WingFlaps wrote ...
As fas as I know ATC radar picks up as many moving
targets as it can "see". Not sure what you mean by primary tho'.

OK then, time to pick up the AIM and look at Chapter 4, Section 5.1,
or look here (courtousy of the gov't nobody wants to pay for) in 4.5.1:

http://www.faa.gov/airports_airtraff...ns/atpubs/aim/

Check for the diff between Primary and Secondary radar.

FYI: Before 1998 the controllers in NY Center would routinely keep the
primary intensity dialed way down on their scopes, to a point where primary
targets could not be seen unless you were really looking for one. The reason
was too many the false reflections (heck, trucks on the bridges and
interstate overpasses would show up). I don't know much after they moved
onto the new scopes, and I can guess that after 9/11 it's all different now
(maybe).