The target's effective echo area — the size of a fictitious perfect reflector that would return the same echo power. It is not physical size, and it varies with aspect, frequency, and polarization.
dBsm — decibels relative to one square meter. \(0\ \text{dBsm} = 1\ \text{m}^2\). The log scale is used because RCS spans many orders of magnitude.
Insect \(\approx -40\) dBsm, bird \(\approx -20\), human \(\approx 0\), fighter \(\approx +7\) (~5 m²), ship \(\approx +30\) to \(+40\), LO "marble-class" aircraft \(\approx -30\) dBsm.
\(\sigma = A_\text{geo} \times \Gamma \times D\): geometric (projected/intercepted) cross section, reflectivity (fraction re-radiated), and directivity (fraction of the echo aimed back at the radar).
Directivity \(D\) — and shaping attacks it. Geometric area and reflectivity only buy factors; directivity buys orders of magnitude, so shaping is the foundation of LO design and absorber is the finishing coat.
Broadside, \(\sigma = 4\pi A^2/\lambda^2\) — a huge specular spike that collapses within degrees off-normal (loud but narrow). A 1 m² plate at X-band (\(\lambda = 3\) cm) gives \(\approx 14{,}000\) m² (~+41.5 dBsm).
Its RCS is \(\sigma = \pi r^2\) and is aspect-independent — no directivity spike — so it returns a known, constant echo from every angle.
Its double-bounce geometry retroreflects energy straight back at the radar over a wide range of angles, whereas a plate is loud only in a narrow specular spike.
Tail-fuselage junctions, open weapons bays, pylons, and inlets (a cavity acts as a corner). First commandment of shaping: do not present a right angle.
Optical (\(L \gg \lambda\), shape rules), resonance (\(L \sim \lambda\), the whole body rings and shaping loses grip), Rayleigh (\(L \ll \lambda\), only gross size matters, \(\sigma \sim f^4\)).
At VHF the wavelength is comparable to airframe features, dragging a fighter-sized shaped target toward the resonance regime, where optical-regime shaping tricks lose much of their leverage.
Narrow specular spikes (tens of dB tall, at facet normals) over a deep, noisy floor of edges and traveling waves. The max lives in the spikes; the median describes the floor a search radar sees most of the time.
Averaging must be done in linear m² — \(10\log_{10}(\overline{10^{\sigma/10}})\). A few narrow spikes drag the linear mean many dB above the median; the median tracks the floor a search radar actually sees, so it is the honest planning number.
\(R_\text{max} \propto \sigma^{1/4}\). Cutting median RCS 10 dB drops range to ~56%; 30 dB to ~18%. Detection area falls as \(\sigma^{1/2}\), collapsing IADS rings. Stealth does not make you invisible — it shrinks the rings.