Electronic warfare: reading the spectrum as terrain

Electronic warfare (EW) covers everything that happens in the radio spectrum once it becomes contested in a conflict. The same spectrum you explore with an SDR — FM, ISM, 2.4 GHz, radars — is, on an operational theatre, disputed ground: each side tries to see the other in it, to blind them, and to protect its own links.

This note is educational and conceptual. It explains publicly known principles (detection, jamming, countermeasures) to make sense of current events and the physics involved — not to build a jammer, which is illegal and dangerous (Légal & sécurité).

The three pillars

Classic doctrine splits EW into three functions:

Pillar	NATO term	In plain words	SDR link
Electronic support	ES (ESM)	listen: detect, locate, identify emissions	exactly what an SDR receiver does, only more sensitive
Electronic attack	EA (ECM)	act: jam, decoy, saturate	transmitting — forbidden outside a military context
Electronic protection	EP (ECCM)	resist: frequency hopping, spreading, directivity	the design of robust waveforms

A consumer SDR does only the first pillar, and only in receive-only mode.

Electronic support: seeing without being seen

Listening to an adversary's spectrum reveals a great deal without ever transmitting:

Emission detection: a radar, a radio, a drone link betrays itself the moment it transmits. It's the principle you use when spotting a peak above the noise floor.
Direction finding (DF): with several antennas or a directional one, you estimate the direction a signal arrives from. Crossing two bearings gives a position (triangulation).
Fingerprinting / SIGINT: an emitter's "signature" (frequency, bandwidth, modulation, hop pattern) lets you identify and catalogue it. Seeing a signal (detecting) and understanding it (decoding) are two different levels.

It is silent, passive and undetectable — which is exactly its value.

Electronic attack: jamming and decoying

Jamming means drowning the useful signal under noise or a deceptive higher-power signal in the target band. Conceptually:

Barrage jamming: flood a whole band with noise → simple but power-hungry and conspicuous.
Spot/follower jamming: concentrate energy on the exact frequency in use → efficient, but you must first find it (back to electronic support).
Spoofing: instead of noise, transmit a fake but plausible signal. The best-known example is GNSS (GPS) spoofing: a counterfeit positioning signal, stronger than the genuine satellites (very weak at ground level), makes a receiver compute a wrong position.

All of this requires transmitting, so it belongs strictly to the military domain and is forbidden to civilians.

Electronic protection: making a link resistant

On the defensive side, waveforms are designed to be hard to jam or intercept:

Frequency hopping (FHSS): the link changes frequency tens to thousands of times per second following a sequence only the correspondents know. Jamming becomes a game of hide-and-seek. Bluetooth is a civilian version.
Spread spectrum (DSSS): the signal is spread below the noise floor; without the spreading code it is almost invisible. This is how GPS and "LPI/LPD" (low probability of intercept/detection) links work.
Directional antennas & nulling: aim a narrow beam at the wanted signal and steer a "null" toward the jammer.
Band agility: hop to a whole different band (2.4 → 5.8 GHz) when one is saturated.

These ideas explain why modern links (drones, military, but also WiFi) are so hard to disrupt cleanly.

Why it matters today

Recent conflicts have made EW a decisive factor: ubiquitous drones, GNSS jamming over entire regions, a constant arms race between jammers and agile links. The drone side is detailed in Drones et spectre sur le champ de bataille moderne.