RCS says how "big" something looks to a radar. Shape and material count for far more than actual size: an inside corner, like the corner of a box, reflects the energy straight back, while an angled surface deflects it away.
In practiceSailing boats and life rafts carry a small radar reflector so that ships see them on their radar. Stealth aircraft such as the F-35 do exactly the opposite: angled surfaces and aligned edges deflect the energy away, so that a large aircraft looks small to the radar.
The Radar Cross Section (RCS, symbol σ) is the measure of how much radar energy a target reflects back to the radar, expressed in m². It is an equivalent area: a target with σ = 5 m² reflects as much as an ideal sphere that would intercept 5 m² of radar energy and scatter it evenly. RCS is not a physical area — a target can "appear" much larger or smaller than it is.
A target's RCS depends on:
- Size: larger objects generally reflect more.
- Shape: flat plates and corners perpendicular to the radar reflect enormously (corner-reflector effect); rounded and angled surfaces deflect the energy away from the radar — the principle behind stealth shaping.
- Material: metal reflects strongly; radar-absorbent material (RAM) converts radar energy into heat.
- Aspect angle: the RCS of the same aircraft can vary by tens of dB between the frontal, side and rear aspects.
- Radar frequency: stealth shaping is optimised against particular bands (usually X-band); against long wavelengths (VHF) it works much less well.
| Target | Typical RCS (order of magnitude) |
|---|---|
| Insect / bird | 0.00001 – 0.01 m² |
| Stealth aircraft (frontal) | 0.0001 – 0.01 m² |
| Cruise missile | 0.1 – 1 m² |
| Human | ~1 m² |
| Fighter aircraft | 1 – 10 m² |
| Bomber / transport aircraft | 10 – 100 m² |
| Frigate | 1,000 – 10,000 m² |
| Corner reflector (physically small) | can "appear" > 1,000 m² |
Reducing RCS (stealth) shortens the radar's detection range; increasing RCS is used deliberately: corner reflectors and Luneburg lenses make training targets, drones and decoys look as large as a real aircraft on the scope. Because detection range scales with the fourth root of the RCS (Inverse square law), an RCS reduction by a factor of 10,000 yields "only" a 10× shorter detection range — but that is often exactly enough.