JED - December 2010 - (Page 64)
EW Against Modern Radars – Part 13
Random Frequency or PRI, Burn-through Modes and Home-on-Jam
By Dave Adamy
The Journal of Electronic Defense | December 2010
n this final column for our series on modern radar, we consider a few radar features that make jamming more difficult and one that makes it dangerous.
A radar can have multiple operating frequencies as shown in Figure 1. Note that a radar needs an efficient antenna and a well-behaved power amplifier, so the range of frequencies used can be expected to be less than 10 percent. As explained in the November 1997 “EW 101” column, a parabolic antenna can have 55 percent efficiency if it operates over less than 10 percent frequency range, but that a wider frequency range antenna will have much lower efficiency. For example, a 2-18 GHz EW antenna can be expected to have about 30 percent efficiency. The simplest case of frequency diversity is a set of selectable frequencies, with the radar operating at the selected frequency for an extended time. As long as a receiver associated with a jammer can measure the operating frequency, the jammer can be set to the frequency in use and can optimize its jamming bandwidth against that signal. This applies to spot jamming with narrowband noise, as well as to deceptive jamming techniques. A more challenging use of frequency diversity is assignment of one frequency per sweep of the radar antenna. For example, if the radar antenna has a helical scan (one circular azimuth sweep at each of several elevation angles) the radar might change frequencies after each circular sweep. This gives the radar the advantage of a single frequency during its coherent processing interval. When a jammer has a digital radio frequency memory (DRFM), it will be able to measure the fre-
Figure 2: Random PRI requires a jammer to cover the full-time excursion of pulse times.
quency (and other parameters) of the first pulse it sees and make accurate copies of all subsequent pulses during the time the radar beam is covering the target on which the jammer is located. (Note that we will be discussing DRFMs in detail in a later “EW 101” column series.) The most challenging case of frequency diversity is pulse-topulse frequency hopping. In this case, each pulse is transmitted at a pseudo-randomly selected frequency. Because the jammer cannot anticipate the frequency of future pulses, it is impossible to optimally jam the radar. Also note that this type of radar can be expected to avoid frequencies at which jamming is detected, so jamming a few of the frequencies is unlikely to improve the jamming performance. If there are only a few frequencies, it may be practical to set a jammer to each frequency, but more typically, it is necessary to jam the whole frequency-hopping range. For example, if the radar operates over a 10 percent frequency range at about 6 GHz and has a 3 MHz receiver bandwidth: • The jammer must cover 600 MHz of frequency range. • The radar only sees the 3 MHz of the jamming signal in its bandwidth • Thus, the jamming effectiveness is only 0.05 percent • This reduces the effective J/S (compared to matched jamming) by 23 dB
If a radar has a pseudo-randomly selected pulse repetition interval, as shown in Figure 2, it is not possible to anticipate the arrival time of radar pulses. Thus it is not possible to use RGPI jamming (see the January 2010 “EW 101” column). If cover pulses are used to deny the radar range information, they must be extended to cover the full range of possible pulse positions. This requires the jammer to have a longer duty cycle in its cover pulse stream, which reduces the jamming efficiency.
Figure 1: Frequency diversity requires a jammer to cover multiple frequencies or an increased frequency range.
Table of Contents for the Digital Edition of JED - December 2010
JED - December 2010
The View From Here
From the President
EW Battle Management
Technology Survey: EW Simulators
2011 EW/SIGINT Resource Guide
Index of Advertisers
JED Quick Look
JED - December 2010