deliberate interference that impairs or disrupts for military purposes the normal operation of radar equipment, such as radar sets, the homing heads of guided missiles and aerial bombs, and proximity fuzes. A distinction is made between active and passive countermeasures; active countermeasures are implemented by special transceivers or transmitters of radio-wave interference—jamming stations—whereas passive countermeasures rely on a variety of artificial reflectors of radio waves. Although passive interference can also result from the reflections of radio waves from local objects and natural formations, such reflections cannot be regarded as countermeasures.

Active countermeasures, according to the nature of their effect, are classified as either jamming or deception (disorienting) techniques. Jamming involves the radiation of noise, which makes it difficult to discriminate the signals reflected from targets; deception techniques make use of signals similar to those reflected from a target but containing false information. Jamming is often in the form of radio-frequency signals modulated with noise or of noise fluctuations similar to those that are internal to the radar set’s receiver. Depending on the width of the frequency band, jamming is subdivided into selective jamming, which has a spectral width commensurate with the pass band of the radar receiver, and barrage jamming, which covers a certain part of the radio-frequency spectrum. Active countermeasures can also take the same form as the scanning radar signals, which are then modulated in amplitude, frequency, phase, time delay, or polarization. These modulated signals are formed from the scanning signals picked up by the jamming stations. The interference thus created, which is called responding interference, can be used both in jamming and in deception techniques.

The jamming stations are placed on the targets being protected or are exterior to them. Modern (mid-1970’s) jammers for aircraft have powers ranging from approximately 10 to 10³ watts under continuous output and an order of magnitude higher in the pulse mode; the antenna gain ranges from 10 to 20 decibels. The power of ground and ship jamming stations is generally higher. The transmitters in the stations employ broadband amplifiers equipped with traveling-wave tubes, amplifiers with distributed constants, oscillators using backward-wave tubes and magnetrons (tunable magnetrons), and other vacuum-tube devices tunable over a broad frequency range. Equipment has been developed with phased array antennas, in which there are amplifiers and oscillators using semiconductor devices and miniature traveling-wave tubes.

Passive countermeasures are carried out with objects such as chaff, strip and corner reflectors, dielectric lens reflectors, array antennas, and balloons with metallized coatings. Chaff, which is widely used, is made up of strips of foil, metallized paper, or metallized glass fibers, each having a length equal to approximately one-half the wavelength of the scanning radiation. Large quantities of chaff, either packaged in pods or in loose form, are thrown or fired into the air and become scattered in flight. As a rule, passive reflectors do not have their own sources of energy. In the 1970’s, however, with the new developments in semiconductor electronics and the miniaturization of electronic components, reflectors can now be equipped with miniature electronic amplifiers and oscillators and thereby converted into instruments for active countermeasures—miniature jammers.

Countermeasures create a noise background or a false target indication either on certain parts or over the entire surface of the cathode-ray display screen, which complicates the detection, classification, and tracking of the targets. The effect on the apparatus that automatically detect and track targets with respect to azimuth, elevation, speed, and range is such as to overload the automatic data-processing equipment, interrupt the automatic tracking, and introduce large errors in the determination of a target’s position and parameters of motion.