Electronic counter-counter-measures
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Electronic counter-counter measures, or ECCM, refers to an electronic system's ability to function in the presence of Electronic counter-measures, or ECM. Because the label ECCM is fairly cumbersome, the synonym, electronic protection, or Electronic Protective Measures, or EPM, is more common in modern usage. Also see Electronic warfare. In practice, EPM often means resistance to jamming. With radar, the following are some examples of EPM.
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Sidelobe cancellation
Radar jamming can be effective from directions other than the direction the radar antenna is currently aimed. When jamming is strong enough, the radar receiver can detect it from a relatively low gain sidelobe. The radar, however, will process signals as if they were received in the main lobe. Therefore, jamming can be seen in directions other than where the jammer is located. To combat this, an omnidirectional pecker antenna is used for a comparison signal. By comparing the signal strength as received by both the omnidirectional and the (directionall) main antenna, signals can be identified that are not from the direction of interest. These signals are then ignored.
Chirp, or Linear Frequency Modulation
This is an electronic method to boost the apparent signal strength as perceived by the radar receiver. The outgoing radar pulses are 'chirped,' that is, the frequency of the carrier is varied within the pulse, much like the sound of a cricket chirping. When the pulse reflects off a target and returns to the receiver, the signal is processed to add a delay as a function of the frequency. This has the effect of 'stacking' the pulse so it seems stronger, but shorter in duration, to further processors. The effect can increase the received signal strength to above that of noise jamming. Similarly, jamming pulses (used in deception jamming) will not typically have the same chirp, so will not benefit from the increase in signal strength.
Polarization
Polarization can be used to filter out unwanted signals, such as jamming. If a jammer and receiver don't have the same polarity, the jamming signal will incur a loss that reduces its effectiveness. The four basic polarities are horizontal, vertical, right-hand circular, and left-hand circular. The signal loss inherent in a cross polarized (transmitter different from receiver) pair is 3 decibels for dissimilar types, and 17 dB for opposites. Aside from power loss to the jammer, radar receivers can also benefit from using two or more antennas of differing polarity and comparing the signals received on each. This effect can effectively eliminate all jamming of the wrong polarity, although enough jamming may still obscure the actual signal.