The course covers methods to intercept radar and other non-communication signals and a then how to analyze the signals to determine their functions and capabilities. Practical exercises illustrate the principles involved.
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3. Copyright
1996-‐2015
Erevno
Aerospace
• The higher the radiated
frequency...
– the smaller/lighter the required
antenna/system
– the less peak power that can
reasonably be radiated by the
radar system
– the more the radiated energy
takes on the propagation
properties of light
• The lower the radiated
frequency...
– the larger/heavier the required
antenna/system
– the more peak power that can
reasonably be radiated by the
radar system
– the less the radiated energy
takes on the propagation
properties of light
Radio Frequency
Some basic rules…
4. Copyright
1996-‐2015
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Aerospace
• A 0 - 250
• B 250 - 500
• C 500 - 1,000
• D 1,000 - 2,000
• E 2,000 - 3,000
• F 3,000 - 4,000
• G 4,000 - 6,000
• H 6,000 - 8,000
• I 8,000 - 10,000
• J 10,000 - 20,000
• K 20,000 - 40,000
• L 40,000 - 60,000
• M 60,000 - 100,000
• VHF 50 - 300
• UHF 300 - 1,000
• L 1,000 - 2,000
• S 2,000 - 4,000
• C 4,000 - 8,000
• X 8,000 - 12,000
• Ku 12,000 - 18,000
• K 18,000 - 27,000
• Ka 27,000 - 40,000
• MMW 40,000 - 100,000
ElectronicWarfare
RadarDesigners
Frequency Band Designations (MHz)
5. Copyright
1996-‐2015
Erevno
Aerospace
Reflection
Occurs when a wave meets a plane object. The wave is
reflected back without distortion.
Refraction
Occurs when a wave encounters a medium with a different
wave speed. The direction and speed of the wave is altered.
Diffraction
Occurs when the wave encounters an edge. The wave has the
ability to turn the corner of the edge.
Scattering
Catch-all description of wave interactions that are too complex
to be described as reflection, refraction or diffraction.
Source: www.cs.ucl.ac.uk/staff/S.Bhatti/teaching/d51/notes.html
Medium 1
Medium 2
Medium 1
Medium 2
Medium 1
Medium 2
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1996-‐2015
Erevno
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TransmitterExciter
Duplexer Antenna
Display
Receiver
Signal
Processor
Signal
Processor
High PRF results in unambiguous velocity
measurements and ambiguous range measurements
Doppler measurements require coherency
LO and
Reference
Signals
Pulse-Doppler Radar
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Aerospace
Pulse Repetition Interval
(PRI)
Pulse Repetition Frequency
(PRF)
Pulse Duration
(PD)
The Pulse Train
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PRF
1
PRI =
PRI
1
PRF =
PD = normally in usec
PRF = normally in pulses per second (pps)
PRI = normally in usec
1.0 second
PD PRI
Pulse Train
PD is the length of time the illuminating power is on for each transmission
PRF is the number of pulses transmitted per second
PRI is the time between the start of consecutive pulses
10. Copyright
1996-‐2015
Erevno
Aerospace
PRF(kHz)
80
Runamb(nm) =
PRF
• Determines radar “data rate”
• Determines Maximum Unambiguous Range (MUR)
- The range at which a radar can receive an echo before the next
pulse is generated
Source:U.S.Navy/NAWC-WDEWHandbook
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PD
• Determines range resolution
• Determines minimum range
• Remember:
ü PD (in feet) = 1000 feet/usec
ü PD (in radar feet) = 500 feet/usec
12. Copyright
1996-‐2015
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Variation of interval between pulses within
the radar’s pulse train
Used to eliminate MTI blind speeds,
main-bang eclipsing and range ambiguities
Improves anti-jamming (EP) capabilities
Interpulse Modulation
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Involves the process of modulating the RF carrier of
a pulsed radar during transmission (within the pulse)
Pulses can vary in frequency, phase or amplitude
Increases range and range resolution
Example: Pulse Compression
Intrapulse Modulation
14. Copyright
1996-‐2015
Erevno
Aerospace
• Gain: Increase/decrease in signal strength as the incoming/outgoing
signal is processed by the antenna.
• Frequency Coverage: The range of frequencies over which the
antenna can operate effectively.
• Bandwidth: Frequency range of the antenna in units of frequency.
• Polarization: Orientation of E and H waves.
• Beam Width: Angular coverage of the antenna in horizontal and
vertical dimensions.
• Efficiency: Percentage of signal power transmitted/ received
compared to a ‘perfect’ antenna.
• Power Rating: The maximum power which can be fed to the antenna
without damaging the antenna and/or reducing antenna
performance from the desired specifications.
Antenna Performance Parameters
15. Copyright
1996-‐2015
Erevno
Aerospace
Antenna gain is the ratio of the power per unit of solid angle
radiated in a specific direction, to the power per unit of solid angle
had that power been radiated using an isotropic antenna
apertureofareaeffectiveA
hwavelengt
mainlobeofcenteratgainantennaG
2
e
e
A
4G
=
=
=
=
λ
λ
π
Source: Introduction to Airborne Radar (2nd Edition)
Used by permission of SciTech Publishing
Antenna Gain
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Polarization
Note: For further information on polarization, see
“Practical Communications Theory” by Dave Adamy
Source: U.S. Navy / NAWC-WD EW Handbook
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Erevno
Aerospace
RF
Input
(Main
Beam
-‐
Primary
Antenna)
RF
Input
(Secondary
Omni
-‐
antenna)
Comparator
Duplexer
Receiver
Signal
Processor
Guard
Receiver
Signal
Processor
A
B
Gate
A
Sidelobe Blanking Concept
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Coherent Sidelobe Cancellers (CSLC)
• Uses auxiliary receivers with
antennas that have low gain and wide
angle coverage
– Most CSLC radars use 3-6 auxiliary
elements
– In a perfect world, one element (antenna)
provides one degree of freedom and can
provide one adaptive null
– The aux receivers operate on the same
frequency as the primary radar receiver/
antenna
• The Howells-Applebaum method is a
common CSLC implementation
technique
CSLC
Processor
Output
Sidelobes
Target
Return
+
-‐
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1996-‐2015
Erevno
Aerospace
Space Time Adaptive Processing (STAP)
• STAP exploits the narrow ridge
that actually forms the clutter
spectrum
• STAP clutter filters have narrow
clutter notches
– Slower targets fall into the
receiver pass band
• Used for Doppler spread
compensation caused by airborne
platform motion/tactical
maneuvering
• Uses a priori data to enhance the
chosen STAP algorithm(s)
• Modern processing capabilities
are allowing for the increased use
(and development) of STAP The Principle of Space-Time Clutter Filtering
(Derived from G. Richard Curry)
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1996-‐2015
Erevno
Aerospace
Knowledge-Based (KB) Radar Systems
• KB radar systems can dynamically change processing
when provided with data from various sources
– Processing power was the inhibiter in the past (no longer the case)
• KB-STAP now possible
– Artificial intelligence (AI) methods can be used to dynamically
choose the best STAP algorithm based upon programmable factors,
vice a set (single) algorithm based upon a priori data
• AI has been used to develop an expert system to
dynamically modify CFAR
• Use of KB techniques to perform filtering, detection,
tracking and target identification is ongoing
– NATO has held conferences on KB radar
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1996-‐2015
Erevno
Aerospace
LPI Systems
LPI systems can (roughly) be broken
into the following technological/
operational approaches:
– Reduced ERP
• Power management based upon current
situation requirements
– Reduced Sidelobes
• Low and Ultralow sidelobes
– Broadband
• Fast becoming common place for COTS
marine and battlefield surveillance radar
systems
– Low peak power capabilities
» Some < 1 Watt
• Natural fall-out of waveform diversity
Image
sources:
Lowrance
/Kelvin
Hughes
/Thales
Group
23. Copyright
1996-‐2015
Erevno
Aerospace
• Sensitivity
– Ability to receive weak signals and
amplify them to usable level. It is
the minimum signal strength that a
receiver can receive and still
operate effectively.
– Three components of sensitivity
are thermal noise, receiver system
noise figure, and signal-to-noise
(S/N) ratio.
• Selectivity
– Ability of a receiver to tune to a
particular station without other
signals/ emissions interfering with
the reception of the desired signal.
• Dynamic Range
– Range of signal levels over
which the receiver can
successfully operate.
– The low end of the dynamic
range is governed by receiver
sensitivity.
– The high end it is governed by
the receiver’s ability to handle
overload and/or strong signals.
• Frequency Stability
– Ability to stay tuned to an
incoming signal for a long period
of time.
22
Receiver Characteristics
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1996-‐2015
Erevno
Aerospace
Low Sensitivity Crystal Video Receiver
High Sensitivity Crystal Video Receiver
RF
Pre-‐amplifier
Crystal
Detector
Video
Amplifier
Antenna
Antenna
Bandpass
Filter
Crystal
Detector
Video
Amplifier
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