JED - May 2014 - 61

E W101
Figure 3. After receiving the first BPSK-modulated
pulse, the processor determines the code clock and
the sequence of 1s and 0s in the code. Then, it
creates a digital model for a 1 and for a 0. Finally, it
outputs a digital signal with the correct code for each
subsequent pulse and outputs this signal to the DRFM
for retransmission with the time and frequency shift
appropriate to the jamming technique employed.

Pulse Width

BPSK Code Clock
Received BPSK
Modulation
on First Pulse

FOR UNIQUE PULSES
Now, consider the more challenging requirement to
Transmitted
reproduce radar signals that change on a pulse-to-pulse
Rebroadcast Pulse Width
Waveform
basis. The primary example is a pulse-to-pulse frequencyhopping radar. There will be multiple frequencies that are
pseudo-randomly selected by the radar. Also, it is reasonFigure 4. If every pulse is unique, the width of rebroadcast pulses will be
able to assume that the radar can sense when it is being
reduced by the processing latency time.
jammed and will have a "least jammed frequency" mode.
The energy of the jamming pulse is reduced by the ratio of
Frequencies at which jamming or other interference is detected
the duration of the jamming pulse (i.e., the radar's pulse width
will be skipped in the hopping sequence. This means that a
less the processing latency time vs. the original pulse width).
jammer without the ability to measure the frequency of each
For example, if the pulse width is 10 µsec and the processing
pulse must cover the whole frequency hopping range and canlatency time is 100 nsec, the reduction in jamming energy is:
not maximize its J/S by concentrating its power in part of the
covered range (a technique called "partial-band jamming").
9.9 µsec/10 µsec = .99
If the jamming frequency bandwidth is spread, the achieved
Which is only 0 .04 dB.
jamming to signal ratio (J/S) is reduced by the ratio of the radar's receiver bandwidth to the pulse-hopping range. Take for
example a radar with a 3-MHz receiver bandwidth operating at
WHAT'S NEXT
6 GHz. The radar's hopping frequency range would typically be
Next month, we will discuss some other techniques that are
10 percent of its operating frequency (i.e., 600 MHz). The ratio
used to handle processing latency issues in DRFMs. For your
of the hopping range to the receiving bandwidth is thus: 600
comments and suggestions, Dave Adamy can be reached at
MHz/3 MHz = 200.
dave@lynxpub.com. a

The Journal of Electronic Defense | May 2014

This reduces the effective RCS by 23 dB.
Now, consider a jammer with a DRFM that can
measure the frequency of each received pulse.
Transmitted BPSK
Modulation
By knowing the frequency of each received
on Subsequent Pulses
pulse, it can jam that pulse at the correct frequency, avoiding this loss of effective J/S.
Since the frequency of each pulse is not
known until it is received at the jammer, a DRFM and its as* The clock rate of the code (a Barker code or some longer
sociated processor must:
maximal length code),
* Determine the radar transmitting frequency,
* The sequence of 1s and 0s in the code,
* Generate a digital representation of the pulse with the cor* The received frequency,
rect frequency and timing (including any frequency and
* And the pulse's time of arrival.
time offset for the chosen jamming technique), and
Then, the processor develops digital signals for a "one bit"
* Begin coherently rebroadcasting at that frequency.
and for a "zero bit." Finally, as shown in Figure 3, the proIt must perform all of these tasks during a small part of the
cessor outputs a digital representation of the BPSK-modulated
radar's pulse width, as shown in Figure 4.
pulse for each subsequent pulse in the received signal. The
generated signal has the correct frequency from the received frequency and the Doppler shift appropriate for the
jamming technique chosen. The signal is delayed by an
Received
amount that will place the pulse at the proper time for
Waveform
Pulse Width
each subsequent pulse considering both the pulse repetition interval (PRI) of the received signal and the time
offsets required for the chosen jamming technique. The
Process
DRFM then coherently rebroadcasts each pulse after the
Latency
Time
first pulse received.

61



JED - May 2014

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

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
Washington Report
World Report
Interview
European Helicopter EW
Advanced ASE And ESM Already On Some US Helicopters
Ground-Based COMINT Solutions
Fighting the Kill Chain: Concepts and Methods of Electronic Attack
EW 101
AOC News
Index of Advertisers
JED Quick Look
JED - May 2014 - cover1
JED - May 2014 - cover2
JED - May 2014 - 3
JED - May 2014 - 4
JED - May 2014 - 5
JED - May 2014 - The View From Here
JED - May 2014 - 7
JED - May 2014 - Conferences Calendar
JED - May 2014 - 9
JED - May 2014 - Courses Calendar
JED - May 2014 - 11
JED - May 2014 - From the President
JED - May 2014 - 13
JED - May 2014 - 14
JED - May 2014 - The Monitor
JED - May 2014 - 16
JED - May 2014 - 17
JED - May 2014 - 18
JED - May 2014 - 19
JED - May 2014 - 20
JED - May 2014 - 21
JED - May 2014 - Washington Report
JED - May 2014 - 23
JED - May 2014 - World Report
JED - May 2014 - Interview
JED - May 2014 - 26
JED - May 2014 - 27
JED - May 2014 - European Helicopter EW
JED - May 2014 - 29
JED - May 2014 - 30
JED - May 2014 - 31
JED - May 2014 - 32
JED - May 2014 - 33
JED - May 2014 - 34
JED - May 2014 - 35
JED - May 2014 - 36
JED - May 2014 - 37
JED - May 2014 - 38
JED - May 2014 - Advanced ASE And ESM Already On Some US Helicopters
JED - May 2014 - 40
JED - May 2014 - 41
JED - May 2014 - 42
JED - May 2014 - 43
JED - May 2014 - Ground-Based COMINT Solutions
JED - May 2014 - 45
JED - May 2014 - 46
JED - May 2014 - 47
JED - May 2014 - 48
JED - May 2014 - 49
JED - May 2014 - Fighting the Kill Chain: Concepts and Methods of Electronic Attack
JED - May 2014 - 51
JED - May 2014 - 52
JED - May 2014 - 53
JED - May 2014 - 54
JED - May 2014 - 55
JED - May 2014 - 56
JED - May 2014 - 57
JED - May 2014 - 58
JED - May 2014 - 59
JED - May 2014 - EW 101
JED - May 2014 - 61
JED - May 2014 - AOC News
JED - May 2014 - 63
JED - May 2014 - 64
JED - May 2014 - Index of Advertisers
JED - May 2014 - JED Quick Look
JED - May 2014 - cover3
JED - May 2014 - cover4
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