JED - July 2010 - (Page 52)
EW Against Modern Radars – Part 8A*
Monopulse Radar and Anti-CrossPol EP
The Journal of Electronic Defense | July 2010
onopulse radars get direction-of-arrival information from every skin-return pulse. Because this makes certain kinds of deceptive jamming ineffective, it can be considered an electronic protect (EP) technique. Cross polarization (CrossPol) is one of the techniques speciﬁcally recommended for use against monopulse radar. Thus, anti-CrossPol EP approaches deserve careful attention.
to jamming signals, which distorts the balance of the antenna feeds. For example, CrossPol jamming causes a radar to point one of its cross-polarized Condon lobes at the target.
Cross Polarization Jamming
CrossPol jamming was covered in the April 2010 “EW 101,” but to better understand Condon lobes, try this: Hold a pencil in your hand oriented 45 degrees to the right and move your hand toward a wall at a 45-degree angle until the pencil touches the wall. Then move your hand in the direction that the pencil would move if it were “reflected” from the Monopulse Radar wall. You will notice that the pencil is now oriented 45 degrees Jamming techniques, such as range gate pull-off or cover left in the direction of travel. The forward geometry of the wall pulses, provide range deception, but because they generate and the angle of the diagonal angle of the pencil have caused strong pulses from the direction of the target, they enhance the angle of the pencil relative to the forward motion of your angle tracking by monopulse radars. Angle-deception techhand to change 90 degrees. niques like inverse gain jamming, which generate strong pulses Now consider the vertically polarized signal arriving in the to fool radar tracking algorithms, likewise enhance monopulse upper right portion of the parabolic dish reflector in Figure angle tracking. 2. The forward geometry of the dish causes a (weak) In general, angle deception horizontally polarized reflection toward the antenna is more powerful than range feed because this part of the dish is about 45 degrees deception. A radar can typiDIRECTION TO TARGET to the signal polarization. This effect causes each cally reacquire in range in milWHEN SIGNALS ARE BALANCED IN ANTENNAS Condon lobe. liseconds, while a significant In his excellent but very technical (and now out pull-off in angle will require of print) set of three books on applied electronic a return to the radar’s acquicountermeasures, Leroy Van Brunt provides detailed sition mode. This may cause discussions of CrossPol jamming. He points out an angle reacquisition time of that CrossPol jamming can be used with either onseconds. frequency or noise jamming and is effective against A chaff cloud or a decoy, both acquisition and tracking radars, including the which creates an actual, tracktwo-beam SA-2 track-while-scan radars in which the able object, works well against beams are cross polarized to each other. monopulse radars. Σ Δ In addition to the two-path repeater type CrossPol Monopulse radars point their jammer described in the April 2010 “EW 101,” there antennas toward targets by adERROR CORRECTION SIGNAL are jammers that sense the polarization of arriving justing in angle to balance the Δ-Σ radar signals and create a cross polarized response power received by multiple anFigure 1: A monopulse radar has multiple with a signal generator as shown in Figure 3. tenna feeds as in Figure 1. Efantenna feeds and generates antenna If a two-channel repeater CrossPol jammer cannot fective angle jamming forces the pointing corrections from the difference achieve adequate antenna isolation, Mr. Van Brunt radar to move its antenna in an of the two received signals normalized to the sum. points out that time gating can be used to isolate the improper direction in response *Editor’s Note: Due to an editorial error, the June 2010 “EW 101” installment was misnumbered as the eighth article in the “EW Against Modern Radars” series. It should have been titled “EW Against Modern Radars – Part 7.” The articles themselves have been published in the correct sequence. The June “EW 101” was simply misnumbered. JED regrets the error.
Table of Contents for the Digital Edition of JED - July 2010
JED - July 2010
The View From Here
From the President
SIGINT for Special Mission Aircraft
Fighter Aircraft EW: Shifting from Defense to Attack
Technology Survey: Airborne Radar Jammers
Index of Advertisers
JED Sales Offices
JED Quick Look
JED - July 2010