JED - December 2013 - 52

E W101

The Journal of Electronic Defense | December 2013

52

Figure 4. If there are, let's say, 25
frequencies used by a radar hopping over a band at 4 GHz, they
would most likely be spread over
400 MHz. This is 10 percent of the
radar's RF frequency. (Note that
Jamming
an operating frequency range of
Frequencies
less than 10 percent allows optimum radar antenna and amplifier
performance.) If we can split our
jamming into 25 transmissions, we
can jam every pulse. However, this
Pulse
would reduce the effective jamming
Frequencies
at each frequency by a factor of 25,
which will reduce the J/S by 14 dB.
[10 log10(25) is 14 dB]
To consider the effect of spreading the jamming signal across the
Frequency
whole hopping range, we first need
to determine the coherent bandFigure 3: If a jammer can be configured to transmit a bandwidth matched jamming signal on each
width of the radar receiver. The
frequency at which a pulse can be transmitted, the jamming power at each frequency is reduced by the
coherent bandwidth will be the
number of frequencies.
inverse of the pulse width. If the
Figure 5. This is a reduction of 11 percent in the energy of the
pulse width is 1 microsecond, the coherent bandwidth would
jamming pulse compared to what it would be if there were no
be 1 MHz. It would be optimum to place spot jamming within
latency in the DRFM. Eleven percent change translates to 0.5
the radar receiver bandwidth. With non-coherent jamming,
dB, so this follower jammer with 100 nanoseconds of process
however, the jamming bandwidth would typically be a little
latency will lose only 1/2 dB of effective J/S. Note that this is
wider - let's say 5 MHz. This means that spreading our jambased on non-coherent noise jamming, and the set on accuracy
ming over the full hopping range (i.e., 400 MHz) reduces the
will be limited. Also, if the radar has leading edge tracking, it
jamming power at each hop frequency by a factor of 80. This
can still track its target in the time before jamming at the new
reduces the J/S by 19 dB. [10 log10(80) is 19 dB]
frequency starts.
We could, of course, cover only some of the frequencies at
If the pulse is longer, so that the DRFM can process the
a higher jamming level, but a "least jammed" capability designal longer, the frequency will be known to much better acfeats this strategy by not transmitting where we are jamming.
curacy. Assuming that the radar's hop frequencies are known,
Because every radar pulse is broadcast at some frequency, the
the jamming can be accurately set to the hop frequency.
skin return energy to the radar would remain the same, so the
jamming would be completely ineffective.

FOLLOWER JAMMING
On the other hand, if we could measure the frequency on each pulse and
jam on that frequency, we would achieve
the full J/S (i.e., 14 to 19 dB more J/S
than would be provided by the two approaches we have discussed).
In order to enable pulse-to-pulse follower jamming, a DRFM (along with its
associated processing components) must
determine the transmission frequency
and set the jamming to that frequency
during a small part of the pulse. Let's
consider a threat radar with 1 microsecond pulse width. If the latency in the
DRFM, including both signal propagation and processing time is less than 100
nanoseconds, the jammer could jam the
last 90 percent of the pulse as shown in

Noise Spread
Across Hopping
Range

JAMMING
Pulse
Frequencies

Frequency
Figure 4: If a jammer spreads its power over the full hopping range, the jamming power at each hop
frequency is reduced by the ratio of the jamming range to the radar receiver coherent bandwidth.



JED - December 2013

Table of Contents for the Digital Edition of JED - December 2013

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
World Report
Seeing the Light: Laser Warning Systems Facing Increasingly Lethal Threats
2014 EW/SIGINT Resource Guide
EW 101
AOC News
Index of Advertisers
JED Quick Look
JED - December 2013 - cover1
JED - December 2013 - cover2
JED - December 2013 - 3
JED - December 2013 - 4
JED - December 2013 - 5
JED - December 2013 - The View From Here
JED - December 2013 - 7
JED - December 2013 - Conferences Calendar
JED - December 2013 - 9
JED - December 2013 - Courses Calendar
JED - December 2013 - 11
JED - December 2013 - From the President
JED - December 2013 - 13
JED - December 2013 - 14
JED - December 2013 - The Monitor
JED - December 2013 - 16
JED - December 2013 - 17
JED - December 2013 - 18
JED - December 2013 - 19
JED - December 2013 - 20
JED - December 2013 - 21
JED - December 2013 - 22
JED - December 2013 - 23
JED - December 2013 - World Report
JED - December 2013 - 25
JED - December 2013 - Seeing the Light: Laser Warning Systems Facing Increasingly Lethal Threats
JED - December 2013 - 27
JED - December 2013 - 28
JED - December 2013 - 29
JED - December 2013 - 30
JED - December 2013 - 31
JED - December 2013 - 32
JED - December 2013 - 2014 EW/SIGINT Resource Guide
JED - December 2013 - 34
JED - December 2013 - 35
JED - December 2013 - 36
JED - December 2013 - 37
JED - December 2013 - 38
JED - December 2013 - 39
JED - December 2013 - 40
JED - December 2013 - 41
JED - December 2013 - 42
JED - December 2013 - 43
JED - December 2013 - 44
JED - December 2013 - 45
JED - December 2013 - 46
JED - December 2013 - 47
JED - December 2013 - 48
JED - December 2013 - 49
JED - December 2013 - EW 101
JED - December 2013 - 51
JED - December 2013 - 52
JED - December 2013 - 53
JED - December 2013 - AOC News
JED - December 2013 - 55
JED - December 2013 - 56
JED - December 2013 - Index of Advertisers
JED - December 2013 - JED Quick Look
JED - December 2013 - cover3
JED - December 2013 - cover4
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