ECE 495 — Electromagnetic Warfare and Survivability

By the end of this lesson, you will be able to:

• Define RCS as an effective echo area and place targets on the dBsm ladder
• Decompose $\sigma$ into geometric cross section, reflectivity, and directivity
• Rank canonical shapes and explain why directivity dominates
• Read an aspect pattern and defend the median as the honest single number

$\sigma$ is the target's effective echo area — the size of the fictitious perfect reflector that would return the same power.

• It is not physical size: a shaped 50,000-lb bomber can echo less than a bird
• Logarithmic currency: dBsm (0 = 1 m$^2$): insects $-40$, fighters $+7$, ships $+30$
• Like L22's $J$, it depends on aspect, frequency, and polarization — it is a function, not a number

• $A_\text{geo}$ — the intercepted power: how much of the wave the body catches
• $\Gamma$ — the fraction re-radiated: materials live here (L27's RAM attacks this knob)
• $D$ — how much of the echo goes back at the radar: shaping lives here

Same physical size, wildly different echoes — directivity at work.

A 1 m$^2$ plate at X-band echoes $\sim$14,000 m$^2$ broadside — and is gone within degrees.

Two flat plates are survivable. Two flat plates at a right angle are not.

• A dihedral retroreflects: double-bounce geometry sends energy straight back over a wide range of angles
• Add a third face and the trihedral returns fire from almost anywhere — which is why they make good calibration targets
• Aircraft grow corners by accident: tail-fuselage junctions, open bays, pylons, and inlets (a cavity is a corner with a grudge)

Plot $\sigma$ around the compass: narrow specular spikes, tens of dB tall, where a facet faces the radar — and a deep, noisy floor everywhere else, where edges and traveling waves whisper.

$\sigma$ carries the radar's fingerprints, not just the target's.

• Optical regime ($L \gg \lambda$): shape rules — everything today assumed this
• Resonance regime ($L \sim \lambda$): the whole body rings; shaping tricks lose their grip
• Rayleigh regime ($L \ll \lambda$): only gross size matters
• Polarization matters too: edges and wires favour one polarization over the other

Why fight for every dB? Because Block 1's fourth-power law now pays us: $R_{\max} \propto \sigma^{1/4}$.

Same site, same radars, same waveforms. One number changed — and every ring is drawn to the same scale as before.

Type this with me — L25_RCSPattern.csv sits beside the script in the code bundle.

M   = readmatrix('L25_RCSPattern.csv');
az  = M(:,1);  sig = M(:,2);
nose = (az <= 30) | (az >= 330);
s    = sig(nose);
mn   = 10*log10(mean(10.^(s/10)));
fprintf('nose: max %.1f  med %.1f  mean %.1f dBsm\n', ...
        max(s), median(s), mn);
fprintf('gap (mean - median): %.1f dB\n', mn - median(s));
parts = [0.03 0.004 0.012];      % three contributors, m^2
tot   = sum(parts);
fprintf('budget: %.3f m^2 = %.1f dBsm\n', tot, 10*log10(tot));

The nose mean and median agree. Which sector breaks that, and by how much?

Then the budget: drop the smallest contributor. How far does the total move?

A flat access panel on a fuselage side, seen by an S-band search radar, with the range’s calibration sphere for scale.