What is Radar Cross Section?
Radar cross section is the electromagnetic equivalent of a target's visual size. Formally, it is the area of a perfectly reflecting sphere that would return the same power as the actual target. A large commercial aircraft might have σ = 100 m², while a stealth fighter can achieve σ = 0.001 m² — a 50 dB (100,000×) difference that translates to a 17× reduction in detection range. Understanding and controlling RCS is fundamental to both radar design and stealth technology.
Scattering Regimes
Target RCS behavior depends on the electrical size parameter ka = 2πa/λ. In the Rayleigh region (ka << 1), the target is much smaller than the wavelength and RCS scales as (ka)⁴ — a strong frequency dependence that makes low-frequency radar nearly blind to small objects. In the resonance region (ka ≈ 1), creeping waves circling the target create complex interference patterns. In the optical region (ka >> 1), RCS approaches the physical cross section and is dominated by specular reflections from surfaces perpendicular to the radar line of sight.
Stealth Design Principles
Reducing RCS is the foundation of stealth technology. The primary technique is shaping: flat surfaces are angled to deflect energy away from the transmitter, edges are aligned to concentrate scattered energy into a few narrow angular sectors, and curved surfaces are used where possible to spread reflections. Radar-absorbing materials provide additional 10–20 dB reduction by converting electromagnetic energy into heat. Modern stealth aircraft combine these approaches to reduce RCS by 30–40 dB across threat radar bands.
RCS Measurement and Prediction
RCS is measured on outdoor ranges, in anechoic chambers, or on compact ranges using scaled models. Computational electromagnetics — method of moments, physical optics, finite-difference time-domain — can predict RCS for complex targets, though full-wave methods for electrically large objects remain computationally challenging. The aspect-angle dependence of RCS creates characteristic signatures used in non-cooperative target recognition.