BTW, a couple of things we tend to take for granted:
Initial conditions- When picking up a new contact, that contact is travelling on a constant course and speed. The contact is also
closing in terms of relative distance.
Constant course and speed is just the way ships operate 95% of the time. You started out somewhere and you want to go somewhere else.
Always assume the contact is closing in terms of distance. Your scopes were clear ten minutes ago and now they.. aren't.
Something changed, in this case the contact is getting close enough to be detected.