(Tentative)
Dictionary of Radar Terms
I am trying to collect here some
radar-releted terms with the relevant explanations. This is not an
exahustive list and will probably never be. If you have some additions
to propose, or something wrong to correct, please mail me.
BACK TO RADAR'S
CORNER
A
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ADPC
Antenna Displaced Phase Centre - a technique used in AMTI
radars to
cope with the doppler spread of the ground
return due to the finite
antenna beamwidth (different clutters
in the beam have different radial
speeds preventing effective doppler compensation). Receivers are
connected to different antenna segments along the flight direction in
order to "shift back" the overall Tx+Rx phase centre PRI
to PRI, thus
having a virtually stationary antenna.
ADT
Automatic Detection and Tracking - Referred to Track-While-Scan (TWS)
tracking where the tracks are initialised automatically by the system
AGC
Automatic Gain Control. Closed-loop control of the receiver
gain. In tracking radars, after target
locking, keep the
target echo constant. In search
radars, generally controls the noise
level at the A/D converter
A-scope
Amplitude-vs-range presentation on radar display. Used in tracking radars, usually in
conjunction with R-scope
AI
Airborne Intercept. General term referring to fighter radars
Ambiguities
phenomenon which leads to an erroneous interpretation of
target parameters, typically range and doppler. This two parameters
are generally conflicting: in pulsed radars, avoiding range ambiguities
requires to reduce the PRF, while it should be
increased to avoid
doppler
ambiguity.
AMTI
Airborne MTI - MTI on airborne platform is
generally more critical than
ground based operation, due to the worse clutter
illumination geometry.
Tight control on nadir sidelobes is generally
required.
Angle Noise
Target-induced error in the angular measurement of a tracking radar. For conical scan radars, the bigger component of
this error is due to the target glint, which
interferes with the conical scan amplitude modulation. Both conical
scan and monopulse radars are instead
affected by errors induced by multiple reflections on complex targets,
which distort the phase front of the echo.
B
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B-scope
A tipe of radar display, with the signal amplitude modulating the
intensity, and presenting azimuth on the x-axis and elevation on the
y-axis.
Barker-code
A 0-180° phase modulation applied within the radar pulse to
increase its bandwidth and, therefore, the relevant possible range
resolution.
Bistatic
A radar having transmitter and receiver located at different
sites.
Blind Range
Range corresponding to an echo delay of one or more PRIs:
the echo then
arrives at the receiver while the radar is transmitting a new pulse and
the receiver is blanked.
Blind Speed
Speed which produce doppler shift which are
integer multiples of the
radar PRF, which are therefore aliased to zero
doppler and cancelled by
the MTI filter.
Burst
Mode of operation sometime used in spaceborn system to deal with
range/doppler ambiguities. The pulse repetition frequency (PRF) within the burst
is high enough to unambiguous sample the max expected doppler shift,
while a new burst is transmitted only after the echo at the maximum
expected range is received.
C
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C-band
Frequency range from 4.0 to 8.0 GHz, according to the "classic"
frequency designation.
C-scope
A type of radar display, used on tracking
radars, presenting de amplitude errors in azimuth and alevation on
a x/y display.
CFAR
Constant False-Alarm Rate. Condition of a constant probability of false
alarms (due to noise and/or clutter)
independently of the
external conditions.
CFAR condition can be achieved by using adaptive threshold detection
logic. Over sea clutter (or gaussian clutter in
general) a log-FTC
receiver allows to achieve the CFAR condition.
Chaff
Small stripes made of metallic (or metallised) material. Thrown in
quantities they produce a strong radar return, and are used to confuse
or deceive hostile radars.
Chirp
a (generally) linear frequency modulation of the carrier
frequency applied within the radar pulse to increase its bandwidth
and therefore the range resolution.
Clutter
unwanted echo (generally from the ground surface) which can obscure
targets.
Coherency
capability of a system to keep a stable phase
reference during the target illumination time in order to properly
exploit the echo phase information for MTI, synthetic aperture or other
purposes.
Coherent-on-Receive
A system using a power oscillator instead of a power amplifier as
transmitter (e.g. magnetron, EIO), transmitting each pulse with
a random phase. Coherency is therefore
recovered by locking the
receiver phase (usually with a COHO) at each
transmitted pulse. Memory
of the
phase (and therefore coherency) from the previous PRI
is therefore lost.
COHO
COHerent Oscillator. Generally referred to the oscillator working at IF
frequency and used as a reference for I/Q demodulation in the Phase
Detector. In coherent-on-receive
systems, the COHO was used
to reconstruct the system coherency by
locking its phase to the phase
of each downconverted transmitted pulse.
Conical Scan
A technique used in tracking
radars to measure target off-boresighth
angle in elevation and azimuth. Derived from sequential lobing
technique, but here the beam is continuously nutating around the
antenna boresighth. The target resulting amplitude modulation carries
the desired information and, demodulated in its sin and cosine
components, provide the angular errors on the two axes. Conical-scan,
as well as sequential lobing systems,
are prone to angle errors induced by the glint of
the target, which overlaps an unwanted modulation on top of that due to
the conical scan. They are also prne to spoofing in angle by the
"reverse modulation" transponders. For these reasons, modern tracking
radars generally use the monopulse technique,
instead.
Cosec^2
An antenna with the elevation beam patter shaped according to the
cosec^2 of the elevation angle, often used in 2-D radars. With this
pattern shape, a target at a given altitude returns always the same
echo, independently of the range.
COSRO
Conical Scan on Receive Only. A conical scan
implementation which use a
fixed transmit beam and a nutating Rx beam to avoid detection of
conical scan information by enemy ECM and their exploitation for angle
deception.
Cross-section
See RCS
CW
Continuous Wave
D
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DBS
Doppler Beam Sharpening - A technique to enhance the azimuth resolution
from a moving platform by discriminating the different
doppler shifts from stationary targets at
different angles wrt the
direction of motion. Often referred to as "unfocused SAR"
Deramping
A technique to process chirped wide-band signals
with limited receiver
bandwidth. The first downconversion LO is replaced with a chirped
signal with the same slope of the received one. The resulting output is
a tone with an offset from the centre frequency proportional to the
time offset between the two chirps. In this way, the bandwidth needed
in the RX stages after Deramping is proportional to the width of the
receive window and non to the chirp bandwidth itself.
Dechirping
Synonymous of deramping
Descalloping
Processing of removal of the scalloping
effect during the processing of SAR data.
Destaggering
processing step used in pulse-doppler
radars to extract the correct
range of targets from range data at different (staggered) PRFs.
Detection
Process in which the received signal is compared with a threshold
(fixed or adaptive), possibly followed by additiona logic, to decide if
a target is present.
Detection, probability of
Probability that, for a given obaservation geometry and target
characteristics, the received signal+noise exceeds the detection
threshold thus achieving a positive detection.
Detection threshold
see threshold
Dickie-fix
ECCM technique used in early radars against
broadband noise jammers,
consisting in a wide-band first stage which saturated on noise (natural
or man-made), followed by a narrow-band stage which allowed recovery of
the target echo from the noise
Doppler
shift in frequency of a wave due to the relative motion between the
emitter and the receiver. Frequency shift is v/lambda. Radars echo are
shifted twice this value because this shift must be accounted for both
for the forward and for the return path.
Down Range
Range between the radar and the ground projection of the target
(for surface based radar - viceversa if the radar is airborne and the
target is on ground)
E
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Eagle scanning
A technique to scan the radar beam by introducing a phase shift along
tha antenna aperture (as in phased
array systems) by mechanical means, developed in the US at the end
of WWII. A single curvature reflector was illuminated by a slotted
waveguide, and the electrical lenght of the waveguide (i.e., its delay)
was controlled by mechanically changing its section (one wall of the
waveguide was movable). As a result, it was possible to control the
direction of the resulting wavefront.
An evolution of this technique was the Foster
Scanning method.
Early-gate/late-gate
Classic technique used in tracking
radars to perform
range tracking. The tracking window is splitted
in two halves and the
relevant amplitudes are compared, providing an error signal (to control
the tracking window position) proportional to the amplitude difference.
ECM
Electronic Counter-Measures. General term to define equipments and/or
techniques used to blind, disturb, deceive radar systems.
ECCM
Electronic Counter-Counter-Measures. All the techniques/features used
by radars (or other systems) to reduce the vulnerability to enemy ECMs.
F
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False Alarm
false detection due to the noise which, due
to its random nature,
temporary exceed the detection threshold.
Fan Beam
The "classic" radar antenna beam, with a beam narrow in azimuth and and
wide enough in elevation to cover all the possible (aerial) targets
elevation angles. A radar with this kind of beam cannot discriminate
target elevation, i.e. is "bi-dimensional"
FM
Frequency modulation
FM-CW
Frequency Modulation - Continuous Wave A type of radar where a
continuous wave instead of pulse is transmitted, in order to avoid
doppler ambiguities.
The range information is derived by frequency
modulating the carrier with a sawtooth waveform and comparing the echo
FM modulation envelope with the reference.
Foster Scanning
A technique to scan the radar beam by introducing a phase shift along
tha antenna aperture (as in phased array
systems) by mechanical means, derived from the Eagle scanning. Here, the control of the
delay along the aperture was achieved by letting the RF signal
propagating in the space between two concentric cones, with a
"shortcut" provided by a slit in the inner cone. Rotating the inner
cone changed the position of this "shortut" and, consequently, the
total delay.
Being the device conical and not cylindrical, both asbolute delay an
its variation where linearly increasing along the antenna aperture,
allowing to control the direction of the wavefront.
Focusing
in SAR processing, the compensation of phase
rotation introduced by the
varying radar-target range along the synthetic
aperture.
Fresnel Ripple
Ripple in a chirp spectrum (which would be
otherwise flat over the
chirp bandwidth) and larger at the chirp band edges, due to the finite
time duration
of the chirp itself. It is proportional to the chirp
Bandwidth/Pulsewidth ratio.
Frequency agility
Techniques of rapidly changing the radar carrier frequency (typically,
for coherent radars, every processing packet) during
the time-on-target, in a random
sequence. Commonly used in military
radars as an ECCM techniques.
FTC
Fast Time Constant. A AC-coupling of the video data. Used in log-FTC
receivers.
G
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GCI
Ground Control of Intercept - A radar (3-D or 2-D + height finder) used
to control fighters operetions and to vector them on their targets.
Glint
Fluctuation of a target RCS (and, therefore, of the
echo amplitude) due
to changes in the target attitude (for complex targets, RCS is normally
strongly
dependent from the angle of observation)
GPR
Ground Penetrating Radar - A type of radar using a low frequency
carrier, capable to penetrate the soil and return the echo of buried
targets.
H
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Height Finder
A type of radar used to detmine the elevation of a target detected by a
2-d radar. Uses a nodding-beam antenna
pointed on the target azimuth
and scanned in elevation.
I
Interferometry
Technique in which the echoes received by two antennae are compared in
phase to detect the angle of arrival. Also called as "phase comparison
monopulse". In SAR
application, phase difference vs range from two
receivers produces "phase bands" which, unwrapped, are used to extract
elevation information of the imaged area. "Dual-pass" interferometry is
performed with a single-channel spaceborn SAR comparing two different
pass (with a slight offset in the orbit), and can be performed only
over targets which remain coherent (i.e., do not change their echo
phase for the same observation geometry as, for example, vegetation
does) between the passes (e.g., rocky
surfaces).
IFF (Identificator Friend/Foe)
Generally associated to search radar to
allow target identification,
the IFF "interrogates" a transponder located on the target, and
classify it as "friend" if the proper reply is received. Similar in
principle to civilian SSRs, modern IFFs use crypto
feature to avoid
exploitation of the system by the enemy to detect friendly aircrafts.
IFSAR
Interferometric SAR
Imaging
Radar
Radar (normally, airborne or spaceborne) used to acquire
high-resolution images of the
surface. Normally used to refer to SAR.
Impulse Equivalent
A radar signal whose correlation function is an impulse. For example,
the chirp pulse is impulse equivalent since its
correlation function is
sin x/x.
INSAR
Interferometric SAR
IRF (Impulserensponse Function)
See System Impulse rensponse
ISAR
Inverse SAR. A fixed radar which use
SAR techniques to enhance azimuth resolution
on airborn target, relying on target motion to provide the
synthetic aperture.
Isodoppler
In SAR application, isodoppler lines
are those connecting points on the
imaged surface having the same doppler
frequency.
Isorange
In SAR, line connecting ground
points having the same slant range.
ISLR
Integrated SideLobes Ratio - ratio between the integrated powers of the
correlation peak and the sidelobes. It
is normally used in
imaging
radars (SAR) as a measurement of the
"contrast" achieavable in an
image.
ISLS
Interrogator Side Lobes Suppressor - see SLS
J
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Jammer
Device generating some form of intentional interference (noise, swept
tone, etc) to degradate the performance of enemy radars.
Janus
Type of configuration used in doppler radar
navigation systems, using
four beams (2 fore and two aft, on the two sides of the ground track)
to compute the aircraft velocity vector referred to the terrain by
measuring the doppler shift of the ground echo from the beams.
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K
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K-band
Frequency band from 18.0 to 26.5 GHz, according to the "classic"
frequency designation
Ka-band
Frequency band from 26.5 to 40.0 GHz, according to the "classic"
frequency designation
Ku-band
Frequency band from 12.4 to 18.0 GHz, according to the "classic"
frequency designation
L
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L-band
Frequency band from 1.0 to 2.0 GHz, according to the "classic"
frequency designation
Logarithmic receiver
logarithmic amplifiers are often used in radar receivers
to handle wide dynamic range. Typical application is on naval surface
detection systems in log-FTC receivers.
Log-FTC
A logarithmic receiver
followed by an AC-coupled video stage. Over a gaussian
clutter (like sea clutter) the output of the log
chain
has a Rayleigh distribution, independent of the amplitude of the
clutter
apart of the dc component (removed by the AC coupling). These receivers
thus allows to achieve CFAR conditions over sea.
Look
In synthetic aperture radars, the part of the time-on-target used
generate the synthetic aperture. To
achieve the theoretical resolution
of half of the antenna aperture, the whole time on target shall be
used, allowing only one "look". When multiple looks are desired, the
time-on-target is split in 2 or more
synthetic apertures, with a
proportional degradation of the azimuth resolution. This "multilook"
technique is often used to achieve multiple images of the same point
to be averaged in order to reduce the speckle noise.
LORO
Lobing on Receive Only. An implementation of sequential lobing which
use a
fixed transmit beam and switched Rx beams to avoid detection of
conical scan information by enemy ECM and their exploitation for angle
deception, using an approach similar to COSRO.
LPI
Low Probability of Intercept - a radar which makes use of low
peak-power, high duty cycle and wideband, noiselike waveforms to reduce
the probability of detection of its emissions by hostile surveillance
receivers.
M
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Matched Filter
A receiver filter having a frequency response which is "matched" of the
transmitted pulse (complex conjugated) in order to optimise
the signal-to-noise ratio. For unmodulated pulse, it can be
approximated by a bandpass filter of bandwidth=1/T, where T is the
pulse width.
When a matched filter is used in the receiver, the S/N is proportional
only to the pulse energy (P * T) and not to the peak power P.
MGC
Manual Gain Control: control of the receiver gain performed manually by
the operator.
Missing detection
Presence of a target which is not detected by the radar,
being the instantaneous value of the signal+noise below the detection
threshold.
Monostatic
A radar having both transmitter and receiver located at the same
sites
(and, generally, sharing the same antenna). The large majority of
radars currently in use are Monostatic.
Multipath
Reception of an echo signals not only direcly from the target, but also
via additional paths produced by reflections. A classical example is an
airplane flying at low altitude over a sea surface: in addition to the
direct radar-target path, the signal can also "bounce" on the sea
surface producing one additional path in Tx and one in Rx. These
signals
add up to the main one in a costructive or destructive way (depending
on the geometry) producing fluctuations of
the echo amplitude and errors in the elevation measurement. Multipath
can
also be produced by ground surfaces, and is strongly dependent from
surface
characteristics and radar-target-surface geometry.
Multistatic
A radar having one transmitter and several receivers located at
different sites (one receiver=bistatic).
Monopulse
A technique used in tracking
radars to extract the off-boresighth angle
(i.e., the X/Y offset wrt the radar electrical axis). Four different
beams, with a slight pointing offset in X and Y wrt the radar axis, are
used. Transmission chain use the sum of these beams, while, on receive,
they are combined by sum/subctaction to extract the amplitude
difference on the two axis. These differences, normalised in amplitude
and phase wrt the sum channel, provide the offset in angle of the
target. The main advantage of this technique vs the simpler conical-scan is the redeced sensitivity to
the errors induced by the target glint.
MTI
Moving Target Indicator. An MTI radar is a coherent
(both fully
coherent or coherent-on-receive)
which discriminates moving targets
from unwanted fixed target by detecting the pulse-to-pulse phase change
wrt an internal phase reference) due to their doppler
shift.
Non-coherent MTI systems uses the clutter phase
as reference instead of
an internal
coherent source.
MTI Improvement Factor
Ratio of clutter amplitude
before and after MTI processing.
Multilook
see Look
Multi-pass Interferometry
A kind of interferometric SAR operation in which interferometric
information are generated using a single antenna with two ore more
passes having slightly different geometries. Multi-pass interferometry
requieres that target coherency is preserved between passes (i.e., the
echo phase delay does not change, like, e.g., vegetation does)
Multiple-time-around echo
Echo of rank N. See Second -time-around.
Multi-sensor-tracking
A kind of TWS where the tracking is performed by
conbining information
from more than one radar and/or other sensors.
N
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Nodding-beam
A nodding beam antenna is basically a fan-beam
antenna tilted by 90
degrees, with a beam wide in azimuth and narrow in elevation, and
scanned in elevation, used in height finders
radars.
Null-steering
Technique used in phased array radars,
consisting in adaptively
change the weights of the radiator to steer the null (minimum level) of
the antenna sidelobes in the wanted direction
(tipically, the direction
of a jammer to minimise the interference)
O
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OTH, OTH-B
Over The Horizon (Backscatter). A radar working in the HF
range (from MHz to tens of MHz) exploiting ionospheric scattering
to achieve over-the-horizon performances. Thousands km ranges are
possible, with multiple bounces between ionosphere and ground.
Performance
are strongly dependent on accurate ionosphere behaviour prediction
and requires huge antenna complexes.
P
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P-band
Frequency band from 0.5 to 1.0 GHz, according to the "classic"
frequency designation
PAR
Precision Approach Radar. A 3-dimensional system composed by two 2-D
radars (one with a fan-beam for azimuth and
the other with nodding beam
for elevation detection) used to guide aircrafts in the final approach
to the landing strip.
Packet
Radar processing is often organised in independent "packets"
characterised by constant radar parameters (PRF,
carrier frequency,
modulation scheme) to allow coherent
processing.
PFA
Probability of False Alarm. Probability
that, for each "decision" taken
by the radar detector, the noise alone can exceed the threshold and be
detected as a target. Higher threshold decrease the PFA at the expense
of an increasing probability of missing
detection (or a shorter
detection range for a given probability
of detection).
Phase Detector
A device used in radar receivers which provide an output with
amplitude
proportional to the received signal amplitude and the phase difference
between received signal and reference signal. Normally,
implemented with mixers, and using two channels, with quadrature
reference
signal in order to extract sin and cosine component of the received
signal.
Phased Array
An antenna which can be electronically steered by changing the
electrical length (phase delay) of the feed path of the different
radiating elements. When all radiators are in-phase, the wavefront is
parallel to the radiator plane; introducing a linear delay in one
direction, the wavefront forms an angle with the radiator plane, so the
beam direction is steered in the direction opposite to that of
increasing delay. Steering can be on one axis only (each radiator
"line" is fed with different phase) or two axes, with individual phase
control of each element.
Polarisation, circular
Circular polarisation, instead of linear one, is sometimes used to
suppress rain and other atmospheric clutter.
Exploits the property of
simple targets such as rain drop to reflect an echo with mirrored
polarisation (rejected by the receiving antenna) while complex target
like aircrafts, due to multiple reflections, scatters the energy about
50/50 in both polarisations.
Polarimetry
Technique used for remote sensing application which exploits
differences in reflectivity for V or H polarisation and/or changes in
the signal polarisation in the reflected echo to extract information
about the target. Full polarimetry implies transmitting alternating
V and H polarisation and simultaneously receiving both on separate Rx
channels.
PPI
Plan Position Indicator. The classic "radar display" providing
range-azimuth information on a polar presentation
PRF
Pulse Repetition Frequency = 1/PRI
PRF Jitter
An ECCM technique which uses pulse-to-pulse
slight, pseudorandom
changes in the PRF to prevent enemy spoofers to
generate a credible
false echo at shorter range than its own by transmitting a delayed
replica of the transmitted pulse from the previous PRI.
PRI
Pulse Repetition Interval For pulsed Radar, the interval
between transmission pulses. Equal to 1/PRF.
Propagation, anomalous
Phenomena which occours under specific atmosphere conditions, and
which causes the radar signal to be "trapped" between the surface and
an atmosphere layer, allowing detection of targets well beyond the
normal radar line-of-sight.
PSF (Point Spread Function)
See System Impulse rensponse
PSLR
Peak sidelobes ratio - ratio of the echo peak wrt the highest sidelobe
(in angle, from the antenna sidelobes, and in
range for pulse-compression systems)
PTR (Point Target Rensponse)
See System Impulse
rensponse
Pulse-Compression
Technique used to achieve a wide pulse bandwidth (and, therefore, range
resolution) using long pulse
(for high pulse energy with limited peak
power) by introducing an intrapulse modulation (e.g., chirp=
frequency
modulation, Barker = discrete phase
modulation) and performing a
correlation on the received echo.
Pulse-Doppler
A pulsed MTI radar which tries to avoid ambiguities in doppler
by using
high PRF. The consequent range ambiguity is handled
by staggering PRFs
and resolving the ambiguities via a destaggering
process.
Q
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R
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RADAR
RAdio Detection And Ranging
R-Scope
Amplitude vs Range presentation display. Differs from A-scope for non
displaying the total sweep but only a limited range around the tracking
tracking window.
Range
Ambiguous range: range corresponding to a delay
greater equal to 1/PRF. If the target is in
ambiguous range it is
detected as having a smaller range equal to the remainder of its delay
divided by (1/PRF), unless ambiguity
resolution techniques (such as PRF staggering)
are used. An ambiguous echo is
also called "Nth time
around" echo, where N is the integer result of the aforementioned
division (echo of the 1st pulse is detected as if is produced by the
Nth pulse)
Slant range: straight range between the radar and
the
target
Down Range: range between a (ground) radar and the
ground projection of the target (and vice versa for airborne/spaceborn
radars)
Range Gate
Sometimes used to refer to the elementary "cell" of range resolution
Range Migration
In a SAR system, the effect, due to the wide sinthetic antenna and the
(usual) fine renge resolution,
of shift of the radar range of a given
target > range resolution for different position of the radar along
the
sinthetic antenna aperture. This prevent correct integration of the
signal and therefore must be compensated for during SAR image
generation.
Rank
Order of "ambiguity" of the echo: Rank "N" =
Nth time around echo.
RAC
Reflective Array Compressor - A type of SAW device
used for generation and compression
of chirp signals.
RAR
Real Aperture Radar. A radar which does not employ beam synthesis
techniques such as SAR or DBS
to enhance angular resolution.
Generally
applied to airborne/spaceborne systems.
RCS
Radar Cross Section (or simply "cross-section"). Measure of the
reflectivity of a given target (for a given frequency/polarisation),
expressed in area units for point targets and its symbol is the greek
letter "sigma". One square meter of RCS reflect an amount of energy
equivalent
to the energy incident on 1 m^2 re-radiated isotropically. For
distributed
surface scatterers the cross section ("sigma-0") is expressed as a pure
number (usually in dB), and is 1/m for volume scatterers (rain, fog,
etc).
Resolution
Capability to discriminate closely-spaced targets. The range resolution
(for a pulsed radar) is given by the amplitude of the (correlated, if
applicable) pulse. The angular resolution depends of the antenna
beamwidth (for SAR,
the beamwidth of the sinthesized antenna applies).
The "Resolution Cell" is defined as the product of range resolution by
the angular resolution.
RHI
Range-Height Indicator. A type of radar display presenting
a range/elevation polar plot of the echo of a nodding-beam
radar (used
in height finders of PAR)
RWR
Radar Warning Receiver - A passive ECM consisting
in a wideband
receiving equipment capable to detect (and, normally, analyse and
classify) radar emission to provide warning of tracking radars
illuminating the vehicle or, more generally, of the presence of hostile
radars in the area.
S
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S-band
Frequency band from 2.0 to 4.0 GHz, according to the "classic"
frequency designation
SAR
Synthetic Aperture Radar. Radar from a moving platform which performs a
coherent summation of the echoes received
while illuminating the target
from different positions along its path ("SAR baseline") to achieve an
angular resolution equivalent to that
provided by an antenna having a
length equal to the baseline. SAR "focusing" is
required to
compensate for the phase change due to target range variation during
the
measurement: unfocused SAR (or "DBS"
systems) are limited to smaller
baselines
with corresponding degraded resolutions.
SAW
Surface Acoustic Wave device - An electro-acoustic device used to
perform processing on IF/RF signals. The main application is in
filters, but they have also been used as correlators (for chirp or
other coding
systems) and as "expanders", to generate the coded signal when excited
by
an appropriate wideband pulse.
Scalloping
In a SAR operating in scanSAR
mode, the effect of the non-uniform illumination of the imaged area due
to the stepping of the beam in elevation, which produces an amplitude
modulation effect in the along-track direction. The process of removing
this modulation in the processing is called descalloping.
ScanSAR
A technique to increase the effective swath width of a SAR system
beyond the limits imposed by the range/doppler ambiguities
trade-off. A smaller swath is electronically scanned in different
position
to cover the larger swath.
Search radar
A radar designed to systematically scan a defined volume of space every
given time. This is normally done using continously rotating antennae
to achieve 360° azimuth coverage
Second-time-around echo
An echo with rank=1 (being 0 the rank at a delay
< PRI), i.e received after a second pulse is
transmitted,
and which can be misinterpreted (if no means to solve the ambiguities
are available) as an echo from the second pulse at shorter range.
Sequential Lobing
First technique used in tracking
radars to extract target
off-boresighth information. Four different beams, slightly squinted on
+/-x and +/-y wrt the antenna boresighth, were used sequentially. A
target exactly at the centre provided echoes of the same amplitude. An
off-boresighth target will provide a stronger return for the beam
squinted
in the relevant direction. Angular errors extracted in this way were
then fed to the tracking servo to keep the target on boresighth.
Sidelobes
1) Antenna Sidelobes - antenna rensponses outside the intended
radiation beam. "untapered" antennae have (theoretically) a sinc
((sin x)/x) rensponse, showing the first sidelobes at -13 dB. Proper
weighting of
the illumination function allows a significant reduction of these
lobes, but some rensponse outside
the intended direction is unavoidable, normally presenting an irregular
pattern with "peaks" and "nulls". This sidelobles may produce
rensponses from targets in unwanted directions, allowing disturbing
signal (intentional or not)
to enter the receiver, and to make the radar detectable also by
receivers
not illuminated by the main beam, thus the efforts to put them under
control.
2) Range Sidelobes - when using pulse
compression, the correlated pulse
always presents rensponse outside the correlation peak (before and
after it)
known as sidelobes. Their main effect is to limit the capability to
discriminate weak returns in proximity of strong ones (with sidelobes
of the same order of magnitude of the weak one).
3) Azimuth sidelobes (SAR) - In
syntetic aperture radars, a correlation
process occours also in azimuth, basically producing the same effect of
range sidelobes (but in the azimuth
direction) and with the same impact
on performance
(Note thet on imaging radars, which
normally look at distributed
targets, the total energy of the sidelobes wrt peak (ISLR)
is more
important than
their peak value (PSLR).
Single-pass Interferometry
A kind of interferometric SAR operation in which interferometric
information are produced with a single pass using two physically spaced
antennae (typical, one Tx/Rx and one Rx only)
Syntetic
Aperture
Portion of the radar trajectory (or target trajectory in ISARs) which is coherently integrated to form the
synthetic antenna in SAR systems
Syntetic Aperture Radar
See SAR
Slant Range
Absolute range between the radar and the target, measured by means of
the echo round-trip time.
SLAR
Side-Looking Airborne Radar. term originally used for airborne
reconnaissance radars (initially RARs, then DBS and SAR). RARs
needed a
long physical antenna which could be only accommodated on aircraft side
(therefore looking to the side) while DBS/SARs relies on tangential
motion and must therefor "look on sides" by their physical nature.
SLS
Side Lobes Suppressor - System which exploit comparison of the
signal with an omnidirectional pattern to detect and cancel azhimuth
sidelobes. On secondary radars
/ IFFs the Interrogator Side Lobes
Suppressor (ISLS) radiates a reference pulse on via an omnidirectional
antenna (with gain in excess of the greatest main antenna sidelobe):
transponders rensond only if the signal
from the main pulse (radiated fron the main antenna) is larger than the
reference,
to avoid rensponding on sidelobes. Similarly, on primary radars, the
signal
received from an omni antenna can be used to set the detection
threshold
for the main channell.
Sounder
In the radar field, referred to airborne/spaceborne groung penetrating
radars used to "sound" underground features.
Speckle
A form of multiplicative noise which occours when imaging distributed
targets. Reflections from individual scatterers within a resolution
cell combines with different phases (depending on scatterers geometry).
As the geometrical resolution is reduced, the lower number of
scatterers in the cell whose rensponses are averaged leads to an higher
fluctuation of reflected power from cell to cell.
Spotlight
A technique used in SAR system
to improve the angular resolution
without increasing the antenna aperture thus reducing its gain, and
consisting in the use of a steerable antenna which is kept pointed
on the target during the synthetic
aperture. The improved resolution
over stripmap is payed in terms of coverage
(only specific "spots" can
be mapped in this way, instead of a continuous strip)
Staggering
To resolve range ambiguities "staggering"
of different values of PRFs is often
used. In this way, ambiguous targets, illuminated with different PRFs,
shows different apparent ranges, allowing to solve the ambiguity and to
extract the correct range information.
SSR (Secondary Surveillance Radar)
A system generally (but not always) associated with search radars
which "interrogates" transponders on board of aircratfs and receives
their replies (containing info like flight code, altitude and other
parameters) for Air Traffic Control Purpouse. Similar to military IFFs
in their mode of operation.
STALO
STAble Local Oscillator. RF oscillator used as 1st local
oscillator in the receiver (and for upconversion of Tx signal in
fully-coherent systems)
STAP
Space-Time Adaptive Processing - used in AMTI
radars to cope with
clutter doppler spread
(like ADPC). It uses partitioned receiving
antennae and doppler beam sharpening to compensate
doppler for each
sub-beam.
STC
Sensitivity Time Control. Uses an attenuator placed in front of the
receiver (usually in the T/R+limiter) to apply an
attenuation
decreasing with time from the transmit instant, in order to prevent
saturation from close-in clutter.
Stripmap
Sasic operation mode of SAR systems, with a fixed
antenna beam pointing
on the platform side while it is moving. The resulting image therefore
a continuous "strip" mapping the terrain on the side of
the flight track. The theoretical angular resolution for a stripmap SAR
is L/2 (where L is the untapered antenna aperture) and in independent
from
the target range.
Subclutter Visibility
Capability of a system to detect targets in presence of strong clutter
returns. Not to be confused with the MTI
improvement factor: saturating
clutter may still be canceled but if target return is lost in the
saturation, it cannot be recovered and detected.
System Impulse Rensponse
The end-to-end (transmit + Rx chain) rensponse of the radar system, to
an "impulse" target (i.e. a point target). Also called PTR (Point
Target Rensponse) o PSF (Pulse Spread Function) or IRF (Impulse
Rensponse Function). It is a fundamental parameter to evaluate the
effect of filtering/distortions, etc.. introduced by the system
hardware.
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Track
On search radars, the sequence of discrete
"plots" relevant to the same
moving targets, scan after scan.
Threshold, detection
Level above which the received signal is considered as a valid echo,
and a detection is achieved. A lower threshold led to improved
probability of detection at the
expenses of the probability of false
alarm (PFA) due to noise, and viceversa.
Time-on-target
The amount of time during which a specific target will remain within
the radar antenna beam (e.g., while rotating, for a search radar, or
when flying over it, as in SARs)
Threshold, adaptive
A threshold whose level is automatically adapted to the operating
conditions in order to optimise the system performances. Normally used
for achieving the CFAR condition.
T/R
Transmit/Receive - The device used to perform the duplexing between
transmitter and receiver ports when they share the same antenna.
T/R Module
Transmit/Receive module. Element of an active antenna which includes
both Tx power amplifier and Receiver Low-noise amplifier
Tracking radar
A radar degned to "track" a given target in order to provide continous
information on its cinematic parameters. A discontinous form of
tracking can also be performed by search radars
using TWS
Tracking
window
Receive "window" opened by tracking radars around the
expected range of
the target. The window position is normally controlled by a closed loop
system which keep the window around the target.
TWS
Track-While-Scan. A technique which allows to automatically "track" a
target from the sequence of plots on a search radar.
TWT
Travelling Wave Tube. Microwave High-power amplifier used as
transmitter in fully-coherent radars
U
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V
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V-beam
A technique to extract taget elevation used in some old radar designs
(mainly russians). Two fan beams set at about
45° in order to form
a "V" are rotated together, and the target elevation is computed from
the time (i.e., rotation angle) elapsed between detection on first and
second beam.
W
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Window
Original british term for chaff (WWII)
X
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X-band
Frequency band from 8.0 to 12.4 GHz, according to the "classic"
frequency designation
Y
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Z
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Last updatedJun, 02, 07