ECE 495 — Electromagnetic Warfare and Survivability

Block 3: IR EW and Signature Management
Lesson 22: IR Emitters

Juan Jurado, Col, USAF, Ph.D.
Permanent Professor and Head
Electrical and Computer Engineering
Fall 2026

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Block 3 · Lesson 22 · Where We Left Off

Where We Left Off

L21 gave you the physics: everything warm glows, hot parts peak in the MWIR and cool ones in the LWIR, and a thermal sensor lives on contrast.

Today we point that physics at the aircraft itself. What, exactly, is glowing?

Today

Five components, one rule to rank them, and how the winner changes with band, aspect, and time of day.

Block 3 · Lesson 22 · Objectives

Lesson Objectives

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

Name the five components of an aircraft IR signature and place each in its band
Rank components with a light in-band intensity rule: emissivity, area, and temperature
Explain how aspect and look-geometry change which component dominates
Predict the thermal crossover where the skin term vanishes in the LWIR

An aircraft glows whether or not anyone is asking.

Block 3 · Lesson 22 · Part One

What Is Glowing?

Break the airframe into parts, then rank the parts. Three of them radiate their own heat; two only borrow someone else's.

Block 3 · Lesson 22 · The Five Components

The Five Components of an IR Signature

Component	Mechanism	Primary band
Hot parts (nozzle, turbine)	emission	MWIR
Exhaust plume (CO$_2$, H$_2$O)	emission	MWIR ($\sim$4.3 µm)
Airframe skin (aero-heated)	emission	LWIR
Reflected sunlight	reflection	VIS–MWIR (day)
Reflected sky / earthshine	reflection	MWIR / LWIR

Three emitters (the aircraft's own heat) and two reflectors (borrowed light)
The first two shout in the MWIR; the skin owns the LWIR; only the solar reflection is day-only

Block 3 · Lesson 22 · Ranking Rule

How Loud Is Each Part?

What a band-limited sensor collects from one part

$$I_{\text{band}} \;\propto\; \varepsilon \,\cdot\, A \,\cdot\, f_{\text{band}}(T) \,\cdot\, \sigma T^{4}$$

Factor	Name	What it does
$\varepsilon$	emissivity	How efficiently the surface radiates — coatings can lower it
$A$	projected area	What the sensor actually sees: this is where aspect enters
$f_{\text{band}}(T)$	band fraction	How much of the glow lands in your band — Wien decides
$\sigma T^{4}$	total output	Explodes with temperature, so hot parts win per unit area

Temperature, area, emissivity — change any one and you change the signature.

Block 3 · Lesson 22 · The MWIR Stars

Hot Parts and the Plume: the MWIR Stars

Hot parts

The nozzle and turbine face run hot ($\sim$700–900 K). Small area, but the $T^4$ term makes them brilliant in the MWIR.

Plume

Exhaust gas radiates in the CO$_2$ band near 4.3 µm, squarely in the MWIR; large but diffuse.

Both are strongest from the rear: a tail-chase sensor looks straight up the tailpipe
This is why classic heat-seekers are MWIR and love the stern hemisphere

Tail-on, an engine is the loudest thing in the sky.

Block 3 · Lesson 22 · The Skin

The Airframe Skin: the All-Aspect LWIR Term

Aerodynamic heating warms the skin; even unheated, the skin radiates at near-ambient temperature in the LWIR.

Lower intensity than the plume, but spread over a large area and visible from every aspect — not just the tail
Speed drives it: a fast jet's skin can run hot, while a subsonic penetrator keeps the skin term modest
This is the component you cannot point away from — it sets the broadside and head-on signature

You can point the tailpipe away. You cannot point the skin away.

Block 3 · Lesson 22 · Reflections

The Reflections You Forget

The skin does not only emit — it reflects:

Sunlight

By day, reflected solar in the MWIR can rival a warm graybody; in the LWIR it is negligible.

Sky and earthshine

Deep sky is very cold, so upward-facing surfaces read cold; the warm ground lights the underside from below.

Reflections flip with geometry: look-up, look-level, and look-down are three different problems.

Block 3 · Lesson 22 · Part Two

From Where, and When

The same aircraft has many signatures. The sensor's vantage picks one — and so, twice a day, does the clock.

Block 3 · Lesson 22 · Aspect

It Depends on Aspect and Geometry

The same aircraft has many signatures — the sensor's vantage picks which:

From the sensor	Stern aspect	Beam / nose aspect
Dominant component	Plume and hot parts	Airframe skin
Band	MWIR	LWIR
Why	The tailpipe is in view	The fuselage masks the hot parts

Look-up (target on cold sky): high contrast, easy detection. Look-down (target on warm, cluttered terrain): low contrast, hard — this is the penetrator's friend.

Block 3 · Lesson 22 · Thermal Crossover

The Diurnal Trap: at Midday the Target Reads Cold

Skin and background do not heat and cool together. An LWIR sensor sees only the gap between them.

terrain hotter
target reads COLD

Block 3 · Lesson 22 · Thermal Crossover

At Night It Flips — and Twice Between, It Vanishes

At night the terrain radiates to the cold sky and drops below the skin. The same gap, the same size, the other sign.

skin hotter
target reads WARM

Twice a day, the airframe vanishes into its own background.

Block 3 · Lesson 22 · Where This Is Headed

Where This Is Headed: Every Component Has a Counter

You manage an IR signature component by component — that is Lesson 24:

Component	The counter
Hot parts	Shield or block line-of-sight to the nozzle; avoid afterburner
Plume	Mix exhaust with bypass air to cool it; route it over the airframe
Skin	Low-emissivity coatings and paints
Reflections	Manage geometry and finish to kill sunglint

For an LO bomber like the B-21, these choices are baked into the airframe, not bolted on.

Block 3 · Lesson 22 · Quick Exercise

Quick Exercise / Type-along: Find the Crossover

Type this with me — the diurnal model behind today's demo.

t    = linspace(0,24,1441);        % 24 h at 1-min steps
Tbg  = 15 + 18*sin(2*pi*(t-8)/24); % terrain, peaks ~14:00
Tsk  = 15 +  6*sin(2*pi*(t-9)/24); % skin lags 1 h, swings less
dT   = Tsk - Tbg;                  % LWIR contrast, skin - bg
ix   = find(sign(dT(1:end-1)) ~= sign(dT(2:end)));
tc   = t(ix) - dT(ix).*(t(ix+1)-t(ix))./(dT(ix+1)-dT(ix));
fprintf('Crossover at %02d:%02.0f and %02d:%02.0f\n', ...
        [floor(tc); 60*rem(tc,1)]);
fprintf('Peak |dT| = %.1f C\n', max(abs(dT)));
r = (300/850)^4;                   % skin vs nozzle, per area
fprintf('Skin is %.2f%% of the nozzle per unit area\n', 100*r);

Nothing here says when the sun rises. So what sets the crossover times?

The skin has a hundred times the area. So why does the nozzle still own the MWIR?

Block 3 · Lesson 22 · Quick Problem

Quick Problem: Who Owns Which Band

A stern-quarter look at a notional airframe: the exposed nozzle against the skin the sensor can also see.

Nozzle: $\varepsilon = 0.9$, projected area $A = 0.5$ m², $T = 775$ K
Skin: $\varepsilon = 0.8$, projected area $A = 25$ m², $T = 310$ K
MWIR (3–5 µm) band fraction: nozzle 0.33, skin 0.016
LWIR (8–12 µm) band fraction: nozzle 0.14, skin 0.27

(a) Over all wavelengths, which part radiates more, and by what factor?
(b) Which part wins the MWIR, by what factor, and by how many dB?
(c) Which part wins the LWIR, and by what factor?

(a) skin, by 1.14× · (b) nozzle, by 18.1× = +12.6 dB · (c) skin, by 2.19×

Two to three minutes, calculators or a MATLAB console; then click. Worked path: one ratio per factor, nozzle over skin. Emissivity 0.9 over 0.8 is 1.125; area 0.5 over 25 is 0.02; sigma T to the fourth is 775 over 310, which is 2.5, to the fourth, 39.1. The product is 0.879, so (a) over all wavelengths the skin wins, by 1 over 0.879, 1.14 — per unit area the nozzle is 44 times brighter, but the skin shows 50 times the area. (b) Multiply by the MWIR band-fraction ratio, 0.33 over 0.016, 20.6: 0.879 times 20.6 is 18.1, so the nozzle wins the MWIR by 18.1, which is +12.6 dB. (c) Multiply instead by the LWIR ratio, 0.14 over 0.27, 0.519: 0.456, so the skin wins the LWIR by 2.19. The so-what: the same two parts give a near-tie in total power, an 18-fold nozzle win in the MWIR and a 2-fold skin win in the LWIR — which one is louder has no answer until you name the band, and that is why the rule multiplies all four factors inside the sensor's band.

Today's Interactive Demo: Thermal Crossover

Companion script: L22_ThermalCrossover.m

Live demo

Thermal Crossover

usafa-ece.github.io/ece-495-ew

Demo hand-off; drive it from here. Model the skin and the terrain background over a 24-hour day, then plot the LWIR contrast, skin minus background, across the day. Read the two temperature curves first: the high-amplitude one is the terrain, the gentler lagging one is the skin, and both ride the same daily mean. Switch to the contrast plot and find the two crossover markers where it passes through zero. Then the slider worth dwelling on: drag the daily mean from one rail to the other and ask what happened to the crossover times. Take answers before saying anything. Ask the operational question: when is the airframe hardest to catch in the LWIR? Then switch to the second tab, pick MWIR, and ask whether a stern-aspect seeker cares about any of this. Companion script runs on their own.

Block 3 · Lesson 22 · Wrap-Up

Wrap-Up

Key ideas

Five components: hot parts, plume, skin, plus reflected sun and sky/earth
Rank them with $I_{\text{band}} \propto \varepsilon A f_{\text{band}}(T) \sigma T^4$ — temperature dominates per unit area, $A$ carries aspect
Stern shows plume and hot parts in MWIR; beam/nose shows skin in LWIR
Thermal crossover zeroes the LWIR skin contrast twice a day — but the MWIR plume does not care

Next lesson

L23: IR Seekers, IRST, and FLIR — the sensors that turn these components into a track
Work the L23 pre-flight before class — you could be the one up front