Before we go on to understand how this problem is very sinister and therefore difficult to detect, I'm going to explain the difference between a tracked target and an untracked target.
A tracked target is associated with an aircraft to whom the controller is providing radar service. In most cases this is an IFR aircraft, but it can also be a VFR aircraft receiving radar traffic advisories. In either case, the tracked target has a Full Data Block.
In the above example, the target on the left has a Full Data Block (FDB). This full data block carries along information like identity, altitudes, speed, etc. In the above FDB, the aircraft is Crown Airways flight 3234. In this example the aircraft is assigned an altitude of zero, which is shorthand a lot of us controllers utilized to show that the aircraft was "cleared for the approach." The mode C altitude data, as received via this aircraft's transponder, reports that the aircraft is at an altitude of 2,800 feet. The 189 is simply a computer identification number, and the 140 represents the ground speed as determined by the radar tracking program. This aircraft is traveling eastbound, and there are five "history" target indications that show where the aircraft has been in the last minute.
An untracked target is typically associated with an aircraft that is flying under Visual Flight Rules (VFR), and has its transponder set to reply with a code 1-2-0-0. Its target is represented by the letter V. It has a Limited Data Block (LDB), so the only thing it shows is the altitude as reported by via the aircraft's transponder. It is reporting at an altitude of 2,800 feet. This aircraft is proceeding southwest bound, as can be ascertained by the five history targets. These aircraft are in close proximity, and if the controller is in communication with the pilot of CRO3234, he'd be saying something like this (with lots of inflection)...
"Crown thirty two thirty four, TRAFFIC ALERT! Traffic ten o'clock, one mile, southwest bound, altitude indicates two thousand eight hundred. Suggest you climb IMMEDIATELY!!!"
If the pilot of the Crown Airways aircraft didn't already have the traffic in sight and know that he could safely pass by the aircraft visually, he'd probably follow the controller's advice and execute an immediate climb.
Okay, so other than the full data block versus the limited data block, what's the big deal?
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