Lesson 25 Flashcards#
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1. What is radar cross section (RCS)?
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.
2. What are the units of RCS, and what is 0 dBsm?
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.
3. Place these on the dBsm ladder: insect, bird, human, fighter, ship, LO aircraft.
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.
4. State the three-factor rule for RCS and name each factor.
\(\sigma = A_\text{geo} \times \Gamma \times D\): geometric (projected/intercepted) cross section, reflectivity (fraction re-radiated), and directivity (fraction of the echo aimed back at the radar).
5. Which of the three RCS factors buys orders of magnitude, and what design technique attacks it?
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.
6. Give the peak-RCS formula and behavior for a flat plate.
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).
7. Why is a sphere the calibration reference target?
Its RCS is \(\sigma = \pi r^2\) and is aspect-independent — no directivity spike — so it returns a known, constant echo from every angle.
8. Why is a corner (dihedral/trihedral) more dangerous to LO than a flat plate?
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.
9. Where do aircraft accidentally grow corners?
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.
10. Name the three scattering regimes and what governs RCS in each.
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, \(\sigma \sim f^4\)).
11. Why are VHF early-warning radars a counter-stealth capability?
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.
12. What does an RCS aspect pattern look like, and where do max and median live?
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.
13. Why must the mean RCS be computed in the linear domain, and why prefer the median?
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.
14. State the fourth-power detection-range law and what a 10 / 30 dB RCS cut buys.
\(R_\text{max} \propto \sigma^{1/4}\). 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.