JED - November 2013 - 45

E W101
will allow the radar to reject the jamming pulses and continue
to track the true target.
A DRFM can change both the time and frequency of a radar
pulse before coherently rebroadcasting it. This makes the jamming signal appear, to the radar, to be a legitimate target return,
so the jamming can break the radar's lock on the target.

RADAR INTEGRATION TIME
A radar receiver is optimized to its own signal. Thus, a pulse
of exactly the right length will have the same integration characteristics as the radar's own signal. This enhances the processing
gain of the jamming signal as compared to a jamming pulse of a
different pulse width. The DRFM can generate jamming pulses of
exactly the right pulse duration, maximizing the achieved J/S.

CHAFF

RGPO & RGPI JAMMING
The bank of Doppler filters
allows the determination of the
rate of change of range to a target.
As shown in Figure 4, a PD radar
can discriminate separating targets
- each associated with its Doppler
shift. The radar processing can look
at the range vs. time history of signals and calculate the radial velocity
of each of the separating targets.
For a legitimate target return, the
rate of change of range will be the
same as the Doppler-derived velocity. If a range gate pull-off (RGPO)
or range gate pull in (RGPI) jamming
technique is used against the radar,
the Doppler shift will not be consistent with the rate of change of
range. This is because the jammer
only delays or advances the pulses
at their transmitted frequency. This

Wind

Radar
transmission
frequency

Doppler
shift from
wind

Chaff
cloud
Reflected
spectrum
spread by
chaff dipole
movement

45

Frequency
Figure 3: The random movement of the dipoles in a chaff cloud causes spreading of the frequency in
radar signals reflecting from the cloud. Wind movement of the cloud causes frequency shift.

Velocity Cells
Velocity cells
from Doppler filter
corresponding to
true relative range
rate of target

Velocity cells
from Doppler filter
corresponding to
range rate of jamming
platform

Range Cells

Radar signals reflected from
chaff are spread in frequency by
the movement of the many chaff
elements as shown in Figure 3. With
proper analysis, the PD radar can
discriminate against chaff returns,
precluding the ability of chaff to
break the lock of the radar on the
true target, and allowing the radar
to select and process the valid target
return in the presence of chaff. This
reduces or eliminates the effectiveness of chaff as a radar countermeasure against PD radars. However, if
coherent jamming signals (from a
DRFM) are used to illuminate chaff,
it can be effective in breaking the
lock of the radar.

The Journal of Electronic Defense | November 2013

Illumination time = Scan period (beamwidth/ 360°)
= 5 sec (5°/360°) = 69.4 msec
The processing gain of a PD radar is its CPI multiplied by its
PRF, so the processing gain is:
Processing Gain = .0694 sec x 10,000/sec
= 694 which equals 28.4 dB.
The bandwidth of a single Doppler filter can be as narrow
as the inverse of the CPI or 14.4 Hz. This means that the skin
return from the radar's own signal will be enhanced by 28.4 dB,
but that a non-coherent jamming signal will not be enhanced.
Hence, a coherent jamming signal (generated by a DRFM) that
falls into a 14.4 Hz filter will provide 28.4 dB better jamming
than a non-coherent noise jamming signal of the same jammer
effective radiated power.

ACTUAL
TARGET

FALSE
TARGET
FROM
JAMMER

Figure 4: A PD radar places separating target pulses in its time/velocity matrix with the appropriate
Doppler shift for each.



JED - November 2013

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

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
World Report
What’s New in Naval ESM
Technology Survey: COMINT/DF Receivers
EW 101
AOC News
Index of Advertisers
JED Quick Look
JED - November 2013 - cover1
JED - November 2013 - cover2
JED - November 2013 - 3
JED - November 2013 - 4
JED - November 2013 - 5
JED - November 2013 - The View From Here
JED - November 2013 - 7
JED - November 2013 - Conferences Calendar
JED - November 2013 - 9
JED - November 2013 - Courses Calendar
JED - November 2013 - 11
JED - November 2013 - From the President
JED - November 2013 - 13
JED - November 2013 - 14
JED - November 2013 - The Monitor
JED - November 2013 - 16
JED - November 2013 - 17
JED - November 2013 - 18
JED - November 2013 - 19
JED - November 2013 - 20
JED - November 2013 - 21
JED - November 2013 - World Report
JED - November 2013 - 23
JED - November 2013 - What’s New in Naval ESM
JED - November 2013 - 25
JED - November 2013 - 26
JED - November 2013 - 27
JED - November 2013 - 28
JED - November 2013 - 29
JED - November 2013 - 30
JED - November 2013 - Technology Survey: COMINT/DF Receivers
JED - November 2013 - 32
JED - November 2013 - 33
JED - November 2013 - 34
JED - November 2013 - 35
JED - November 2013 - 36
JED - November 2013 - 37
JED - November 2013 - 38
JED - November 2013 - 39
JED - November 2013 - 40
JED - November 2013 - 41
JED - November 2013 - 42
JED - November 2013 - 43
JED - November 2013 - EW 101
JED - November 2013 - 45
JED - November 2013 - 46
JED - November 2013 - AOC News
JED - November 2013 - 48
JED - November 2013 - Index of Advertisers
JED - November 2013 - JED Quick Look
JED - November 2013 - cover3
JED - November 2013 - cover4
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