# Lesson 23 Flashcards Click a question to reveal the answer.

1. Name the three passive IR sensors and each one's job.

Seeker — puts a weapon on target (feeds a guidance loop); IRST — searches and tracks like a passive radar (feeds a computer); FLIR — images the scene for a human to interpret. All three are passive and emit nothing.

2. Compare seeker, IRST, and FLIR by field of view and who reads the output.

Seeker: narrow FOV, output feeds a guidance loop. IRST: wide FOV, output feeds a computer. FLIR: selectable FOV, output read by a human. All three are passive.

3. How does a reticle seeker turn a target into a steering signal with only one detector?

A spinning patterned disk (the reticle) chops the target's IR energy into an AM signal; the modulation's phase encodes the bearing of the pointing error off boresight, which guidance then steers to null.

4. What is the reticle seeker's fundamental weakness?

It sees a point of energy, not a shape — so a hotter point (a flare) just wins the modulation contest and seduces it away.

5. What does conical scan add over a simple spin-scan reticle?

A fixed reticle with a nutating image produces PWM instead of pure AM, giving better tracking accuracy near boresight (center). Rosette scan is a further step, sweeping a petal pattern for crude pseudo-imaging.

6. Why did imaging (FPA) seekers change the countermeasure game?

They see the target as a shape and weigh spatial, spectral, and temporal cues together — "does it look, glow, and move like an aircraft?" — which a flare or simple decoy struggles to fake, unlike a point-tracking reticle.

7. In one line, what are the two seeker bins?

Reticle seekers track a point of energy (fooled by a brighter point). Imaging FPA seekers track a shape (force a decoy to look, glow, and move like an aircraft). Con-scan and rosette are stepping stones between them.

8. What guidance law do modern IR missiles use, and what does it null?

Proportional navigation — it nulls the line-of-sight rate to the target, leading the intercept rather than simply pointing the missile at the tail. Modern seekers also bias the aim point off the plume toward the airframe.

9. Why is an IRST called a "passive radar," and what is its biggest limitation?

It scans wide, tracks autonomously, and shows a radar-like display while emitting nothing — so the target's RWR hears no warning. Limitation: angles come easy but range does not; passive ranging needs target kinematics or multi-ship geometry.

10. What operator tools shape a FLIR image, and what mission problem limits it during identification?

Gain and level adjustment plus white-hot / black-hot polarity, with a video tracker (centroid/correlation) holding the aimpoint. Limitation: the narrow-FOV "soda-straw" problem and operator workload while identifying.

11. Contrast IRST and FLIR across FOV, frame time, resolution, and consumer.

IRST: small instantaneous FOV scanned over a wide field of regard, many frames per second, high angular resolution, read by a computer. FLIR: large FOV, seconds per frame, lower resolution, read by a human.

12. State the passive IR trigger condition and the maximum-range rule.

Detection when \(\frac{J\,\tau(R)}{R^{2}} \ge \text{NEI}\). Solving gives \(R_{\max} \propto \sqrt{J\,\tau/\text{NEI}}\), where \(J\) is in-band source intensity, \(\tau\) is atmospheric transmittance, and NEI is the sensor's noise-equivalent irradiance.

13. How does IR range scaling differ from radar's, and why does that favor IRST against stealth?

IR: \(R_{\max} \propto \sqrt{J\,\tau/\text{NEI}}\); radar: \(R_{\max} \propto \sigma^{1/4}\). RCS reduction shrinks \(\sigma\) and starves the radar, but does not reduce the aircraft's radiated IR intensity \(J\) — so the IRST detects it just as far away.

14. What technology change made IR missiles all-aspect instead of tail-chase only?

Cooled MWIR detectors, which see the CO₂ exhaust plume near 4.3 micrometers — visible from nearly any aspect. First-generation uncooled detectors saw only hot metal (the tailpipe), which shows only from behind.

15. In the demo, the vacuum rule predicts a 4.5× stern-over-beam range advantage but the atmosphere delivers only ~1.4×. Why the compression?

The exponential transmittance \(\tau(R)=\exp(-\alpha R)\) crushes the extra intensity at long range; every doubling of \(J\) buys less range than the last, so the \(\sqrt{20}\) advantage collapses. To truly hide you must attack \(J\) by orders of magnitude, component by component (the L24 argument).