JED - December 2009 - (Page 56)

EW 101 EW Against Modern Radars – Part 1 Radar Jamming Equations or the next few months, we will be discussing modern radars from an EW point of view. A part of this discussion is, of course, the vulnerability of these radars to electronic attack (EA), and their electronic protection (EP) features. Radar jamming was initially covered in the May 1996 to April 1997 “EW 101” columns, and radars were covered in the May 2000 to February 2001 “EW 101” columns. We will begin this series with a brief review of some of the important issues raised in these earlier columns. Transmitter Power RADAR JAMMING Radar jamming approaches are differentiated by geometry and by techniques. First we will cover the geometric considerations: self protection and remote jamming. This includes dB formulas for the jammer to signal ratio (J/S) and burn-through range associated with both types of jamming. In the following discussion, “log” is the logarithm to the base 10, all jamming power is assumed to be within the radar receiver’s bandwidth, and the radar is assumed to use a single antenna for transmit and receive. More complex cases will be considered later in this series. You will note that each of the dB formulas in this series includes a number (for example -103). This number combines conversion factors allowing values to be input in the most convenient units. The rather large resulting number is converted to dB form. A very important consideration in the use of all dB formulas is that the input values must be entered in the specified units to get the correct answer. First, consider the power a radar receiver receives from the skin return from a target. As shown in Figure 1, the transmitted power is focused toward the target by the radar’s antenna. The effective radiated power (in dB form) is the transmitter power increased by the main beam bore-sight gain. Because a typical radar uses a directional antenna to transmit and receive signals, the propagation mode is line of sight (see the July 2007 “EW 101” column). The skin return power in the radar receiver is called “S” and is given (in dBm) by the formula: S = -103 + ERPR – 40 log R – 20 log F + 10 log o + G ˉ Where: ERPR is the radar effective radiated power toward the target in dBm R is the range from the radar to the target in km F is the radar’s transmitting frequency in MHz o is the radar cross section of the target in square ˉ meters G is the main beam boresight gain of the radar antenna in dB The power received by the radar from the jammer is called “J” and is given (in dBm) by the formula: J = -32 + ERPJ – 20 log R J – 20 log F + GRJ Where: ERPJ is the jammer effective radiated power toward the radar in dBm RJ is the range from the jammer to the radar in km F is the jammer’s transmitting frequency in MHz GRJ is the gain of the radar’s antenna (in dB) in the direction toward the jammer Antenna Gain Range The Journal of Electronic Defense | December 2009 SELF PROTECTION JAMMING As shown in Figure 2, a self protection jammer is located on the target being detected or tracked by a radar. This means that the distance from the jammer to the radar is “R” and the gain of the radar antenna toward the jammer and the target are the same (we will call this gain “G”). By subtracting the expression for “S” from the expression for “J” and simplifying, we get the following formula for the J/S produced by a self protection jammer: J/S = 71 + ERPJ – ERPR + 20 log R - 10 log o ˉ Received Skin return power Radar Signal Jammer Signal J RADAR Figure 2: Self Protection Jamming protects a target by use of an onboard jammer. Antenna Gain Transmitter Power XMTR RCVR Radar Cross-section of Target Range to Target REMOTE JAMMING In remote jamming, the jammer is not located at the target. The classical case of remote jamming is stand-off jamming as shown in Figure 3. The jammer (typically in a special jamming aircraft) is beyond the lethal range of the weapon controlled by a tracking radar. The jammer protects target aircraft that are within that lethal range. The stand-off jammer typically protects multiple targets from acquisition by multiple radars. Received Skin return power Figure 1: Radar skin return power is calculated from the radar transmitter power and antenna gain, the range to the target, and the target radar cross section.

Table of Contents for the Digital Edition of JED - December 2009

JED - December 2009
Table of Contents
The View From Here
Calendar
From the President
The Monitor
Washington Report
World Report
2010 EW/SIGINT Resource Guide
What’s New in SIGINT Software?
EW 101
AOC News
AOC Industry/Institute/University Members
Index of Advertisers
JED Sales Offices
JED Quick Look

JED - December 2009

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