Lesson 17 Flashcards#
Click a question to reveal the answer.
1. Define electromagnetic attack (EA).
The use of EM energy to deny, degrade, or disrupt the enemy's use of the spectrum. It is offensive EW — where ES listens and EP shields, EA reaches out and breaks the threat's picture. Its three reaches are jamming, deception (L18), and directed energy.
2. Cover jamming vs deception — what is the difference?
Cover (noise) hides the real return by raising the noise floor, so the radar sees nothing clearly — it denies information. Deception creates false returns by mimicking or replaying the signal, so the radar sees the wrong thing — it corrupts information. This lesson is the noise half.
3. What single trade-off governs all noise-jamming types?
A jammer's power is fixed; only its distribution in frequency is free. Spread it wide for coverage and you lose density per hertz; concentrate it for density and you lose coverage. Spot, barrage, and swept-spot are three answers to that trade.
4. Describe spot jamming and its weakness.
All of the jammer's power on one narrow band matched to the threat frequency — very high power density. Its weakness: a frequency-agile radar simply hops out from under the spot, leaving the jammer shouting into an empty channel.
5. Describe barrage jamming and the dilution penalty.
Power spread across the whole band at once — immune to hopping, since the radar cannot escape a band jammed everywhere. The penalty: finite power divided over a wide bandwidth drops the power per hertz in the threat's passband. Coverage costs density.
6. Describe swept-spot jamming.
A high-density spot that sweeps across the band. At any instant it is a full-density spot; over time it covers the whole band. It trades dwell time for coverage — any given channel is jammed only a fraction of the time.
7. Self-protect (SPJ) vs stand-off (SOJ) jamming geometry?
SPJ rides on the target it defends, so jammer and skin echo share one range to the radar. SOJ is a dedicated platform jamming from a safe range outside the threat ring while strikers ingress, so its range to the radar stays roughly fixed. Geometry sets \(R\) in the J/S equation.
8. Define J/S and give the self-protect formula.
The jam-to-signal ratio: effective jamming power in the receiver passband over the signal (skin echo) power. For a self-protect jammer, \(\frac{J}{S} = \frac{4\pi\,P_j G_j\,R^2}{P_t G_t\,\sigma}\). Only power in the passband counts.
9. Why does self-protect J/S scale as \(R^2\)?
The skin echo is two-way (round trip), so \(S \sim 1/R^4\); the jammer is one-way, so \(J \sim 1/R^2\). The ratio is \((1/R^2)/(1/R^4) = R^2\). The jammer wins far out and loses as the target closes.
10. The target halves its range. What happens to J/S?
Since \(J/S \sim R^2\), halving \(R\) scales J/S by \((1/2)^2 = 1/4\) — a factor-of-four drop, or \(-6\) dB. Closing the range helps the radar.
11. What is the burn-through range \(R_{bt}\)?
The range at which the closing skin echo overtakes the jam and J/S drops back through the radar's tolerable threshold — inside \(R_{bt}\) the radar "burns through" and re-detects. A modern pulse-Doppler radar tolerates J/S up to about \(+10\) dB, so burn-through occurs while the jammer still nominally "wins."
12. How does lowering RCS affect burn-through?
RCS \(\sigma\) is in the denominator of J/S, so a smaller RCS means a weaker echo and a higher J/S at every range. The radar must get closer to recover the track, so burn-through moves inward. LO and jamming reinforce each other.
13. Why doesn't a stand-off jammer get the same range relief as it closes?
An SOJ sits at a roughly fixed standoff range; its \(R\) to the radar barely changes as the strike package presses in, so its J/S does not fall the way an SPJ's does. It also typically jams through the radar's side lobes rather than the main beam.