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Deceptive jamming against monopulse radars (part two).

This is the second in a three-part series on the jamming of monopulse radars. Last month's tutorial went into some detail on the nature of the radar resolution cell and covered the formation- and blinking-jamming techniques. This article covers several more techniques: terrain bounce jamming, skirt jamming, image jamming and cross-polarization jamming.

Terrain Bounce

The terrain-bounce technique [ILLUSTRATION FOR FIGURE 1 OMITTED] is particularly powerful against active or semi-active missile-guidance systems. A strong simulated radar return is generated and directed at an angle that will cause a reflection from the ground. The jammer transmission must have sufficient effective radiated power (ERP) to reflect a signal from the ground that reaches the missile-tracking antenna with significantly more signal strength than the skin return from the aircraft being attacked. If done properly, this will cause the missile to be guided below the protected aircraft.

Skirt Jamming

Figure 2 shows the amplitude pass band of a bandpass filter. Filters are designed to pass all frequencies within a pass band with as little attenuation as possible, while causing as much attenuation as possible to all signals outside the pass band. An ideal filter (sometimes called a "stone wall filter") would provide infinite attenuation to any signal even a tiny bit out of band. However, real-world filters have "skirts" in which input signals are attenuated by an amount proportional to the amount by which they are out of band. The slope of the skirt is 6 dB per octave - that is, the attenuation increases by a factor of four for each doubling of the frequency "distance" from the center of the filter's pass band - for each stage of filtering. Filters also have an "ultimate rejection" level, a maximum attenuation applied to signals that are far out of band. The ultimate rejection is often about 60 dB. This, means that a very strong out-of-band signal can get through the filter with some rejection if it is very close to the pass band and more if it is far out of band.

The other curve in Figure 2 shows the phase response of the filter. Across the pass band, a well designed filter will usually have a fairly linear phase response. However, beyond the band edges, the phase response is undefined and can be extremely nonlinear. This means that if a strong jamming signal is received in the "skirt" frequency range, it will have an erroneous phase, causing the radar's tracking circuitry to malfunction. The J/S ratio must, of course, be very high, because the jammer must overcome the filter's rejection and still have significantly more power than the true skin return.

Image Jamming

Figure 3 is a frequency spectrum diagram. As you will recall from an "EW 101" column several months ago, a superheterodyne receiver uses a local oscillator (LO) to convert an input RF frequency to an intermediate frequency (IF). This frequency conversion occurs in a mixer, which generates harmonics and the sums and differences of all signals input to it. The output of the mixer is filtered and passed to an IF amplifier (and then perhaps to another frequency-conversion stage). The frequency of the LO is either above or below the frequency of the desired receiver tuning frequency by an amount equal to the IF frequency. For example, in an AM broadcast receiver tuned to 800 kHz, the LO frequency is 1,255 kHz (because the IF frequency is 455 kHz). In this case, the "image" frequency is 1,710 kHz, and a signal received into the mixer at this frequency would also appear in the IF amplifier, causing severe degradation to receiver performance. To prevent such "image response," the receiver design almost always includes a filter to keep the image frequency away from the mixer.

Incidentally, the reason that wide-frequency-range reconnaissance receivers typically have multiple conversion designs is often to avoid image-response problems.

Assume for a moment that a particular radar receiver uses an LO whose frequency is above the frequency to which the receiver is tuned, as in Figure 3. The receiver is, of course, tuned to the appropriate frequency to receive the skin return; the IF frequency is equal to the difference between the skin-return frequency and the LO frequency. If a signal that looks like the skin return were received at the image frequency with enough power to overcome input filtering, it would also be amplified by the radar's IF amplifiers and processed along with the skin return. However, it would be reversed in phase from the true skin return, which would cause the radar's tracking error signal to change its sign (i.e., move the radar away from the target rather than toward it). Unfortunately, this technique requires a great deal more knowledge about the radar's design than just its transmitted frequency (which will, of course, be the skin-return frequency without Doppler shifts). Does it use high-side or low-side conversion - i.e., is the LO above or below the frequency of the skin return? If the radar receiver has little or no tuned front-end filtering, this technique requires only moderate J/S ratio, but if there is significant tuned filtering, 60 dB or more J/S may be required.

Cross-Polarization Jamming

Cross-polarization jamming can be effective against some radars that use parabolic dish antennas. The effectiveness is a function of the ratio of the antenna's focal length to its diameter, because the smaller this ratio, the greater curvature the antenna will have. If illuminated by a strong cross-polarized signal, the antenna will provide false tracking information because of cross-polarization lobes called "Condon" lobes. If the cross-polarization response is dominant over the matched polarization response, the radar tracking signal will change signs, causing the radar to lose track on the target.

To produce a cross-polarized signal, the jammer has two repeater channels with orthogonally oriented antennas (i.e., each with linear polarization, but 90 [degrees] to each other) as shown in Figure 4. Although any set of orthogonal polarizations would work, they are shown as vertical and horizontal in the figure. If the vertically polarized component of the received signal is retransmitted with horizontal polarization, and the horizontal with vertical polarization, the received signal will be retransmitted cross-polarized to the received signal, as shown in Figure 5.

This technique requires 20 to 40 dB of J/S, depending on the design of the radar's antenna. It must be noted that an antenna protected by a polarization screen will have little vulnerability to cross-polarization jamming.

What's Next

Next month we'll finish our discussion of deceptive jamming techniques used against monopulse radars with a discussion of cross-eye jamming. For your comments and suggestions, Dave Adamy is at Internet:
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Article Details
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Author:Adamy, Dave
Publication:Journal of Electronic Defense
Date:Mar 1, 1997
Previous Article:A sampling of SIGINT systems.
Next Article:Washington report.

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