Protecting our own use of the spectrum against both enemy attack and friendly interference: make radars/links resistant to jamming and EMI, harden against large EM disturbances, build LPI systems, and reduce signature (the low-observable half, deferred to Block 3). Legacy term: ECCM.
ES listens, EA denies — EP keeps us in the fight while both happen. EP is the protect branch of the move–countermove tree, answering specific attacks.
A radar must transmit to work, but a one-way listener hears it (\(1/R^2\)) long before its two-way echo (\(1/R^4\)) closes the loop. Every watt radiated can be intercepted and used to locate and jam you; turning the radar off is safe but useless. EP is staying useful while giving the threat as little as possible.
One designed to be hard to intercept in the first place — through emitting less and later, and through waveforms whose energy is spread so thin that an intercept receiver sees only noise-like, sub-threshold signal.
EMCON (lean on passive sensors — RWR, IR/EO, off-board data link — and radiate only when you must); delay radiation (go active at the last moment); power management (emit only enough to close this shot, not the maximum).
The threat's intercept range scales with the radar's transmit power, so emitting only enough to close the current shot shrinks the RWR's reach — the listener has to come closer to detect you.
A long coded pulse (chirp or phase code) spreads energy over a wide bandwidth, so an intercept receiver sees weak, noise-like signal (poor wideband SNR). The radar's matched filter re-concentrates that energy into a sharp peak — high SNR for the radar only.
\(G_p = B\tau\) (bandwidth × pulse duration). The matched filter pays it back to the radar that owns the code; the eavesdropper, lacking the code, never claims it. "Spread to hide; compress to see."
Hopping the carrier pulse-to-pulse. A spot jammer parked on one channel cannot follow, and an RWR struggles to sort a moving emitter. Bonus: hopping decorrelates clutter and target glint between pulses.
Changing transmit polarization to beat a cross-polarization jammer matched to the old polarization. Anti-cross-pol design refuses energy arriving on the wrong polarization.
A jammer rarely sits in the main beam, so it pours energy through off-axis side lobes. EP responds with ultra-low side lobes (starve every off-axis path), a side-lobe blanker (reject wideband side-lobe pulses via a guard channel), and a side-lobe canceler (an auxiliary antenna that adaptively nulls a narrowband jammer).
The side-lobe blanker rejects wideband pulses arriving through the side lobes (a guard channel blanks the receiver); the side-lobe canceler uses an auxiliary antenna to place an adaptive null on a narrowband side-lobe jammer.
No single trick wins — each technique counters one specific attack (e.g., agility↔spot jamming, pulse compression↔code-less deception, ultra-low side lobes↔side-lobe entry, PRF jitter↔cover pulses). Stacking layers buys robustness but costs complexity, weight, and dollars.