JED - March 2010 - (Page 49)

EW 101 EW Against Modern Radars – Part 4 Jamming Mono-Pulse Radars The Journal of Electronic Defense | March 2010 By Dave Adamy ast month, we discussed the angle deception of radars that must determine the angular position of a target from multiple pulse returns. Now we consider mono-pulse radars, which get angular information from every pulse return. Mono-pulse radars determine target angle by comparing signals in multiple receiving sensors. Figure 1 shows only two sensors, however actual mono-pulse radars have three or four sensors to allow two-dimensional angle tracking. The sensor outputs are combined in sum and difference channels. The sum channel establishes the level of the returned signal and the difference channel provides angle tracking information. Note that the difference response is typically linear across the 3 dB width of the sum response. The guidance input is the difference response minus the sum response. L Formation Jamming If two aircraft fly formation inside the radar’s resolution cell as shown in Figure 2, the radar will be unable to resolve them, seeing in effect a single target between the two real targets. The difficulty with this technique is that it can be very challenging to keep both aircraft within the resolution cell. Figure 1: Mono-pulse radars derive angle information from each pulse by use of multiple sensors. Figure 2: Formation jamming involves flying two aircraft within the radar’s resolution cell. The radar will “see” only one target halfway between the two real targets. Jamming techniques shown in the last two columns actually improve the angle tracking effectiveness of mono-pulse radars by increasing the signal strength received from the target location. However, there are several techniques that do work against mono-pulse radars. These include the following: • Formation jamming • Formation jamming with range denial • Blinking • Terrain bounce • Cross polarization (Cross-Pol) • Cross eye The width (i.e. cross range) dimension of the resolution cell is: W = 2 R sin (BW/2) Where W is the width of the cell in meters, R is the range from the radar to the target in meters, and BW is the 3 dB beam width of the radar antenna. The depth (i.e., down range) dimension of the cell is: D = c (PW/2} Where D is the depth of the cell in meters, PW is the radar pulse width in seconds, and c is the speed of light (3 x 108 meters per second). For example, if the target is 20 km from the radar, the ra-

Table of Contents for the Digital Edition of JED - March 2010

JED - March 2010
Contents
The View From Here
Calendar
From the President
Letters
The Monitor
Washington Report
World Report
Modernizing EW Ranges
Shooting Down the Good Guys
USAF EW Sustainment
Technology Survey: TWTs and MPMs
EW 101
AOC News
Industry/Institute/University Members
JED Sales Offices
Index of Advertisers
JED Quick Look

JED - March 2010

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