# Reading — Active Deception By the end of this lesson you should be able to: 1. Contrast **deception jamming** with the **noise jamming** of L17. 2. Explain how a **DRFM** repeater builds believable false returns. 3. Walk through **range-gate pull-off (RGPO)** step by step. 4. Describe **velocity-gate pull-off (VGPO)**, **false-target injection**, and **towed/expendable decoys**. 5. Name the radar **counters** that blunt each deception technique. ## Two ways to attack a tracker L17 taught you to *shout*. Noise jamming pours energy into the threat receiver until its noise floor climbs above the real echo — the return drowns, the track breaks. It is brute force, and it announces itself: a strobe of noise on the bearing of the jammer, defeated in the end by burn-through as the geometry closes. Deception is the surgical alternative. Instead of hiding the target, you hand the radar a **convincing lie** and let it track *that*. Where noise denies the picture, deception paints a fake one — and a radar busy tracking a false return is, in some ways, worse off than a radar staring at snow, because it does not even know it has been beaten. To see how the lie works, you have to know what a tracker keeps books on. A tracking radar follows its target through a small set of **bins** — a range cell, a Doppler (velocity) cell, an angle cell — one per quantity it measures. Each scan, it expects the target's energy to land in the cell it predicted; it nudges its gate to stay centered on the strongest return inside that cell. Noise jamming **masks** a bin by burying it in energy. Deception **fills** or **shifts** a bin with a false but believable return. Win the bin — own what the radar thinks is true. :::{admonition} Key Concept :class: key-concept Noise jamming attacks the **power budget**: it raises the threat's effective noise floor so the real echo cannot be seen. Deception jamming attacks the **measurement**: it puts a plausible return where the target is *not*, so the radar tracks a lie. Noise shouts; deception lies. ::: ## From transponder to DRFM The crudest deceiver is a **transponder** (repeater): hear the threat's pulse, wait a fixed delay, and reply. The echo arrives late, so the radar computes a target *farther out* than the real one — a range-false target. But a transponder is non-coherent: its reply does not preserve the exact phase and frequency structure of the pulse it copied, so a half-decent radar can flag the imposter and the false target sits at one clumsy, fixed offset. The modern tool is **digital RF memory (DRFM)**. A DRFM jammer **captures** the incoming pulse, **digitizes** it, **stores** the samples, and **retransmits** modified copies under software control. Because it works from a sampled copy, it can reproduce the threat's own waveform almost perfectly and then dial in exactly the lie it wants: - **Delay** — set precisely, and *ramped* over time to walk a false target in range. - **Doppler** — a programmed frequency shift to place the copy at any apparent velocity. - **Amplitude** — make the false return as loud, or louder, than the skin echo. - **Replication** — emit many copies from one captured pulse: instant false targets. The payoff of getting the copy *right* is subtle but decisive. A radar pulls weak echoes out of the noise with a **matched filter** — a correlator tuned to its own transmitted waveform. A DRFM copy is a near-exact replica of that waveform, so it earns the radar's *own* processing gain. The lie comes through the matched filter as cleanly as the truth — and just as loud. :::{admonition} Key Concept :class: key-concept The skin return from a real target is a faint, passive reflection. A DRFM false return is an *active, phase-coherent replica* of the radar's own pulse. It passes the matched filter with full processing gain, so the fake can be made to look exactly like — or brighter than — the real thing. ::: ## False-target injection The simplest use of DRFM is volume. Replay the captured pulse at several delays and Doppler shifts and the radar's range/Doppler map blooms with believable contacts: - A **search** radar drowns in plausible targets. Which of the dozen blips is the bomber? Operators and trackers must spend time, and time is the point. - A **tracking** radar can be seduced into locking the wrong return — a brighter false target sitting a few cells away from the real one. False targets are cheap and they buy confusion, but they do not by themselves *break* an existing lock. For that you have to reach into the gate. ## Stealing the gate: range-gate pull-off RGPO is the centerpiece of deception and the clearest illustration of "win the bin." Picture a tracker that has already locked the true target, its **range gate** centered on the skin return. The DRFM walks that gate off the target in three moves: 1. **Cover pulse — capture the gate.** The jammer replays the captured pulse at (essentially) zero added delay, sitting right on top of the real skin return but *louder*. The gate's automatic-gain control and centroid logic now lock onto the strongest return in the cell — which is the jammer, not the target. The lie and the truth are momentarily co-located, so the radar never notices the handoff. 2. **Walk it out — ramp the delay.** The DRFM now *increases* its delay smoothly, scan after scan. The false return drifts outward in range, and because the gate chases its strongest return, the gate is dragged along with it — pulled steadily away from the true target. If the false return moves at a believable closing or opening rate, the radar's track-rate logic accepts it as real motion. Quantitatively, an added delay $\Delta t$ moves the apparent range by $$ \Delta R = \frac{c\,\Delta t}{2}, $$ the same two-way factor as the range equation: light travels out and back, so a $1\ \mu\text{s}$ delay buys $150\ \text{m}$ of pull-off. 3. **Blink off — abandon the gate.** Once the gate has been dragged far enough off the target, the jammer **switches off**. There is now *nothing* in the gate. The track has no return to center on, so it **coasts** — drifting on its last estimated velocity through empty space — while the true target, which the gate left behind several cells ago, has escaped. By the time the radar reacquires, the geometry has changed. The whole maneuver is a confidence trick: co-locate the lie with the truth, slide the truth's "shadow" away, then vanish and leave the radar holding nothing. **Velocity-gate pull-off (VGPO)** is the exact same idea in the Doppler bin. Start the false return at the target's true Doppler, then ramp a programmed frequency shift so the apparent velocity walks away, dragging the **velocity gate** off the target before blinking off. Run RGPO and VGPO together and the false return walks a **range-Doppler diagonal** — pulling both gates at once and defeating a tracker that cross-checks range against velocity. ## Angle deception, in brief Range and Doppler are not the only bins; the **angle** track can be attacked too. A monopulse radar measures angle error from the ratio of its difference and sum channels, $\Delta/\Sigma$. **Inverse-gain** jamming transmits a phase-inverted $\Delta$ component that drives that ratio toward zero, so the radar reads *zero* angle error and the angle track freezes or drifts. Modern digital and phase monopulse blunt the classic version, but the same principle — give the angle tracker a false null — still drives decoy design. ## Decoys: when you cannot fool the gate from on board If on-board deception is not enough, give the radar a *better* target to lock: - A **towed decoy** is a small repeater trailed on a cable behind the aircraft. Brighter than the skin return and offset in space, it seduces the lock off the platform onto the decoy. - **Expendable decoys** (e.g., MALD, ITALD) are launched to saturate the picture, bait radars into radiating so they can be located, and transfer locks away from real aircraft. Active decoys beat **chaff** on two counts: chaff blooms slowly and sits near **zero Doppler**, so a pulse-Doppler radar's clutter notch rejects it, whereas an active decoy can show any Doppler and looks like a real, moving target. ## Deception isn't free Every trick has a counter — this is the ECCM half of the duel, and DRFM only works *until* the radar adapts: - **Leading-edge tracking** beats RGPO. The real skin return always arrives *first*; the cover pulse and walked-off false return arrive *after* it. A radar that gates on the leading edge of the return stays glued to the true target while the pull-off slides harmlessly behind it. - **Digital/phase monopulse** and track-history logic blunt angle deception. - **Waveform agility, polarization, and processing gain** expose the copy — a DRFM that captured the *last* pulse cannot perfectly fake the *next* one if the radar changes it. - Even a coherent DRFM replay leaves **fingerprints**: small timing and phase artifacts a sophisticated receiver can learn to discriminate. The honest summary: deception buys **time** and **doubt**. It rarely kills a track permanently — it makes the radar work harder, look the wrong way, and waste the seconds the platform needs. ::::{admonition} Quick Exercise :class: quick-exercise For each observation, name the deception technique and one radar counter: 1. The tracked range slowly walks away while the real target holds steady. 2. The scope fills with a dozen identical contacts at different ranges. 3. The monopulse angle error reads zero, yet the target is clearly off-boresight. 4. A bright return separates from the aircraft and the missile follows it. :::{admonition} Solution :class: dropdown 1. **Range-gate pull-off (RGPO)** via DRFM. Counter: **leading-edge tracking** — the real skin return always arrives first, so gating on the leading edge ignores the walked-off false return. 2. **False-target injection (DRFM)**. Counter: **waveform agility**, feature discrimination, and track-history logic to expose returns that don't behave like real targets. 3. **Inverse-gain angle deception**. Counter: **digital/phase monopulse** and ECCM that the classic null cannot defeat. 4. **Towed or expendable decoy** seducing the lock. Counter: **home-on-jam** and kinematic/feature gating to reject the decoy's flight profile. ::: :::: ## Wrap-Up Deception attacks the tracker's bookkeeping, not its power budget: it **fills** or **shifts** the range, Doppler, and angle bins with believable lies instead of masking them with noise. **DRFM** is what makes the lie believable — it captures, stores, and replays a phase-coherent copy of the radar's own pulse, so the false return earns full matched-filter gain and arrives as loud as the truth. **RGPO** is the signature move: cover pulse to capture the gate, ramped delay to walk it off the target, blink off to leave the gate coasting on nothing — with **VGPO** doing the same in Doppler. **Decoys** extend the idea off-board, giving the radar a brighter target to chase, while **ECCM** — led by leading-edge tracking — fights back. Deception buys time and doubt, not a permanent kill. This is the last teaching lesson of Block 2. Next, **L19** is the **Project 2 work day**: you put the whole Block 2 toolkit to work characterizing a B-21's standoff emitter-geolocation problem from RWR angle-of-arrival measurements — turning the listener's advantage into a real targeting product.