# Reading — Electromagnetic Attack By the end of this lesson you should be able to: 1. Define **electromagnetic attack (EA)** and place it against ES and EP. 2. Classify the three families of **noise jamming** — spot, barrage, and swept-spot — and explain the barrage dilution penalty. 3. Compute the **jam-to-signal ratio (J/S)** for a self-protect jammer and explain why it grows as $R^2$. 4. Find the **burn-through range** $R_{bt}$ and explain how lowering RCS moves it. 5. Distinguish **self-protect** from **stand-off** jamming geometry and why range relief applies to only one of them. ## Switching sides The last two lessons were defense. EP protects *our* use of the spectrum — hide, hop, spread, survive. Now switch seats one more time. EA is the offense: it uses electromagnetic energy to **deny, degrade, or disrupt** the enemy's use of the spectrum. Everything the threat did to us in ES and EP, we now turn around — we listen to their radars, we hide from them, and today we reach out and break their picture. EA splits into three reaches. **Jamming** raises the noise floor until real returns vanish. **Deception** injects false returns so the radar believes a lie — that is L18. **Directed energy** burns the receiver itself, and is beyond our scope. This lesson is the jamming half, and the math that governs it. ## Two ways to lie to a radar Jamming and deception are both EA, but they corrupt the threat's picture in opposite ways. **Cover** — noise — *hides* the real return by raising the noise floor until the radar sees nothing clearly. **Deception** *creates* false returns by mimicking or replaying the signal until the radar sees the wrong thing. Noise **denies** information; deception **corrupts** it. The distinction matters because it sets what the operator achieves: a noise jammer buys you invisibility, a deception jammer buys you a decoy. Today is noise. ## The noise-jamming taxonomy All noise jamming faces the same trade: a jammer has a finite amount of power, and it must decide how widely to spread that power across frequency. Spread it thin and you cover more of the band but put less energy on any one channel; concentrate it and you dominate one channel but miss the rest. The three classic types are three answers to that trade. - **Spot jamming** puts all of the jammer's power on one narrow band, matched to the threat's operating frequency. Power density is high — it overwhelms that channel. But it is brittle: a frequency-agile radar (L15) simply hops out from under the spot, and the jammer is left shouting into an empty channel. - **Barrage jamming** spreads power across the *whole* band at once. It is immune to hopping — the radar cannot escape a band that is jammed everywhere — but it pays for that coverage. The same finite power now divides over a much wider bandwidth, so the power *per hertz* in the threat's actual receiver passband drops. This is the **barrage dilution penalty**: coverage costs density. A barrage jammer that must cover ten times the bandwidth puts roughly one-tenth the power into the channel that matters. - **Swept-spot jamming** is the compromise: a high-density spot that sweeps across the band. At any instant it is a spot — full density — but over time it covers the whole band. It trades *dwell time* for coverage, jamming any given channel only a fraction of the time. :::{admonition} Key Concept :class: key-concept A jammer's power is fixed; only its *distribution in frequency* is free. Spot concentrates it (high density, beaten by agility); barrage spreads it (hop-proof, but diluted per hertz); swept-spot time-shares a spot across the band (full density, fractional dwell). Choosing among them is choosing what you are willing to give up. ::: ## Where the jammer sits Before the J/S math, fix the geometry — because geometry sets the range $R$ that drives everything, and it decides who is at risk. - **Self-protect jamming (SPJ)** rides on the very target it defends. The jammer and the skin echo travel together, so they share one range to the radar. - **Stand-off jamming (SOJ)** uses a dedicated platform that jams from a safe range, well outside the threat ring, while the strikers ingress. Its range to the radar stays roughly fixed as the strike package presses in. - **Escort and stand-in** jam from alongside the strike or from deep inside the threat ring, respectively. The geometry is the whole story for the range dependence we are about to derive. Hold onto the difference between SPJ (shares the target's range) and SOJ (fixed standoff range) — it is the answer to one of the Quick Exercise questions. ## J/S: the jamming scorecard The figure of merit for cover jamming is the **jam-to-signal ratio**, $J/S$: the *effective* jamming power in the receiver passband divided by the signal (skin echo) power. For a self-protect jammer it works out to $$ \frac{J}{S} = \frac{4\pi\,P_j G_j\,R^2}{P_t G_t\,\sigma}, $$ where $P_j G_j$ is the jammer's effective radiated power, $P_t G_t$ the radar's, $\sigma$ the target RCS, and $R$ the shared range. The whole lesson is hiding in the exponent on $R$. Recall the two propagation laws. The skin echo makes a *round trip* — radar to target and back — so the signal falls off as $S \sim 1/R^4$ (this is the range equation from L3 and L8). The jammer's energy makes a *one-way* trip — jammer to radar — so it falls off as only $J \sim 1/R^2$. Take the ratio: $$ \frac{J}{S} \sim \frac{1/R^2}{1/R^4} = R^2. $$ This is the central result. **J/S grows as $R^2$.** The jammer's advantage is *strongest far out* and *fades as the target closes*. Far away, the two-way echo has been crushed by $1/R^4$ while the one-way jam has only been softened by $1/R^2$, so the jammer buries the return. Up close, the echo recovers four powers of range while the jam recovers only two — and the radar wins. There is a second consequence in the same formula: only the jamming power *inside the passband* counts. That is exactly why barrage dilution hurts — power spread outside the receiver's bandwidth does nothing to $J/S$. Bandwidth, gain, RCS, and range all move the number. :::{admonition} Key Concept :class: key-concept Because the skin echo is two-way ($S \sim 1/R^4$) while the jammer is one-way ($J \sim 1/R^2$), the self-protect ratio scales as $J/S \sim R^2$. The jammer wins far out and loses close in. This is the same one-way/two-way asymmetry that gives the passive listener its advantage in ES — here it sets the limit of jamming instead. ::: ## Burn-through If $J/S$ falls as the target closes, then somewhere there is a range at which the echo overtakes the jam and the radar re-detects the target. That range is the **burn-through range**, $R_{bt}$ — the point where $J/S$ drops back through the radar's tolerable threshold. Inside $R_{bt}$, the radar "burns through" the jamming and gets its track back. The threshold is not zero. A modern pulse-Doppler radar does not need the echo to *exceed* the jam — its processing tolerates a J/S of roughly $+10$ dB and still extracts a usable track. So burn-through happens while the jammer is still nominally "winning" on raw power. And the collapse is steep on the way in: as $J/S$ falls it does not just restore detection, it also strips the *advanced* techniques that ride on top of noise — range-gate pull-off and other deception (L18) need the cover of a high J/S to operate, and they fail first. The RCS term sits right in the denominator of the J/S equation, which gives low-observable design and jamming a way to reinforce each other. **Lowering the target's RCS weakens the skin echo, which raises $J/S$ at every range — so the radar has to get closer to burn through. Burn-through moves *inward*.** A stealthy platform that also jams forces the radar deeper into the engagement before it recovers a track. LO and EA are not competitors; they pull in the same direction. ::::{admonition} Quick Exercise :class: quick-exercise A self-protect jammer faces a threat radar. Reason about each: 1. The target halves its range to the radar. What happens to J/S, in dB? 2. Why does a stand-off jammer not get this same range relief as it closes? 3. The target also cuts its RCS by 10 dB. Which way does burn-through move? 4. When would a barrage jammer beat a spot jammer against the same threat? :::{admonition} Solution :class: dropdown 1. $J/S \sim R^2$, so halving $R$ scales J/S by $(1/2)^2 = 1/4$ — a factor-of-four *drop*, which is $-6$ dB. Closing the range helps the radar, not the jammer. 2. A stand-off jammer sits at a roughly fixed standoff range; its $R$ to the radar barely changes as the strike package closes, so its J/S does not fall the same way. (It also jams through the radar's side lobes rather than the main beam, a different geometry entirely.) 3. Lower RCS means a weaker skin echo, which means *higher* J/S — so the radar must get closer to recover the track. Burn-through moves **inward**. LO and jamming reinforce. 4. Barrage beats spot when the threat is frequency-agile or its frequency is unknown, so a spot cannot stay on it — at the cost of power per hertz (the dilution penalty). ::: :::: ## Wrap-Up EA denies, degrades, or disrupts the enemy's use of the spectrum, and noise jamming is its blunt instrument. A jammer's power is fixed; spot, barrage, and swept-spot are three ways to distribute it in frequency, each trading density against coverage, with barrage paying the dilution penalty. The self-protect J/S grows as $R^2$ because the skin echo is two-way ($1/R^4$) and the jammer is one-way ($1/R^2$) — so the jammer's edge is greatest far out and fades as the target closes. The burn-through range $R_{bt}$ is where the echo overtakes the jam and the radar re-detects; lowering RCS raises J/S everywhere and pulls $R_{bt}$ inward, so LO and jamming reinforce each other. Next, **L18 — Active Deception** trades shouting for lying: DRFM, false-target injection, and range-gate pull-off, which corrupt the tracker's measurement instead of drowning it in noise.