraft will eventually fly all the way around the world and catch up
with you.