JED - January 2010 - (Page 44)
EW 101 EW Against Modern Radars – Part 2 Radar Jamming Techniques By Dave Adamy TThe Journal of Electronic Defense | January 2010 his month, we will review jamming techniques. These techniques can be divided into cover and deceptive jamming. The jamming effectiveness of both types of techniques is stated in terms of jamming-to-signal ratio (J/S) as discussed last month. Cover Jamming The object of cover jamming is to reduce the quality of the signal in the radar’s receiver enough that the radar cannot acquire or track its target. It can be used in either self-protection or remote-jamming geometry. Cover jamming usually has a noise waveform, but sometimes other waveforms are used to overcome electronic protection (EP) features of the radar. These EP techniques will be covered later in this series. The equations for J/S and burn-through presented last month assumed that all of the jammer’s power was within the bandwidth of the radar receiver. If a jammer uses noise that is wider in frequency than the effective bandwidth of the radar receiver, only the part that is within the radar’s receiver bandwidth is effective. Jamming efficiency is the total jammer effective radiated power (ERP) divided by the effective jammer ERP. This is equal to the radar receiver bandwidth divided by the jamming bandwidth. For example, if the radar receiver bandwidth is 1 MHz and the jamming signal bandwidth is 20 MHz, the jamming efficiency is 5 percent. is tuned to the radar broadcast frequency – this is called spot jamming. As shown in Figure 1, spot jamming wastes little of its jamming power, so the jamming efficiency is increased significantly. Spot width is enough to cover the uncertainty in target signal and set-on frequencies. (We will cover coherent jamming in a later column.) Efficiency is still the radar bandwidth divided by the jamming bandwidth – but the ratio is more favorable. Dr. Schleher, in his book Electronic Warfare in the Information Age, defines spot jamming as jamming over a bandwidth less than five times the radar’s bandwidth. JAMMER RECEIVER LISTENING NOISE COVERAGE FREQUENCY UNCERTAINTY TIME Figure 1: Spot Jamming concentrates noise around the radar’s operating frequency. Swept Spot Jamming If a narrowband jammer is swept across all of the frequency range that is expected to contain threat signals, as shown in Figure 2, it is called a swept spot jammer. The swept spot jam- Barrage Jamming Barrage jamming is generated by a wideband jammer that broadcasts noise over a whole band of frequencies that is expected to contain one or more threat radars. This technique was frequently used in early jammers, and is still an appropriate approach for many jamming situations. The great advantage of barrage jamming is that it does not require real-time information about radar operating frequencies. Look-through (i.e., interruption of jamming to look for threat radar signals) is not necessary. The problem is that barrage jamming typically has very low jamming efficiency. Most of the jamming power is wasted because the effective J/S is reduced by the efficiency factor, and the burn-through range is correspondingly increased. Swept Spot Jamming FREQUENCY Spot Jamming When the bandwidth of the jamming signal is reduced to a little more than the target radar bandwidth and the jammer Fi 2 S tS tJ i TIME j i b d th h l b di Figure 2: Swept Spot Jamming moves a narrow jamming band across the whole band in which the radar might operate.
Table of Contents for the Digital Edition of JED - January 2010
JED - January 2010
The View From Here
From the President
Asia-Pacifi c EW: All Eyes on China?
Maneuver in the EM Domain
Technology Survey: Surface Naval Expendables and Launchers
AOC Industry/Institute/University Members
Index of Advertisers
JED Sales Offices
JED - January 2010