The difference lies in the number of coordinates measured (see Azimuth, elevation and range):
- A 2D radar measures azimuth and range. The antenna has a fan beam: narrow in azimuth (precise direction), wide in elevation (everything from low to high is illuminated, but without knowing at what height). Simple and reliable; standard for air traffic control and many surveillance tasks.
- A 3D radar measures azimuth, range and elevation and calculates the target's height from them. This can be done in several ways: a stack of narrow beams one above the other (stacked beam), a beam scanning electronically in elevation, or a phased array that steers a pencil beam per target.
height ≈ R × sin(El) + corrections
corrections for earth curvature and atmospheric refraction; R = slant range, El = elevation angle
Why height matters militarily: for air defence, a target's height determines which weapon can reach it, how the intercept has to be flown and whether the target is a threat (an airliner at 11 km flies differently from a cruise missile at 50 m). In the past a 2D search radar was combined with a separate height finder that "measured up" one target on command; modern 3D radars do it all at once for hundreds of targets.
Remember
2D = where and how far. 3D = where, how far and how high. The price of 3D is complexity: the energy has to be divided over several elevation beams, at the expense of range or scan speed — or the system becomes (much) more expensive.