JED - February 2014 - 46

E W101

The Journal of Electronic Defense | February 2014

46

nal that does not have the Barker code
modulation will not be shortened. Since
the radar processing is optimized for the
much shorter compressed skin return
pulses, it will process jamming pulses
only during 1/13th of the time. This reduces the effective jamming power by 11
dB relative to the skin return power at
this point in the processing...so the J/S
is reduced by 11 dB. (Note that the ratio
13 converts to 11 dB.) If there were 1000
bits in the code, the J/S reduction would
be 30 dB.
The answer to this problem is, of
course, to add Barker code to the jamming pulses. The only practical way to
accomplish this is by use of a DRFM in
the jammer.
As shown in Figure 5, pulses from
the radar are input to a DRFM, which
digitizes the first pulse received and
passes it to a processor. The processor
determines the code bit duration and
the sequence of ones and zeros in the
code. The processor generates a digital representation of a "one bit" and a
"zero bit." It outputs these code blocks
in the correct sequence to form a digital representation of the Barker coded
radar pulse back to the DRFM. This output can be delayed or frequency shifted
as required to perform the desired jamming functions.
The DRFM produces RF jamming
pulses and coherently converts them
to the operating frequency of the
jammed radar. The jamming pulses
are modified from the received radar
pulses in amplitude, Doppler shift frequency and timing to produce the desired jamming techniques.

IMPACT ON JAMMING EFFECTIVENESS

HALF PULSE
DURATION
(x SPEED OF LIGHT)

3 dB beamwidth

UNCOMPRESSED
RADAR RESOLUTION
CELL

1 ft/nsec

COMPRESSED
RADAR RESOLUTION
CELL

Figure 3: With Barker code compression, the resolution cell depth is reduced to one half of the
period of a code bit multiplied by the speed of light.

Jamming Pulse
Without
Matched
Barker Code

TAPPED
DELAY
LINE

Skin Return Pulse
With Matched
Barker Code

PROCESSING
WINDOW

Figure 4: Unless jamming has the correct BPSK modulation, the effective J/S is reduced by the
compression factor.

RF
PULSES
FROM
RADAR

DRFM

Digitized
Radar
Pulses

PROCESSOR
Determine bit duration and
code from 1st pulse

COHERENT
RF
JAMMING
PULSES
TO
RADAR

Digitized
Jamming
Pulses

Generate digital "one" & "zero"
code bits
Assemble correct bits

When the BPSK coded jamming pulsto reproduce radar code
es are received by the jammed radar, the
Modify pulse amplitude,
radar's processing circuitry compresses
frequency and timing for
them just as it does the skin return
jamming modulation
pulses. This means that the J/S ratio is
not reduced by the compression factor,
which can improve the jamming effecFigure 5: A jammer with a DRFM can produce jamming pulses which have the same Barker code as
the skin return pulses and thus produce the full J/S ratio in the jammed radar.
tiveness (relative to non-coherent jamming) by many dB.
Another benefit of jamming with a DRFM is that the constructed jamWHAT'S NEXT
ming pulses have exactly the correct pulse duration. The radar receiver's
Next month, we will continue our discussion
processing circuits are optimized for pulses with a specific pulse duration,
of DRFMs as target generators. For your comments
so the jamming pulses benefit from the same processing features as skin
and suggestions, Dave Adamy can be reached at
return pulses.
dave@lynxpub.com. a



JED - February 2014

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

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
World Report
Antennas: Shaping the Future of RF Applications
DARPA – No Surprise is the Prize
EW 101
AOC News
Views from the AOC 50th Annual International Symposium and Convention
Index of Advertisers
JED Quick Look
JED - February 2014 - Cover1
JED - February 2014 - Cover2
JED - February 2014 - 3
JED - February 2014 - 4
JED - February 2014 - 5
JED - February 2014 - The View From Here
JED - February 2014 - 7
JED - February 2014 - Conferences Calendar
JED - February 2014 - 9
JED - February 2014 - Courses Calendar
JED - February 2014 - 11
JED - February 2014 - From the President
JED - February 2014 - 13
JED - February 2014 - 14
JED - February 2014 - The Monitor
JED - February 2014 - 16
JED - February 2014 - 17
JED - February 2014 - 18
JED - February 2014 - 19
JED - February 2014 - World Report
JED - February 2014 - 21
JED - February 2014 - Antennas: Shaping the Future of RF Applications
JED - February 2014 - 23
JED - February 2014 - 24
JED - February 2014 - 25
JED - February 2014 - 26
JED - February 2014 - 27
JED - February 2014 - DARPA – No Surprise is the Prize
JED - February 2014 - 29
JED - February 2014 - 30
JED - February 2014 - 31
JED - February 2014 - 32
JED - February 2014 - 33
JED - February 2014 - EW 101
JED - February 2014 - 35
JED - February 2014 - 36
JED - February 2014 - AOC News
JED - February 2014 - Views from the AOC 50th Annual International Symposium and Convention
JED - February 2014 - 39
JED - February 2014 - 40
JED - February 2014 - Index of Advertisers
JED - February 2014 - JED Quick Look
JED - February 2014 - 43
JED - February 2014 - 44
JED - February 2014 - 45
JED - February 2014 - 46
JED - February 2014 - 47
JED - February 2014 - 48
JED - February 2014 - 49
JED - February 2014 - 50
JED - February 2014 - Cover3
JED - February 2014 - Cover4
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