Published on Nov 23, 2015
Electronic counter-countermeasures (ECCM) is a part of electronic warfare which includes a variety of practices which attempt to reduce or eliminate the effect of electronic countermeasures (ECM) on electronic sensors aboard vehicles, ships and aircraft and weapons such as missiles.
ECCM is also known as electronic protective measures (EPM), chiefly in Europe. In practice, EPM often means resistance to jamming.
With the technology going into modern sensors and seekers, it is inevitable that all successful systems have to have ECCM designed into them, lest they become useless on the battlefield. In fact, the 'electronic battlefield' is often used to refer to ECM, ECCM and ELINT activities, indicating that this has become a secondary battle in itself. Today, more powerful electronics with smarter software for operation of the radar might be able to better discriminate between a moving target like an aircraft and an almost stationary target like a chaff bundle
Electronic counter-countermeasures (ECCM) is a part of electronic warfare which includes a variety of practices which attempt to reduce or eliminate the effect of electronic countermeasures (ECM) on electronic sensors aboard vehicles, ships and aircraft and weapons such as missiles. ECCM is also known as electronic protective measures (EPM), chiefly in Europe. In practice, EPM often means resistance to jamming
Ever since electronics have been used in battle in an attempt to gain superiority over the enemy, effort has been spent on techniques to reduce the effectiveness of those electronics. More recently, sensors and weapons are being modified to deal with this threat. One of the most common types of ECM is radar jamming or spoofing. This originated with the Royal Air Force use of what they code named window during World War II, which is now often referred to as chaff. Jamming also may have originated with the British during World War II, when they began jamming German radio communications.
In perhaps the first example of ECCM, the Germans increased their radio transmitter power in an attempt to 'burn through' or override the British jamming, which by necessity of the jammer being airborne or further away produced weaker signals. This is still one of the primary methods of ECCM today. For example, modern airborne jammers are able to identify incoming radar signals from other aircraft and send them back with random delays and other modifications in an attempt to confuse the opponent's radar set, making the 'blip' jump around wildly and be impossible to range.
More powerful airborne radars means that it is possible to 'burn through' the jamming at much greater ranges by overpowering the jamming energy with the actual radar returns. The Germans were not really able to overcome the chaff spoofing very successfully and had to work around it (by guiding the aircraft to the target area and then having them visually acquire the targets).
Today, more powerful electronics with smarter software for operation of the radar might be able to better discriminate between a moving target like an aircraft and an almost stationary target like a chaff bundle
SPECIFIC ECCM TECHNIQUES
The following are some examples of EPM (other than simply increasing the fidelity of sensors through techniques such as increasing power or improving discrimination):
Sensor logic may be programmed to be able to recognize attempts at spoofing (e.g., aircraft dropping chaff during terminal homing phase) and ignore them. Even more sophisticated applications of ECCM might be to recognize the type of ECM being used, and be able to cancel out the signal.
Pulse Compression By "Chirping", or Linear Frequency Modulation
One of the effects of the pulse compression technique is boosting 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.
Frequency agility ('frequency hopping') may be used to rapidly switch the frequency of the transmitted energy, and receiving only that frequency during the receiving time window. This foils jammers which cannot detect this frequency switch quickly enough, and switch their own jamming frequency accordingly during the receiving time window.
This method is also useful against barrage jamming, in that it forces the jammer to spread its jamming power across multiple frequencies in the jammed system's frequency range, reducing its power in the actual frequency used by the radar at any one time. The use of similar spread-spectrum techniques allow signals to be spread over a wide enough spectrum to make jamming of such a wideband signal difficult.
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 antenna is used for a comparison signal. By comparing the signal strength as received by both the omnidirectional and the (directional) main antenna, signals can be identified that are not from the direction of interest. These signals are then ignored
Polarization can be used to filter out unwanted signals, such as jamming. If a jammer and receiver do not 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.
The other main aspect of ECCM, is to program sensors or seekers to detect attempts at ECM and possible even to take advantage of it. For example, some modern fire-and-forget missiles like the Vympel R-77 and the AMRAAM are able to home in directly on sources of radar jamming if the jamming is too powerful to allow them to find and track the target normally.
This mode, called 'home-on-jam', actually makes the missile's job easier. Some missile seekers actually target the enemy's radiation sources, and are therefore called "anti-radiation missiles" (ARM). The jamming in this case effectively becomes a beacon announcing the presence and location of the transmitter. This makes the use of such ECM a difficult decision; it may serve to obscure an exact location from a non-ARM missile, but in doing so it must emit signals which can be exploited by an ARM type missile.
Tracking radars provide good resolution and precise measurement of the kinematic parameters (position, velocity, and acceleration) of targets. The estimation, updated with measurements, and prediction of the kinematic parameters as the time runs are the processing steps used to build up the tracks of targets. Tracks allow guidance and control of friendly forces, threat assessment, and enemy target engagement by weapons. Tracking can be accomplished in four ways:
(i) The dedicated radar tracker (sometimes called a single target tracker and denoted STT) continuously points its antenna at a single target by sensing errors from the true target position and correcting these errors by a servo control system. Then there are two different types of radars called, in the past, track-while-scan.
(ii) One is a limited angle scan as in some air-defence radars and aircraft landing radars, which search a limited angular sector at a rapid rate (e.g., 10 or 20 times a second).
(iii) The other type of track while scan(TWS) was what is now called automatic detection and tracking (ADT). The ADT system generates tracks of more than one target by using a series of scan-to-scan target measurements taken as the antenna samples the target paths.
(iv) The multifunctional phased-array radar tracks multiple targets by multiple independent beams, formed by the same array aperture, that are allotted to different targets.
There are two types of imaging radars: synthetic aperture radar (SAR) and inverse SAR (ISAR).
SAR allows us to have a high-resolution mapping of the EM backscatter from an observed scene. More precisely, the radar data is obtained in polar coordinates, i.e., slant range and azimuth, while a two-dimensional image in the rectangular coordinates ( x, y) is provided. High resolution in slant-range is obtained by trans-mitting a coded waveform, with a large value of the time-bandwidth product, and coherently processing—in a filter matched to the waveform—the echo signals. High resolution along the transversal direction is achieved by forming a synthetic aperture. This requires
(i) To put the radar on board a moving platform, e.g., an aircraft or a satellite.
(ii) To record the EM signals from each scattered that is illuminated by the moving antenna beam in successive instants of time, and
(iii) To coherently combine the signals via a suitable azimuthal matched filter thus focusing the sliding antenna pattern in a narrower synthetic beam.
Radiometric resolution, another key parameter, is related to the capability of SAR of distinguishing different objects in the scene on the basis of their EM reflectivity.
Radiometric resolution determines how fine a sensor can distinguish between objects with similar EM reflection properties. It is a parameter of great importance, especially for those applications oriented to extended target exploitation like polarimetry and classification. Thus, the radiometric resolution should be optimized mainly for good extended target interpretation, accounting for all kind of back-scatterers. Multi look processing is commonly used in SAR image formation in order to reduce the speckle noise. Traditional digital multi look processing consists of an incoherent addition of independent images (looks) of the same scene. The looks can be obtained by partitioning the available signal bandwidth (range and/ or azimuth) and processing each look independently. The final image is produced by adding the looks incoherently, pixel by pixel
Conclusion and Future Scope
An important defence-related role of high frequency (HF) over-the-horizon (OTH) radar is to provide a capability for early warning detection and tracking of air and ship targets. By using the ionosphere as a propagation medium, sky wave OTH radars can operate at very long distances to achieve detection and tracking at ranges of 500–3000 km.
On the other hand, surface-wave OTH radars exploit vertically polarized HF signals (3–30 MHz) and the conductive properties of sea water to detect targets at ranges limited to about 250 km. This upper limit generally applies to large ships and frequencies in the lower HF band
1. Merril. I. Skolnik, Introduction to radar systems, 2nd ed., Mc-Graw Hill International Editions, 1981.
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