Here's what occurs on the controller's display when two radar sites are allowed to display the data from the same aircraft at the same time. The slash marks are the targets that represent the airplane. If two radar sites are utilized for display in a given sort box, and the aircraft is detected by both of those sites, two targets will appear, very close to each other. Concentrate on the target symbols (the slash marks and the V symbols). The top left target is what we call a tracked target with a full data block (in this case, the full data block shows that this target is Crown Airways flight 3234). However, with two targets, it looks like two airplanes are in formation flight, westbound. But it is really only that one aircraft, displayed differently by two different radar sites. The bottom target is that of an aircraft flying under VFR, with its transponder set to reply on code 1-2-0-0, and it is reporting an altitude of 4,500 feet. Because the aircraft is detected by both the preferred and the alternate preferred radar sites like the one above it, it also appears that two airplanes are flying in formation, westbound. If the radar sort box only used one radar site for display as is shown on the right side (Single Preferred), the target symbols would appear clean and crisp, showing only one aircraft each (assuming, of course, that the preferred radar actually detected the aircraft). Double Preferred adaptation is specifically kept to an absolute minimum because of this problem of "clutter."
So, why aren't the targets in the exact same location you ask? Are the radar sites that bad at determining an aircraft's point in space, that they don't agree? No, the radar sites are actually pretty darn good, at least from my point of view behind the scope. For example, when I observe an aircraft inbound on an ILS, the center of the target is usually right on the center of the localizer. Also, I can't help but feel that determining an aircraft's position would be much more accurate when two or more radar sites are involved. But it isn't done that way...apparently because the computers can't crunch all that real-time data. Remember, the way it is done to this day, most of the data goes unused...
So, again, why do the targets appear displaced from each other, when in fact they are one in the same airplane?
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