Surely a modern radar must be technically able to do this?
Military radars are able to give an
approximation of the altitude based on primary.
We had once in France (last year, IIRC) an aircraft with a false altitude report (mode C).
When the pilot said he wasn't sure about his real altitude (FL 270 ?), controller asked the military for a check. They used their primary radar and said the actual level was between 250 and 290.
Great approximation, surely precise enough to lead an interceptor to a target : the plane was actually at FL 290, which was confirmed by a passing-by traffic who had to avoid it at the last second.
Modern technology radars are very precise : they were right in their calculation based on primary (
), but this is not enough to provide any vertical separation, ATCwise.
As there is a doubt, the result is inevitably a lateral avoidance clearance.