Introduction
Noise jamming is shouting in a hall so that nobody can understand the speaker any more. It is crude but it works — except that everyone can also hear where the shouting is coming from.
In practiceJamming aircraft such as the EA-18G Growler and ground jammers such as the Russian Krasukha-4 transmit noise that makes whole sectors of an enemy radar "snow up". The operator sees a streak (strobe) in the direction of the jammer: he knows where it is, but not at what range.
Noise jamming attacks the radar's most basic mechanism: the detection threshold (Threshold, Constant False Alarm Rate and Automatic Gain Control). The jammer transmits noise — random energy — on the frequency of the victim radar. That noise adds to the receiver's natural noise floor; the CFAR threshold moves up with it, and echoes that previously rose well above the threshold disappear below it. The radar is not deceived but blinded.
The effect on the display: a bright, noisy sector in the direction of the jammer (a "strobe"). The operator therefore knows that jamming is taking place, and from which direction — just not from how far away or from how many platforms. That is also the fundamental weakness of noise jamming: it is a shout that gives away your position in azimuth (and on which home-on-jam weapons can aim, Home-on-jamming).
Its effectiveness depends on the jamming-to-signal ratio (J/S) in the radar's receiver — and on how cleverly the jammer distributes its power across the spectrum. That is the trade-off that structures the next three sections: a jammer has finite power, and the radar can change frequency. Spread your power wide (barrage) and you are weak everywhere; concentrate it (spot) and you are strong but miss every change of frequency.
Barrage jamming
In barrage jamming the jammer transmits noise across a wide frequency band at once — wide enough to cover all the (suspected) operating frequencies of the threatening radar(s), including all the channels a frequency-agile radar can hop to.
- Advantage: no knowledge of the exact radar frequency is needed, and the radar's frequency agility (Jamming avoidance) does not help — every channel is already jammed. Several radars are hit at the same time.
- Disadvantage: the power is spread out. The power density (W/MHz) in each individual radar channel is low:
Barrage jamming is therefore typically the work of large stand-off platforms that generate a lot of power, or it is used as an "umbrella" under which more precise techniques operate. Against a single, known radar it is a waste; against an unknown or frequency-hopping threat picture it is the only option.
Spot jamming
Spot jamming is the opposite: all the jamming power concentrated in a narrow band around the exact operating frequency of a single victim radar. The same watts thus deliver an order of magnitude more power density in the channel that matters — maximum J/S, maximum suppression of that one radar.
The conditions and weaknesses follow directly from the principle:
- You have to know the frequency. Spot jamming relies entirely on ESM (Electronic Support Measures): first measure where the radar is, then jam there. Modern jammers do this automatically (set-on receiver).
- Frequency agility is the natural counter (Jamming avoidance): if the radar hops to another channel, the spot jammer jams empty air until its receiver has followed the hop. With pulse-to-pulse hopping, following becomes practically impossible.
- Only one radar (or one frequency) at a time — against a network of radars on different bands, a single spot jammer falls short.
Spot and barrage are therefore not competitors but the extremes of a sliding scale: power density versus bandwidth coverage. In practice the choice usually lies somewhere in between — or it is the compromise of the next section.
Swept-spot jamming
Swept-spot jamming (sweep jamming) tries to get the best of both: a narrow, powerful spot that sweeps rapidly back and forth across a wide band. Each radar channel is periodically hit with full spot density, and averaged over time the whole band is still covered.
- Against each individual radar the interference is intermittent: short, hard bursts of noise each time the sweep passes. That saturates receivers, triggers false alarms and can throw AGC loops (Threshold, Constant False Alarm Rate and Automatic Gain Control) off balance — sometimes more effective per watt than continuous weak noise.
- Against frequency-agile radars it works better than pure spot jamming: wherever the radar hops to, the sweep comes by periodically.
- The weakness: between two passes the channel is clean. A radar that places its processing (integration, Pulse compression and pulse integration) in those clean windows, or times its transmissions to them, looks straight through. The effectiveness depends heavily on the sweep rate compared with the radar's time constants.
To sum up the three forms: barrage = a little everywhere, spot = all in one place, swept-spot = everywhere in turn. Which one wins depends on what you know (ESM!), how much power you have and how agile the radar is.
Side Lobe jamming
The previous sections were about frequency; this one is about direction. A radar receives not only through its main lobe but also — attenuated — through its side lobes (Main lobes, side lobes and back lobes). Side lobe jamming exploits this: a jammer with enough power pushes its noise or false echoes in through the side lobes, regardless of where the antenna happens to be pointing.
The effect is fundamentally different from main-lobe jamming:
- With main-lobe jamming only the sector towards the jammer is disrupted — the operator sees a single strobe and knows the direction.
- With side-lobe jamming the interference comes in at every antenna position: the whole display becomes cluttered, and false targets appear at random azimuths (after all, the radar attributes everything it receives to the direction of its main lobe). Directional information becomes worthless.
The price: side lobes are 25–40 dB below the main lobe, so the jammer needs thousands to tens of thousands of times more power for the same effect than through the main beam — or it has to be close by (stand-in jammer, Radar Jamming).
Countermeasures on the radar side exist and are effective (Main lobes, side lobes and back lobes): side lobe blanking (an auxiliary antenna with a wide pattern compares — if the signal arrives more strongly at the auxiliary antenna than at the main antenna, it came in through a side lobe and is discarded) and adaptive null steering (phased arrays steer a deep null in the pattern towards the jammer). The race between main lobe and side lobe is thus a classic example of the ECM–EPM cycle from The three parts of Electronic Warfare.