JED - February 2010 - (Page 48)
EW 101 EW Against Modern Radars – Part 3 Frequency and Angle Deceptive Jamming By Dave Adamy LThe Journal of Electronic Defense | February 2010 ast month, we discussed cover jamming and deceptive jamming in range. This month, we will continue our coverage of deceptive jamming with techniques that break the angle track or frequency track of hostile radar. When a radar’s range track is broken, several milliseconds may be required to reestablish tracking, after which the range track must be broken again. However, if angle track is broken, the radar must typically return to a search mode to locate the target in angle – which can take seconds. SKIN RETURN times. However, if the jammer times its bursts slightly faster or slower than the known scanning rate of the radar antenna, the jamming can still break up the angle tracking by the radar. This will still allow effective jamming, although not as effective as though the burst were optimally timed. Figure 2 shows angle jamming of a “track while scan” (TWS) radar. On the first line, the skin return from the TWS Angle Deceptive Jamming Older radars required movement of the antenna beam to track targets in angle. Consider the received power vs. time diagram for a conically scanned radar shown on the top line of Figure 1. The antenna movement describes a cone. When the antenna is pointed closer to the target, the received signal is stronger and when it is pointed away from the target, the signal is weaker. The radar moves the center of its scanning pattern in the direction of the maximum return power to center the target in the scan. Both the radar receiver and a radar warning receiver on the target see this same power vs. time plot. If a jammer located on the target transmits a burst of strong pulses (synchronized with the radar’s pulses) at the weakest signal strength time (see the second line of Figure 1), the radar will see a power vs. time plot as shown on the third line of Figure 1. Because the radar develops guidance signals from this information, the processing will see the power data in its (narrow) servo response bandwidth as shown by the blue dash line. Hence, the radar will direct its scan axis away from the target – breaking the angle track. This is called “inverse gain” jamming. If the radar has a non-scanning illuminator, but scans its receiving antenna, the jammer on the target will be unable to know the phase of the sinusoidal power variation with time. Thus, the jammer is unable to time its pulse bursts to the minimum received power JAMMING SIGNAL SERVO RESPONSE RADAR RECEIVED SIGNAL Figure 1: Inverse gain jamming causes a radar to correct its angle guidance in the wrong direction. Angle Gate Skin Return Jammer Radar Return Figure 2: Inverse gain jamming causes a “track while scan” (TWS) radar to move away from its target in angle.
Table of Contents for the Digital Edition of JED - February 2010
JED - February 2010
The View From Here
From the President
New Solutions in Shipboard SIGINT
Technology Profile: What’s New in Antennas?
AOC Industry and Institute/University Member Guide
JED Sales Offices
Index of Advertisers
JED Quick Look
JED - February 2010