Under the Horizon

Three ways the ground hides a drone

How far a radar can see over a round Earth

Radar travels in nearly straight lines. In a normal atmosphere the beam bends slightly downward, which radar engineers model by treating the Earth as if its radius were four-thirds of its true value.Wikipedia With that model, the distance to the radar horizon works out to roughly:

d ≈ 4.12 × (√h_radar + √h_target) km, with both heights in metres.

That gives these limits (our arithmetic, smooth Earth):

• Radar on a 10 m mast, drone at 10 m: about 26 km.
• Same radar, drone at 30 m: about 36 km.
• Same radar, drone at 100 m: about 54 km.

Beyond those distances the drone is in the Earth's shadow, however powerful the radar. Raising the radar helps, which is why air-defence radars sit on masts, hills and aircraft. But the target's own height counts just as much, and the attacker decides that.

Hills, trees and buildings

The horizon figures assume a billiard-ball Earth. Real ground is not like that. Every ridge, tree line, building and embankment casts a radar shadow behind it, and a drone flying a few tens of metres up can stay inside those shadows for most of its route. Shadows in the open may be kilometres long. In a town or a forest the radar's view of low targets can shrink to a few hundred metres.

Siting therefore matters as much as performance. A radar on a hilltop sees further but looks down on more ground and more clutter. A radar in a valley is shielded from clutter but blind beyond the next ridge. The Humvee-mounted counter-mortar radar in the photo above, built to detect mortar and rocket fire from 500 m to 10 km, shows the compromise: a compact sensor on a mast that can be moved to the best vantage point.Wikimedia Commons

Clutter and ghost reflections

Even inside line of sight, a low drone is hard to see. When the radar beam points low, it hits the ground as well as the target. Unwanted echoes are called clutter: from ground, sea, rain, animals and insects. How strong surface clutter is depends on the grazing angle of the beam, the frequency and the polarisation.Wikipedia A hillside or a town at the same range as a 0.01 m² drone returns a far larger echo, which can bury the drone completely unless the radar can separate the two by motion, the subject of Part 4.

The ground also acts as a mirror. Energy can reach a low target directly and also by bouncing off the surface. The two paths interfere, so the echo can strengthen or almost cancel as the drone changes height or range. Ground-reflected multipath echoes are especially troublesome because they appear to move like real targets.Wikipedia A low drone can therefore fade in and out of view, and generate ghosts, even in clear line of sight.

Low flight needs no special technology. It turns the planet, the hills and the ground's own echo into the drone's camouflage.

High or low, the attacker chooses

Ukraine's air war shows the trade playing out. Low flight hides drones from radar but brings them within range of guns. Ukraine's mobile fire groups, gun crews on pickup trucks, were built to exploit exactly that. In 2025 Russian forces began launching Shahed-type drones at 1.5–2 km altitude, high enough that "ground-based interception efforts" became "far less effective", according to an analysis by the Ukrainian outlet Texty.Euromaidan Press At that height the drones are easier for radar to see but out of reach of machine guns. The same analysis reports Russian decoys carrying devices that make them look like much larger aircraft on radar.

The pattern is the point. An attacker can pick the altitude that beats the defender's sensors and weapons, high against guns and low against radar. A defence that relies on one band of altitude will be flown around.

What radar can and cannot do about it

Radar designers have several answers, all partial:

• Height. Masts, towers, aerostats and airborne radars push the horizon back. They cost money and are visible targets themselves.
• More radars. A dense net of short-range radars fills shadows that one long-range radar cannot. The cost multiplies with the number of sites.
• Doppler processing. Separating moving targets from stationary ground clutter by their motion is the main tool. It has its own blind spot for slow and hovering targets, the subject of Part 4.
• Higher frequencies and finer beams. These reduce the patch of ground competing with the target, at the cost of range in bad weather.

None changes the basic geometry. Below the line of sight a radar sees nothing, and close to the ground it sees a great deal it does not want.