Lesson 37 Flashcards — Electronic Warfare and Jamming#
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1. Define ES, EP, and EA.
ES (Electromagnetic Support) senses and characterizes emissions. EP (Electromagnetic Protection) makes our own systems resistant. EA (Electromagnetic Attack) jams, deceives, or destroys threat sensors.
2. Why must ES come before EA?
You cannot jam intelligently a signal you have not first intercepted and characterized.
3. What does a jammer actually do to a receiver?
It does not break it. It adds its own received power to the noise, so the receiver works only when \(S/(N + J) \ge SNR_{\min}\) — the same \(SNR_{\min}\) as with the jammer off. The jamming dwarfs the thermal noise \(N\), so in practice the test is \(S/J \ge SNR_{\min}\).
4. Write the jamming SNR of a communications link in full.
The ratio of two Friis calculations into the same receiver: \(SNR_\text{comm} = \dfrac{P_{T,\text{sig}}G_{T,\text{sig}}G_R\lambda^2/(4\pi R_\text{sig})^2}{P_{T,\text{jam}}G_{T,\text{jam}}G_R\lambda^2/(4\pi R_\text{jam})^2}\).
5. What cancels when you simplify the jamming SNR, and what is left?
\(G_R\), \(\lambda^2\), and \((4\pi)^2\) all cancel, leaving the equation sheet's row, \(SNR_\text{comm} = P_{T,\text{sig}}G_{T,\text{sig}}R_\text{jam}^2/(P_{T,\text{jam}}G_{T,\text{jam}}R_\text{sig}^2)\). The reading writes \(R_\text{txrx}\) for \(R_\text{sig}\).
6. What does that cancellation assume?
That the receive antenna has the same gain toward both the transmitter and the jammer, and that the jammer is on the victim's frequency, inside its bandwidth. A receive antenna that nulls the jammer's bearing breaks it. What is left to fight over is power, gain, and above all the ratio of ranges, squared.
7. In a jamming problem, where is every range measured to?
The victim receiver. \(R_\text{sig}\) runs from the transmitter to it and \(R_\text{jam}\) from the jammer to it — never from the jammer to the transmitter.
8. Why does a stand-in jammer beat a stand-off jammer of the same power?
Because \(SNR_\text{comm} \propto R_\text{jam}^2\). A jammer twice as far away is four times less effective. Geometry beats power.
9. Write the SNR of a radar whose target carries the jammer.
The echo (radar range equation) over the jamming (Friis into the radar's own antenna): \(SNR_\text{rad} = \dfrac{P_T\,G_T\,RCS}{P_{T,\text{jam}}\,G_{T,\text{jam}}\,4\pi R^2}\). It assumes the jammer is at the target's range: self-screening (the target carries it) or an escort flying with the target, so one \(R\) serves both paths. True stand-off radar jamming needs two ranges, which is outside this course.
10. Why does jamming favor the jammer against radar specifically?
The jammer's signal travels one way (\(1/R^2\)) while the radar's echo travels two (\(1/R^4\)). So \(SNR_\text{rad} \propto 1/R^2\): far out the jamming wins, and the echo gains on it only as the target closes.
11. What is the burn-through range?
The range at which \(SNR_\text{rad}\) reaches the radar's \(SNR_{\min}\): \(R_{BT} = \sqrt{\dfrac{P_T\,G_T\,RCS}{P_{T,\text{jam}}\,G_{T,\text{jam}}\,4\pi\,SNR_{\min}}}\), in meters because \(RCS\) is in m². Inside it the radar sees through the jamming.
12. How does jamming change a radar's detection range?
It does not move the radar equation's detection range. It adds a second, tighter limit, the burn-through range: a jammed radar detects only inside it.
13. A burn-through range comes out smaller than the aircraft's altitude. What does that mean?
The aircraft is never closer to the radar than its own altitude (30,000 ft is \(30{,}000 \times 0.305 = 9{,}150\) m), so that range is never reached: the radar never burns through. A small burn-through range is a jammer that works, not a mistake.
14. Name four low-probability-of-intercept techniques.
Spread spectrum (power density below the noise floor), frequency hopping, power management (transmit the least that closes the link), and narrow beams.
15. Why is brute transmit power usually the worst answer to jamming?
Because \(SNR_\text{comm}\) is only linear in \(P_{T,\text{sig}}\) but goes as the square of range. Shortening the link, or using EP so the jammer cannot concentrate power in your band, buys far more.
16. Name four electronic-attack countermeasures beyond noise jamming.
Chaff (false returns), flares (IR decoys), towed and active decoys, and anti-radiation missiles that home on the radar's own emissions.