2-16. JAMMING AND ECM PROCEDURES. (CONT)
Transmission jamming Is caused by interfering signals generated by a jamming transmitter. The Jamming transmitter
may be fixed ground-based, airborne, free-falling (expendable transmitters dropped from aircraft), or transported by other
means. Jamming signals may be continuous wave, amplitude modulated, frequency modulated, pulsed transmission, or
a combination thereof.
A continuous wave (cw) jamming signal is a pure rf carrier at one fixed frequency with no modulation or intelligence.
When a high-power cw jammer Is operating at close ranges, the radar display can be devoid of all radar returns, or have
severe clutter when the jamming signal produces beat frequencies. At greater ranges, cw jamming tends to reduce the
sensitivity of the radar receiver, as if the IF GAIN controls were rotated counterclockwise. Clutter usually occurs in
wedge-shaped areas In the direction of the cw jamming source.
Amplitude-modulated (am) jamming signal may be amplitude modulated by one or a combination of various methods
such as with sine waves, square waves, sawtooth wave, or mechanical means, the most common type used in noise
modulation. The result of am jamming on the crt display is similar to cw jamming. The clutter is similar to that produced
by intense rain or snow.
Frequency-modulated (fm) jamming will cause unwarranted signals to appear irregularly on the crt display as the
modulating frequency varies about the operating frequency of the radar set. Fm jamming usually causes random strobing
on the crt display (intermittent intensified sweeps). At close ranges, the strobing can become intense enough to
completely saturate the crt display. At longer ranges, the strobing occurs mostly in the direction of the jammer and
occasionally occurs 180 degrees away from the jammer (reception on the backside of the antenna).
In pulsed jamming, the signal is turned on abruptly for short periods of time. The jammers can operate in a synchronous
or nonsynchronous mode at varied pulse widths and repetition rates. When in the synchronous mode, the jamming signal
is exactly the same as the pulse repetition rate of the radar set, or an exact multiple or submultiple of it. This could cause
intense range rings, similar to range marks, varying in width (range) to appear on the crt display. These rings could vary
from a fine line to a width sufficient to block the complete radar display, depending on the power and range of the
jammer. Nonsynchronized jammers can cause an unlimited number of patterns, ranging from a light sprinkling of
interference, to a spiraling pinwheel design (or running rabbits), or saturation of the crt display, depending upon the pulse
width and repetition rate of the jammer.
Reflected jamming Is caused by use of reflecting materials that reflect the radar signal just as the target does. One
common technique is "window" jamming. Window is the name given to thin strips of aluminum or other metal, cut to a
length that results in maximum signal reflection at the frequency of the radar being jammed. The strips may be dispersed
by aircraft at high altitude or missiles and projectiles with timed dispersions. As the strips flutter to earth, they appear as
intense clutter or saturated video over large areas of the crt display, similar to heavy rain or snow clutter.
Refer to TM 11-750, Electronic Countermeasures for the Operator, for a detailed
discussion of jamming, deception signals, and tactics employed against radar