JED - November 2013 - 44

EW 101

Digital Radio Frequency Memories

DRFMs - Part 2
By Dave Adamy

Determined by
Doppler filter
in which signal
is received

Range Cells

COHERENT JAMMING

The Journal of Electronic Defense | November 2013

44

One of the advantages of using a
digital RF memory (DRFM) is that it can
generate a coherent jamming signal. This
is particularly important when jamming
a pulse-Doppler (PD) radar. Figure 1
shows a range vs. velocity matrix which
is part of the PD radar processing - for
all signals entering the radar receiver.
The velocity dimension of the matrix is
generated by a bank of narrow filters,
usually implemented in software. Since
the transmitted signal is coherent, the
legitimate return signal from a target
will fall into one filter, and those filters are quite narrow. However, a noncoherent jamming signal like barrage
or spot noise will enter several filters.
This allows the radar to reject jamming
in favor of the return of its own coherent signal.

Velocity Cells

Determined by
pulse return
time

Figure 1: The processing hardware and software of a PD radar includes a matrix of time vs. radial
velocity for each received pulse.

Angular coverage
of antenna scan

INCREASED EFFECTIVE J/S
Noise jamming can have its effective
Jamming to Signal Ratio (J/S) against
a PD radar reduced by many dB by the
radar's processing gain. Consider the case
of an acquisition radar that has its coherent processing interval (CPI) equal to the
time that its scanning beam will illuminate a target. The radar has a circular
scan with a period of 5 seconds, a beam
width of 5 degrees and a pulse repetition frequency (PRF) of 10,000 pulses per
second. The beam illuminates the target
(a time equal to the CPI) for 69.4 milliseconds, calculated from the following
formula. (See Figure 2.)

Target
Antenna
beam width

Figure 2: The amount of time a scanning radar illuminates a target depends on its beam width,
scan rate and angular scan coverage.



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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