Typical is a 3D system where 2 antennas handle the azimuth and bearing detection individually. The bearing/distance antenna axis is horizontal and the azimuth antenna axis is vertical. The newest digital 3D systems can detect a target's azimuth position by ramping the pulse frequencies and correlating small changes in the returning beam angle which can then be used to compute altitude. Only one antenna rotating in the horizontal plane is needed.
Well, if you count 30 years as new, perhaps.
3D planar array radars work by generating multiple height stacked beams; older types would use 4-10 transmitters and be relatively inaccurate, later modules use hundreds of Tx modules in the actual array, but the basic principle is the same.
An aircraft is painted by only some beams, using trigonometry the height is calculated for each beam and averaged. Radar beams bend up or down, hence anaprop; an aircraft can also fail to paint in a higher or lower beam, pushing the average up or down. Suffice it to say that the average accuracy is about +/- 5000ft. When 2 aircraft are in close proximity, obviously the comparative height is much more accurate, perhaps 1-2000ft, but for safety a minimum of 5000ft should be used for separation, and the blips should never be allowed to merge.