Most are described in the AIM, for those with a desire to pursue the issues on their own. To start with, there are two radar sources. The secondary displays overlay the primaries on the screen, and either can be dimmed or brightened in order to best suit controllers needs. In order to begin understanding the limitations of traffic advisories, it is best to have a clear picture of what the controller sees. Assume you are flying in Class E airspace, receiving radar service.
Primary returns are even less reliable when the aircraft are fabric covered or made of plastic.
Don t count on any calls about these. In between sweeps, aircraft information on the screen remains static, despite the fact that the aircraft themselves clearly continue to move. It takes significant skill and experience to translate the result into a smooth mental projection of movement and position. Because the radar screen displays the area radiating out and away from the radar antenna itself, the area closest to the center of the screen tends to get cluttered. Display information there is reduced in size to help with this problem, but deciphering the signals remains a challenge for controllers.