In article ,
(blanche cohen) wrote:

Um....Bob & Chip...could you explain in more detail the reason
for the "turn into traffic"? I'm having problems visualizing
it. And I have the most horrible feeling that someday I'll run
in the same problem and want to understand it.

thanks


That one's easy.

Let's say you've got two objects moving in space (doesn't matter if
they're airplanes, boats, or cars). They are on intercept courses,
something like this:

#1------

^
|
|
#2

Assume for the moment you're object #2, trying to decide how to avoid
hitting #1. Assume that #1 is going to keep moving at a constant speed
and direction. Further assume that the problem is reduced to 2D (i.e.
nobody can change altitude).

You've basicly got 4 choices: you can speed up, slow down, turn left, or
turn right. In boats or planes, speed changes usually happen pretty
slowly, so they're really not effective for short-term collision
avoidance. This leaves you with turn left or turn right.

Let's say I make an 45 degree turn to the right (away from the target).
Now the situation is this:

#1------

^
/
/
#2

Quick, is this an intercept situation? Can you work out in your head if
#2 will cross in front of #1? They're still moving towards each other,
so it's hard to tell.

But, what if #2 turned to the left, toward the target? Now you've got:

#1------

^
\
\
#2

It should be obvious in the picture above that #2 is going to pass
behind #1. Look at the arrow heads (the lengths of the arrows are how
far each target will move in, say, 1 minute). By the time #2 gets to
the head of its arrow, #1 has already moved from it left side to its
right side.

The basic rule is that if you're aiming right at a crossing target,
you'll never hit it, because while you're moving towards it, it's moving
cross-wise to you and will move away from where you're aiming. The
correlary of this is that if you *want* to hit the target, you need to
lead it (this is the "firing solution" stuff they're always doing in
submarine movies when they launch a torpedo).