JED - August 2014 - 47

E W101

The Journal of Electronic Defense | August 2014

XMIT FREQ SHIFT

VELOCITY

coy may need to return radar signals
with an appropriate frequency shift to
simulate the correct Doppler shift of a
real target. Figure 3 shows the velocity vs. time for an object (e.g., a decoy)
expended from a moving aircraft and
the frequency shift required to make a
radar believe that the expended object
TIME
has the same velocity as the aircraft
that expended it.
Jet engine modulation (JEM) is
complex amplitude and phase modulation from the motion of the moving internal parts of a jet engine. It
changes with aspect angle, and can be
detected in the skin return to a radar
TIME
which is up to 60 degrees off of the
Figure 3: A decoy expended from an aircraft is slowed by atmospheric drag that reduces as the
flight path of a jet powered aircraft. If
object slows. To simulate the velocity of the aircraft that expended the decoy, the transmitted
a hostile radar can detect JEM modulafrequency from that decoy must be increasedaircraft is slowed amount.
Figure 3: A decoy expended from an by a time varying by atmospheric drag that reduces
tion, it will notice that a decoy which
as the object slows. To simulate the velocity of the aircraft that expended the
does nottime varying amount. will not have this modulation
have a jet engine,
AIRBORNEthe transmitted DECOYS from that decoy must be increased by a
decoy, SATURATION frequency
feature and can easily discriminate against it. Thus, JEM
The first figure in the July "EW 101" column showed a
modulation may need to be placed on simulated skin returns
large number of airborne targets, including one real tarfrom decoys.
get and a number of decoys. In order for a hostile radar
In the March 2014 "EW 101" column, we discussed the
to take the decoys seriously, they must look (to the racomplex RCS of a tactical aircraft (as shown in Figure 4). If
dar) much like the real target. This means that they must
a hostile radar has the capability of analyzing the frequency
have approximately the same RCS. However, there are also
spectrum of a skin return, it will determine that a decoy reother considerations.
turn has a much simpler waveform than that returned by an
The October 2010 "EW 101" column discussed pulse-Dopaircraft. This would allow for the rapid rejection of a decoy as a
pler radars, which are widely represented among modern
potential target. To overcome this capability of a sophisticated
threat radars. The processing circuitry of a pulse-Doppler raradar, the decoy must modulate its output signal to create a
dar includes a time vs. frequency matrix, as shown in Figure
complex, realistic RCS characteristic.
2, in which the time of arrival and received frequency of multiple targets is captured. For each target, the time of arrival represents the
range to the target, and the received
frequency is determined by the Doppler
shift in the received signal. Because
Discon
the Doppler shift is a function of the
tinuity
rate of change of range to the target,
this chart can be considered a range vs.
velocity matrix . The frequency data
comes from a bank of filters, usually
implemented in software. Note that
this bank of filters can also analyze the
spectrum of a received signal.
Because aircraft have significant
velocity, they present significant Doppler shifts. If decoys are expended
from an aircraft, they will slow down
quickly in response to atmospheric
Figure 4: There are many contributing factors to the RCS of an aircraft. Together, they cause an
drag. This will make a significant
RCS with complex amplitude and phase components.
change in Doppler shift for which the
retransmitted radar signal must compensate. A hostile radar
WHAT'S NEXT
Figure 4: There are many contributing factors to the RCS of an aircraft.
may be able to reject a decoy thatTogether, a time-varyingRCS with complex amplitude and phase components. distraction
returns they cause an
Next month, we will complete our discussion of
signal frequency with the characteristic shape of the atdecoys and cover detection decoys. For your comments and sugmospheric drag deceleration curve. This means that a degestions, Dave Adamy can be reached at dave@lynxpub.com. a

47



JED - August 2014

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

The View From Here
Conferences Calendar
Courses Calendar
From the President
Letters
The Monitor
Washington Report
World Report
Back to the Future With HF SIGIHT
HF SIGIHT Battles the Ionosphere
Technology Survey: Solid-State Power Amplifiers
Book Reviews
EW 101
Index of Advertisers
JED Quick Look
JED - August 2014 - cover1
JED - August 2014 - cover2
JED - August 2014 - 3
JED - August 2014 - 4
JED - August 2014 - 5
JED - August 2014 - The View From Here
JED - August 2014 - 7
JED - August 2014 - Conferences Calendar
JED - August 2014 - 9
JED - August 2014 - Courses Calendar
JED - August 2014 - 11
JED - August 2014 - From the President
JED - August 2014 - 13
JED - August 2014 - Letters
JED - August 2014 - The Monitor
JED - August 2014 - 16
JED - August 2014 - 17
JED - August 2014 - 18
JED - August 2014 - 19
JED - August 2014 - 20
JED - August 2014 - 21
JED - August 2014 - Washington Report
JED - August 2014 - 23
JED - August 2014 - World Report
JED - August 2014 - 25
JED - August 2014 - 26
JED - August 2014 - 27
JED - August 2014 - Back to the Future With HF SIGIHT
JED - August 2014 - 29
JED - August 2014 - 30
JED - August 2014 - 31
JED - August 2014 - HF SIGIHT Battles the Ionosphere
JED - August 2014 - 33
JED - August 2014 - 34
JED - August 2014 - 35
JED - August 2014 - 36
JED - August 2014 - Technology Survey: Solid-State Power Amplifiers
JED - August 2014 - 38
JED - August 2014 - 39
JED - August 2014 - 40
JED - August 2014 - 41
JED - August 2014 - 42
JED - August 2014 - 43
JED - August 2014 - 44
JED - August 2014 - Book Reviews
JED - August 2014 - EW 101
JED - August 2014 - 47
JED - August 2014 - 48
JED - August 2014 - Index of Advertisers
JED - August 2014 - JED Quick Look
JED - August 2014 - cover3
JED - August 2014 - cover4
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