A radar cannot see what is behind a hill — any more than you can see round a corner. Whoever uses that terrain cleverly stays out of sight.
In practiceAttack helicopters therefore fly nap-of-the-earth: low through valleys and along tree lines, coming up only briefly (pop-up) to look or to fire. The defender does the opposite and puts radars on hilltops or tall masts.
Besides the curvature of the earth (Radar horizon), the terrain itself also blocks the line of sight. Terrain masking is the deliberate use of mountains, hills and valleys to stay out of reach of an enemy radar. It is one of the oldest and most effective tactics against ground radar — and it comes in two forms.
Direct terrain masking
The terrain stands completely between radar and target: the target sits in the radar shadow behind a ridge. There is no line of sight, so no detection — regardless of radar power. A combat aircraft or helicopter flying through valleys and behind ridges ("nap-of-the-earth") thus makes itself literally invisible until it crosses the crest.
Indirect terrain masking
Here there is (partial) line of sight, but the target "hides in the clutter": by flying very low over the terrain, the target's echo merges with the strong ground clutter of the surrounding ground (Ground clutter). The radar does receive a signal, but cannot distinguish the target from the returns of hills, woods and buildings — unless it has good Doppler discrimination (Pulse-Doppler radar and Moving Target Indicator). So the target is not blocked by terrain, but masked by the background.
Direct = no line of sight (terrain blocks). Indirect = line of sight, but the echo drowns in ground clutter. Against direct masking only height helps (airborne radar, AWACS); against indirect masking, pulse-Doppler processing helps by pulling moving targets out of the stationary clutter (Pulse-Doppler radar and Moving Target Indicator).