Hi tansis,
This sounds very interesting.
It is difficult to figure out what you want to do,
but it seems that you want to make some kind of mechanism
which will direct a security camera toward a detected
motion.
For high speed movements, you could use pneumatic piston
type drives, they can track a fly.
For the movements that you want to detect, you could
consider using the image display, they make software that
will pick out various shapes, such as a vehicle or a
person. Such software is quite sophisticated, and you
could probably set it to watch the nearest or the farthest.
Making a sonar based system sounds very difficult.
If you want to track something that moves around very quickly
the picture refresh time might be a problem.
If the area you are scanning has other motions in its field,
these could be a problem, such as leafy branches blowing
about, or washing on the line, or just nearby traffic.
Let us know how you get on,
Cheers, John