JED - June 2014 - 49

EW 101

Digital RF Memories

DRFMs - Part 9
By Dave Adamy
Radar tracks on leading edge

Since we will be leaving the DRFM series (at least for a while), it is
good to summarize the role of the DRFM in EW.

Radar pulses

iv

lses

g pu

mmin

ed ja

elay
ely d

Frequency

ress
Several radar techniques are difficult for traditional jamProg
mers to counter. These include:
* Coherent radar
Figure 2. If a radar uses leading-edge tracking, a DRFM-equipped
jammer can generate jamming pulses with leading edges matching the
* Leading-edge tracking
skin return within 50 nsec.
* Pulse-to-pulse frequency hopping
capture the leading edge tracker. This will make both range* Pulse compression
gate-pull-off and terrain-bounce jamming effective.
* Range rate/Doppler shift correlation
Frequency hopping, either from coherent processing inter* Detailed analysis of target radar cross section
val (CPI) to CPI or from pulse to pulse,
Coherent radars expect their skin
requires a conventional jammer to cover
returns to fall within a single frequency
Non-coherent
Coherent
jamming
the whole hopping range of the radar.
cell as shown in Figure 1. This refers to
skin
signal
return
(The radar uses only one frequency at
a pulse-Doppler radar, which has a bank
a time, but the jammer does not know
of filters in its processing circuitry.
which.) This reduces the J/S that the
Because non-coherent jammers, even
jammer can produce.
in spot jamming modes, spread their
Doppler frequency cells
By measuring the radar's frequency
power across multiple filters, the radar
during the first 50 nanoseconds of each
can detect jamming and can go into a
pulse (as in Figure 3), a DRFM-equipped
home-on-jam mode. It will also reduce
Figure 1. A coherent radar produces a skin
return in a signal frequency cell, whereas a
jammer can produce jamming signals,
the achieved J/S by coherent pulse pronon-coherent jamming signal occupies several.
which follow the frequency hopping
cessing gain.
and cover an extremely large percentSince DRFM-equipped jammers can
age of the skin-return pulses.
generate coherent jamming signals, the pulse-Doppler radar
Pulse compression, in addition to improving the range
provides the same processing gain to jamming signals and canresolution of a radar, also reduces the J/S that a jammer can
not detect the presence of jamming. This both improves the J/S
produce by the same amount as the compression ratio. This
and prevents the activation of home-on-jam modes.
Leading-edge tracking makes range-gatepull-off jamming ineffective, because the jamDRFM captures frequency
ming pulses are progressively delayed from the
Radar pulse
radar's skin return. The radar tracks targets
Jammer
using only the leading edges of pulses. Since
pulse
the leading edges of the jamming pulses are
DRFM captures frequency
later than those of the skin return, the radar
continues to track its skin return pulses, igRadar pulse
noring the jamming pulses as shown in Figure
Jammer
pulse
2. Depending on the geometry, leading-edge
DRFM captures frequency
tracking also may allow the radar to ignore
Radar pulse
the jamming pulses of terrain-bounce jamJammer
ming, which are delayed because of their lonpulse
ger transmission path.
Time
Modern DRFMs, since they have very short
latency times (of the order of 50 nanoseconds),
Figure 3. A DRFM-equipped jammer captures the frequency of a frequency-hopped pulse
can generate jamming pulses quickly enough to
in the first 50 nsec and matches the frequency in the corresponding jamming pulse.

The Journal of Electronic Defense | June 2014

RADAR TECHNIQUES THAT CALL FOR DRFM-BASED
COUNTERMEASURES

49



JED - June 2014

Table of Contents for the Digital Edition of JED - June 2014

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
World Report
50 Years of Airborne SIGINT
ACE Program Lays Out Plans for New EW Capabilities
Technology Survey: Spectrum Analyzers
EW 101
NAWCWD Hosts 43rd Annual Collaborative Electronic Warfare Symposium
AOC News
Index of Advertisers
JED Quick Look
JED - June 2014 - cover1
JED - June 2014 - cover2
JED - June 2014 - 3
JED - June 2014 - 4
JED - June 2014 - 5
JED - June 2014 - The View From Here
JED - June 2014 - 7
JED - June 2014 - Conferences Calendar
JED - June 2014 - 9
JED - June 2014 - Courses Calendar
JED - June 2014 - 11
JED - June 2014 - From the President
JED - June 2014 - 13
JED - June 2014 - 14
JED - June 2014 - The Monitor
JED - June 2014 - 16
JED - June 2014 - 17
JED - June 2014 - 18
JED - June 2014 - 19
JED - June 2014 - World Report
JED - June 2014 - 21
JED - June 2014 - 50 Years of Airborne SIGINT
JED - June 2014 - 23
JED - June 2014 - 24
JED - June 2014 - 25
JED - June 2014 - 26
JED - June 2014 - 27
JED - June 2014 - 28
JED - June 2014 - 29
JED - June 2014 - 30
JED - June 2014 - 31
JED - June 2014 - 32
JED - June 2014 - ACE Program Lays Out Plans for New EW Capabilities
JED - June 2014 - 34
JED - June 2014 - 35
JED - June 2014 - 36
JED - June 2014 - Technology Survey: Spectrum Analyzers
JED - June 2014 - 38
JED - June 2014 - 39
JED - June 2014 - 40
JED - June 2014 - 41
JED - June 2014 - 42
JED - June 2014 - 43
JED - June 2014 - 44
JED - June 2014 - 45
JED - June 2014 - 46
JED - June 2014 - 47
JED - June 2014 - 48
JED - June 2014 - EW 101
JED - June 2014 - 50
JED - June 2014 - 51
JED - June 2014 - 52
JED - June 2014 - NAWCWD Hosts 43rd Annual Collaborative Electronic Warfare Symposium
JED - June 2014 - AOC News
JED - June 2014 - 55
JED - June 2014 - 56
JED - June 2014 - Index of Advertisers
JED - June 2014 - JED Quick Look
JED - June 2014 - cover3
JED - June 2014 - cover4
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