In military terms, stealth is the ability to avoid or prevent hostile forces from detecting aircraft, land vehicles, missiles, and other strategic objects. Stealth is achieved by the application of electronic countermeasures and by the use of designs and materials that minimize vulnerability to visual observation and detection by systems such as radar and heat-detecting infrared cameras.
Jamming techniques
Jamming techniques constitute a major part of the armory of electronic warfare (EW). They use the basic premise that radio-frequency systems—radar and radio-communications systems included—are vulnerable to degradation or total disruption by electronic interference.
Jamming was first used on a large scale by the British Royal Air Force in an attempt to defeat the Knickebein (dogleg) radio bombing system used by the German Luftwaffe in its Blitzkrieg (lightning war) bombing campaign of 1940–1941 against British cities. Thereafter, jamming became an increasingly important aspect of modern warfare in direct response to the proliferation of military electronic systems.
Jamming techniques are divided into active and passive types. Active systems use transmitters to produce signals that either swamp a target system’s receiver, making it impossible to interpret incoming signals, or they deceive the system with false but intelligible signals. Active jamming systems are also called electronic countermeasures, or ECMs. Passive systems do not transmit signals, but they provide false targets for hostile detector systems, helping friendly targets evade detection.
Brute-force jammers. Jamming systems that use electronic noise to swamp hostile detectors are described as brute-force jammers. In this context, noise is defined as any unwanted electric signal that affects an electronic apparatus and results in spurious signals occurring at the output. In a radar system, for example, the use of radio-frequency noise as a jamming agent produces obscuring spoke traces (the spurious signals) on the radar’s display (its output). These traces prevent the operator from discriminating a true target amid a host of ghost targets on the screen.
Brute-force jammers are relatively simplistic pieces of equipment. They are produced in a number of distinct forms, the most common of which are the barrage and spot types. Barrage jammers broadcast electronic noise over a range of target frequencies. They have the advantage of being effective against various types of systems within their frequency ranges. Barrage jammers are useful in aerial combat, when an airborne attacker may be prone to detection by a multiplicity of radars; they are also useful where the attacker may anticipate being confronted by an unexpected type of electronic threat.
The disadvantage of barrage systems lies in the breadth of their frequency coverage, which reduces the power output available for use against a specific system. In order to be effective, a noise jammer must emit a signal around 10 times stronger than that emitted by the detector system. However, the wide coverage of the barrage unit often reduces this ratio unfavorably, allowing some elements in a multitype radar system the chance of seeing through the jamming because of the relative weakness of the disruptive signal. This limitation is overcome by spot jammers, which tune to specific target frequencies and can therefore concentrate their outputs against individual systems. Of course, a single spot jammer is unable to counter a multitype detector system; multiple spot jammers must then be used, each one targeting a specific subsystem. This approach requires extensive knowledge of hostile detector systems—information that is not always available—so spot jammers are of little use where an unexpected radar system could be in use.
Sweep-lock jammer. The individual limitations of barrage and spot jammers led to the development of a third type of system, sometimes called a sweep-lock jammer. Such systems are capable of broadcasting over a broad range of frequencies, and that range is monitored—manually or automatically—for incoming signals from hostile detector systems. When threat signals are identified, the entire available jamming output is directed against them. Sweep-lock jammers therefore have more flexibility than spot jammers, and greater efficiency than barrage jammers.
The sweep-lock jammer illustrates a key maxim of electronic warfare—that the electronic "battlefield" is a constantly changing environment. Under such circumstances, no single measure can be fully effective or retain its effectiveness indefinitely in the face of an ever-changing threat.
Deception jammers. Deception jammers are more complex than the three types of brute-force jammers. As their name suggests, deception jammers are designed to deceive rather than to blind, and they are used primarily against radars. Their effect is achieved by carefully monitoring the hostile radar’s signal and matching the jamming signal to it so that the spurious input is perceived by the radar as being a genuine response.
Having achieved this goal, the fake signal can be adjusted to provide the radar with range and target data advantageous to the attacker. In this way, the radar displays more than one target trace—the true target plus the deception signal. The radar system is therefore deceived into "detecting" a particular target farther away or nearer than is actually the case. This technique, known as range-gate stealing, depends on capturing the radar’s range-measuring circuitry range gate by presenting a fake signal strong enough to attract it away from the true echo signal. Once the range gate has been deceived in this manner, the phase of the deception signal can be subtly changed so that the signal appears to emanate from a location other than its real source.
Deception jammers incorporate receivers that monitor hostile radar emissions; these receivers work in tandem with circuits that either produce a matched signal that appears real to the radar and is powerful enough to attract it away from true radar echoes or amplify and modify the received signal for retransmission. The great advantage of deception jammers over brute-force jammers is that the former do not appear to be jamming at all and are therefore harder to overcome.
