| Across-track |
An across-track sensor
is one that uses a mirror system
that moves from side to side in the range to obtain remote sensing
data. ( See also "Imaging
Geometry" in the Geometry
glossary ) |
| Active Microwave Instrument (AMI) |
Ordinarily referred to
as a radar, this is the term commonly
used to describe the radar
instrument onboard the ERS satellites. (See
Microwave) |
| Active Remote Sensing System |
A system that provides
its own source of energy and
illumination (i.e. radar system). A
remote sensing system that transmits its
own electromagnetic emanations at an
object(s) and then records the
energy reflected or refracted back to
the sensor. |
| Advanced Synthetic Aperture
Radar (ASAR) |
This the newest SAR
instrument from ESA. The Advanced
Synthetic Aperture Radar ( ASAR )
instrument onboard the ENVISAT satellite
extends the mission of the Active Microwave
Instrument ( AMI ) Synthetic Aperture
Radar ( SAR ) instruments flown
on the European Remote
Sensing ( ERS ) Satellites ERS-1
and ERS-2. ASAR uses an active phased-array antenna,
with incidence
angles between 15 and 45
degrees. Applications for this
sensor will include the study of
ocean waves, sea ice extent and
motion, and land surface studies
such as deforestation and
desertification, to name a few.
( see also chapter 1 "ASAR User
Guide"(Chapter 1. ) ).
|
| Along-track |
An along-track sensor is
made up of a linear detector array
of CCDs (Charge Coupled Device) that
obtains data in the platform's
direction of motion (azimuth or along-track
dimension). The sensor's
instantaneous field of view extends the
length of the swath width. The
dimension parallel to the path of the
platform carrying the sensor.
The along-track dimension is the imaging
direction of the sensor that is parallel
to the direction in which the satellite
or aircraft is moving . For side-looking radars
(SLARs), this dimension is
sometimes called the cross range. The
typical two-dimensional remotely sensed
image is created by the movement of the
platform in the along-track direction,
while the sensor scans or aims at the
orthogonal direction. ( See also
"Imaging
Geometry" in the Geometry
glossary ) |
| Alternating Polarisation (AP) |
These products are
similar to Image Mode products but
include a second image acquired
using a second polarisation
combination. The raw data is acquired
in bursts of alternating polarisation.
See Products glossary AP_MODE. |
| Amplitude |
Measure of the strength
of a signal, and in particular the
strength or height of an electromagnetic
wave (units of voltage). The amplitude
may imply a complex signal,
including both magnitude and the phase. |
| Antenna |
Part of the radar
system, which transmits and/or receives
electromagnetic energy. (See Radar Antenna ) |
| Antenna Array |
An arrangement of
several individual antennas so spaced and
phased that their individual
contributions add in the
preferred direction and cancel in other
directions. SAR systems, employ a short
physical antenna, but through modified
data recording and processing
techniques, they synthesise the effect
of a very long antenna. The result of
this mode of operation is a very narrow
effective antenna beamwidth, even at far
ranges, without requiring a physically
long antenna or a short operating
wavelength. For example, in a
SAR system, a 2m antenna can be made
effectively 600 m long. |
| Array Antenna |
See Antenna Array
|
| Attenuation |
Decrease in the strength
of a signal. The decrease in the
strength of a signal, is usually
described by a multiplicative factor in
the mathematical description of signal
level. A signal is attenuated by
application of a gain less than unity.
Common causes of attenuation of an
electromagnetic wave include losses
through absorption and by volume
scattering in a medium as a wave passes through.
|
| Azimuth Ambiguity |
A form of ghosting that
occurs when the sampling of
returned signals is too slow. |
| Azimuth Bandpass Filtering |
Bandpass filtering
selects a certain band of frequency components
in the signal. Azimuth bandpass
filtering refers to filtering in the azimuth
direction of the two-dimensional SAR signal. The
location of signal energy in the
azimuth frequency domain depends on the
antenna pointing angle (Doppler centroid 2.6.1.2.2. ),
so bandpass filtering is necessary to
maximize the signal energy in the
processed image. |
| Azimuth Beam Inversion |
The azimuth Impulse
Response Function (IRF) has an
amplitude which is modulated by the
position of the echo in the azimuth
beam pattern as the instrument
passes the target. This
modulation is removed by modifying
the reference function by the
inverse of the azimuth beam pattern
(expressed as a function of Doppler frequency).
This equalises the echo data across
the Doppler bandwidth but
exaggerates the noise - thereby
degrading the noise figure by the
so-called Beamshaping loss.
|
| Azimuth Compression |
In the SAR signal
domain, the raw data is spread out
in the range and azimuth
directions and must be coherently
compressed to realise the
full-resolution potential of the
instrument. Azimuth
compression consists of coherently
correlating the received signal with
the azimuth replica function. (See
Azimuth Beam
Inversion). The appropriate
Hamming weighting is
applied also to the reference
function. Subsequent correlation has
the effect of modulating both signal
and noise by similar amounts
and hence the signal-to-noise ratio
is unchanged by this process
|
| Azimuth Descalloping |
Radiometric correction
(descalloping) is performed with a
vector multiply in the azimuth
direction. See Descalloping in
products glossary. |
| Azimuth Frequency |
See Doppler Frequency
|
| Azimuth Time |
The time along the
flight path. |
| Backscatter |
Backscatter is the
portion of the outgoing radar signal
that the target redirects
directly back towards the radar
antenna.
Backscattering is the process by which
backscatter is formed. The
scattering cross section in the
direction toward the radar is called the
backscattering cross section; the usual
notation is the symbol sigma . It is a
measure of the reflective strength of a
radar target. The normalised
measure of the radar return from a
distributed target is called the
backscatter coefficient, or sigma nought , and
is defined as per unit area on the
ground. If the signal formed by
backscatter is undesired, it is called
clutter. Other portions of the incident
radar energy may be reflected and
scattered away from the radar or absorbed. |
| Band |
A selection of wavelengths or range
of radar frequencies. |
| Bandwidth |
A measure of the span of
frequencies available
in the signal or passed by the band
limiting stages of the system.
Bandwidth is a fundamental parameter of
any imaging system and determines the
ultimate resolution available. |
| Beam |
A focused pulse of
energy. The antenna beam of a side-looking radar
(SLAR) is directed
perpendicular to the flight path and
illuminates a swath parallel to the
platform ground track. Due to the motion
of the satellite, each target element is
illuminated by the beam for a period of
time, known as the integration
time. (See also chapter 1 "Principles of
Measurement" 1.1.3. ) |
| Beam Mode |
The SAR operating
configuration defined by the swath width and resolution. |
| Beam Position |
The area within the
total possible swath that is actually
illuminated while being governed by the
characteristics of a specific beam mode. |
| Beta Nought |
(ß°A radar
brightness coefficient. The reflectivity
per unit area in slant range
which is dimensionless. |
| Bragg Scattering |
Bragg Scattering is
the enhanced backscatter due to
coherent
combinations of signals
reflected from a rough surface
having features, with periodic
distribution in the
direction of wave propagation, and
whose spacing is equal to half of
the wavelength as
projected onto the surface.
Bragg Scattering
( See also ) |
| Brightness |
Property of an image in
which the strength of the radar
reflectivity is expressed as
being proportional to a digital number
(digital image file) or to a grey scale
(photographic image), which for a
photographic positive shows bright as
white. The attribute of visual
perception in accordance with which an
area appears to emit more or
less light. Brightness may be a result
of variations in tone, texture, or in
the case of radar imagery, radar
artefacts. The topography and
surface roughness of the terrain will
affect the image brightness. Where the
local incidence angle is large, the
image will be dark. Conversely,
the image will be brighter where the
local incidence angle is small. |
| Cardinal Effect |
A tendency of a radar to
produce very strong echoes from a city
street pattern or other linear feature
oriented perpendicular to the radar beam. |
| C-Band |
A nominal frequency
range, from 8 to 4 Ghz (3.75 to 7.5 cm
wavelength) within the microwave
(radar) portion of the electromagnetic
spectrum. C-band has been the frequency
of choice for several experimental
aircraft SAR systems as well as a series
of single-band satellite SAR systems,
including the European ERS-1, ERS-2 and
Envisat SAR systems and
Canada'"s RADARSAT SAR. The
corresponding wavelength for these
systems is on the order of 5.6 cm, which
has been found useful in sea ice
surveillance as well as in other
applications. Imaging radars equipped
with C-band are generally not hindered
by atmospheric effects and are capable
of 'seeing' through tropical
clouds and rain showers. Its penetration
capability with regard to
vegetation canopies or soils is limited
and is restricted to the top layers.
C-band is also used in range
instrumentation radars |
| Chirp |
Typical phase
coding or modulation applied to the
range pulse of an imaging radar
designed to achieve a large
time-bandwidth product. The
resulting phase is quadratic in
time, which has a linear derivative.
Such coding is often
called linear frequency modulation,
or linear FM.
Chirp Frequency (1)
Chirp Frequency (2)
Chirp Phase
The Real Part of Chirp
The Imaginary Part of Chirp
|
| Chirp Compression |
The echo signal is
correlated with a suitable reference
function. This correlation is
performed in the frequency domain
after suitable Fast Fourrier
Transform from the time domain.
The reference function of interest
should represent the chirp signal
which illuminates the target.
This chirp varies from one swath to
another due to the phase and
amplitude settings introduced to
produce beam steering. The beam
steering (and shaping if necessary)
is achieved by changing the
excitations - phase and/or amplitude
- to the many phase centres of
the planar array antenna. The
reference function should reflect
these differences.
In ASAR the phase of the excitation
to any one row is constant along its
length. Thus any row can be
considered to be homogeneous.
The characteristics of such a row
are determined by exciting just that
row - and hence deriving a reference
function for it. A total
reference function can then be
derived by combining 32 such
reference functions - using the
commanded excitation phases.
|
| Circular Polarisation |
A polarisation state in
which the two perpendicular
components of the electric field have
equal magnitudes and a 90-degree phase
difference. In this case, the tip of the
electric field vector traces a
circle on a plane that is perpendicular
to the wave propagation direction. |
| Circularly Polarised Antenna |
An antenna that is
designed to radiate a left-hand
or right-hand circularly polarised
electromagnetic wave in its far field. |
| Clutter |
Unwanted echoes in a
radar return. See also Backscatter.
|
| Coherence |
Coherence is the fixed
relationship between waves in a beam of
electromagnetic (EM) radiation. Two wave
trains of EM radiation are coherent when
they are in phase. That is, they vibrate
in unison. In terms of the
application to things like radar, the
term coherence is also used to describe
systems that preserve the phase of the
received signal. |
| Coherence in an Interferogram |
In an interferogram,
coherence is a measure of
correlation . It ranges from 0.0,
where there is no useful information
in the interferogram; to
1.0, where there is no noise in the
interferogram (a perfect
interferogram). Both extremes are
rarely seen-- most images
lie somewhere in between.
Coherence is affected by, (in
approximate order):
- Local slope (steep slopes lead
to low coherence)
- Properties of the surface being
imaged (vegetated or moving
surfaces have low coherence).
- Time lag between the passes in
an interferogram (long lags lead
to low coherence)
- The baseline (large baselines
lead to low coherence)
- Technical details of the
generation of the interferogram
(poor co-registration or
resampling leads to low coherence)
Coherence can serve as a measure of
the quality of an interferogram;
tell you more about the surface type
(vegetated vs. rock); or
tell you when a tiny, otherwise
invisible change has occured in the
image, and it is only visible in the
phase image of an interferogram.
High coherence makes for attractive,
not-noisy interferograms. Here is a
coherent phase image of a mountain.
Coherent Phase Image
of a Mountain
Low coherence makes
unattractive, noisy interferograms,
as shown below. Often, these
interferograms are difficult to
phase unwrap.
Incoherent Phase
Image of a Mountain
Coherence is the
magnitude of an interferogram's
pixels, divided by the product of
the magnitudes of the original
image's pixels. It is
usually calculated on a small window
of pixels at a time, from
the complex interferogram and images.
Interferometric
SAR (InSAR) techniques make
use of the coherence of
the radar signal and
the fact that the signal phase is equal to
twice the path length between the
sensor and the earth's
surface. The phase difference
between measurements generated from
two SAR
images with the sensor separated by
a baseline, allows measurement of
the slant range
difference to fractions of a radar
wavelength. The
slant range difference can be
geometrically related to the terrain
height. By subtracting two
signals, you generate an
interference pattern. By subtracting
the phases of two co-registered SAR
images, you generate an interferogram.
There are two basic SAR
interferometry methods. In the
first, two antennas are placed on
the same platform and simultaneously
acquire images of the scene from two
different angles. The relative phase
difference may then be used to
construct a Digital Elevation
Model (DEM).
In the second, a pair of images from
the same sensor are taken at
different times. This is now a
well-proven concept and has been
demonstrated with several spaceborne
SAR sensors, including RADARSAT. For
this repeat-pass interferometry, the
scenes are acquired at different
times, so there is a time difference
as well as viewing
geometry to consider. The passes
must have rather similar geometry in
order to allow extraction of the
relative phase difference.
This usually requires that the
satellite be on an exact repeat orbit.
|
| Coherent Reflector |
Simple or complex
surface, such as a corner reflector,
from which reflected wave components are
coherent with respect
to each other, and thus combine to yield
larger effective power than
would be observed from a diffuse
scattering surface of the same area. |
| Complex Number |
For radar systems,
a complex number implies that the
representation of a
signal, or data file, needs both
magnitude and phase measures. In
the digital SAR context, a
complex number is often represented
by an equivalent pair of
numbers, the real in-phase component
(I) and the imaginary quadrature
component (Q). For coherent systems
such as SAR, the role of complex
numbers is an essential
part of the signal, since signal
phase is used in the processor to
obtain high-resolution.
Complex Number
( See also ). |
| Conservation of Confusion |
Principle, for imagery
derived from a given SAR, that
the amount of information in the data is
a constant. One expression of this rule
is that the product of the range and the azimuth
resolution divided by the number of
statistically independent looks is a
constant, which serves as a figure of
merit of the system. (In this
context, information is related to the
statistical degrees of freedom in the
data ensemble, and not necessarily to
knowledge about objects in the scene.) |
| Conservation of Co-ordinates |
Principle, for Synthetic
Aperture Radar (SAR) imagery,
that image position is not changed by
pitch, roll, or yaw rotations of the
radar, since range is determined by
the speed of light, and azimuth is
determined by the along-track radar velocity. |
| Conservation of Energy (Radar) |
Principle, assuming that
all available data is used for
each case, that the average value of the
estimated reflectivity from a scene is a
constant for a given SAR and processor,
independent of the number of looks used,
and independent of any time varying
noncoherence in the scene, such as from
a moving surface of water, or in
the radar/processor combination. |
| Co-polarisation Maxima |
The antenna polarisation
state for which maximum
backscattered power is received from a
particular target. For co-polarisation,
the transmit and receive antennas are
the same. |
| Co-polarisation Nulls |
The antenna polarisation
state for which zero backscattered
power is received from a particular
target. For co-polarisation, the
transmit and receive antennas are the
same. Co-polarisation nulls may
not correspond to the maximum cross
polarisation received power. |
| Co-polarisation Signature |
The received signature
when the transmit and receive antennas
have the same polarisation properties. |
| Corner Reflector |
A combination of two or
more intersecting specular
surfaces that combine to enhance the
signal reflected in the direction of the
radar. The strongest reflection is
obtained for materials having a
high conductivity (i.e. ships, bridges). |
| Cross Polarisation Maxima |
The antenna polarisation
state for which maximum
cross-polarised backscattered power is
received from a particular target.
Co-polarisation nulls may not correspond
to the maximum cross
polarisation received power. |
| Cross Polarisation Nulls |
The antenna polarisation
state for which zero
cross-polarised backscattered power is
received from a particular target. For
co-polarisation, the transmit and
receive antennas are the same.
Note that for cross polarisation nulls
the co-polarisation power is maximum. |
| Cross Polarisation Signature |
The received signature
when the transmit and receive
antennas have orthogonal polarisations. |
| Cross-polarised Waves ( or
Orthogonal Waves ) |
Each wave in a pair of
cross-polarised waves are
completely polarised. However, an
antenna optimised to receive the
co-polarisation maximum of one wave will
receive no power from the other
wave. Note that, in general, an
arbitrary wave may be treated as the sum
of two cross-polarised waves. |
| Degree of Polarisation |
An electromagnetic wave
can have a polarised and a
nonpolarised component. The degree of
polarisation is given by the ratio of
the power in the polarised part of an
electromagnetic wave to the
total power in the electromagnetic wave |
| Depolarisation |
The polarisation state
of an electromagnetic wave can change
when the wave scatters from a
target. Depolarisation is a measure of
the change in the degree of polarisation
of a partially polarised wave upon
scattering. For example, a target may
scatter a wave with a greater degree of
polarisation than the incident wave, in
which case the depolarisation is
negative. Depolarisation is also used to
indicate spatial or temporal variation
of the degree of polarisation for a
completely polarised wave |
| Detection ( Radar ) |
Processing stage at
which the strength of the signal is
determined for each pixel value.
Detection removes phase information from
the data file. The preferred detection
scheme uses a magnitude squared
method, which is energy conserving, and
has units of voltage squared per pixel. |
| Dipole Sheet Transform |
Also called radon
transform. A three dimensional transform
used in scattering of radar
signals, optical data processing, and
the like. |
| Doppler Frequency |
The Doppler frequency
depends on the component of satellite
velocity in the line-of-sight direction
to the target. This direction changes
with each satellite position along the
flight path, so the Doppler
frequency varies with azimuth time. For
this reason, azimuth frequency is often
referred to as Doppler frequency. |
| Doppler Radar |
A radar system which
differentiates between fixed and moving
targets by detecting the change
in frequency of the reflected wave
caused by the doppler effects. The
system can also measure target velocity
with high accuracy. |
| Dynamic Range |
A description of the
variety of signal amplitudes
available in a system. Dynamic range is
specified either (i) to be within
minimum and maximum values or (ii) with
respect to the ratio of maximum
to minimum values. |
| Electromagnetic Displacement |
Image distortion in the
range-direction caused
Infra Red, microwave, and radio waves. By
terrain features in the scene being
above, or below, the reference
elevation contour and, in fact, being
closer to, or farther from, the radar than their
planimetric position. The effect may be
used to create radar stereo images. |
| Electromagnetic Spectrum |
The ordered array of
known electromagnetic energy extending
from the shortest rays, through gamma
rays, X-rays, Ultra Violet, visible,
Infra Red, microwave, and radio waves.(
see Scientifc Background 1.1.2.1. ) |
| Elliptical Polarisation |
A polarisation state in
which the two perpendicular components
of the electric field have
unequal magnitudes and a non-zero phase
difference. In this case, the tip of the
electric field vector traces an
ellipse on a plane that is transverse to
the wave propagation direction. |
| Elliptically Polarised Antenna |
An antenna that radiates
elliptically-polarised
electromagnetic waves in its far field |
| European Remote Sensing (ERS) |
Satellite series (ERS-1
and -2) launched by the European Space
Agency (ESA) in July 1991 and
April 1995. One instrument (AMI) includes a C-band
SAR, VV
polarisation, and
23° incidence angle,
and 30-metre resolution. Predecessors to
the ENVISAT satellite. |
| Frequency |
Number of oscillations
per unit time or number of wavelengths
that pass a point per unit time. Rate of
oscillation of a wave. In remote
sensing, this term is most often used
with radar. The frequency bands
used by radar (radar frequency bands)
were first designated by letters for
military secrecy. In the microwave
region, frequencies are on the
order of 1 GHz (Gigahertz) to 100 GHz.
("Giga" implies multiplication
by a factor of a billion). For
electromagnetic waves, the product of
wavelength and frequency is equal to the
speed of propagation, which, in free
space, is the speed of light. *
In the microwave region, frequencies are
on the order of 0.3 GHz-300 GHz, having
wavelengths of 1mm - 1 m respectively. |
| Frequency Assignment |
Frequency at which a
sensor (especially radar) operates. |
| Frequency Modulation |
A
technique in which the frequency of a
signal is changed about a fundamental
or carrier frequency.
FM
signals are used in many applications in
remote sensing and in the radio
broadcasting community. In SAR, the
outgoing chirp signal has a
linear FM of several megahertz. GPS
signals are broadcast in FM. |
| Frequency Rate |
The frequency rate is
the coefficient of quadratic phase
variation in the linear FM signal |
| Horizontal Polarisation |
Linear polarisation with
the lone electric vector oriented in the
horizontal direction in antenna co-ordinates. |
| Horizontal Transmit -
Horizontal Receive Polarisation (HH) |
A
mode of radar
polarisation where the
microwaves of the
electric field are oriented in the
horizontal plane for both signal
transmission and reception by
means of a radar antenna. |
| Horizontal Transmit -
Vertical Receive Polarisation (HV) |
A
mode of radar
polarisation where the
microwaves of the
electric field are oriented in the
horizontal plane for signal
transmission, and where the
vertically polarised electric field of
the backscattered energy
is received by the radar antenna. |
| Imaging Radar |
Most imaging radars
produce two-dimensional images. The
two dimensions are called range, and azimuth. |
| Impulse Response |
Also known as the
point spread function, impulse
response is the
two-dimensional brightness pattern
in an image (after processing)
corresponding to the signal
reflected by an object whose sigma
falls within the dynamic range
of the system, and for which the
width of the imaged pattern is
determined by the radar and
processor rather than by the size of
the object. A trihedral corner
reflector is the most commonly
used object for generating an
impulse response in a test image. A
good impulse response has a
relatively large value for the pixel that maps
the point scatterer location, and
very small values for all
surrounding pixels. The impulse
response is a basic building block
in describing a given radar's
imaging performance, since an image
is built up from the linear
combination of impulse responses
from all individual scatterers
illuminated by the radar. The
impulse response width (IRW, or
resolution) of the central peak is
the most important characteristic of
the impulse response, together with
the shape of the impulse
distribution, both close to and
remote from its centre.
Impulse Response to a
Point Target
|
| Incoherent |
Property of a signal or
data set in which the phases of the
constituents are not statistically
correlated, or systematically related in
any fashion. Also known as non-coherent.
( See Coherence ) |
| In-Phase (I) |
The real component of
the signal that has the same phase as
the complex reference frequency.
In-phase is represented by the constant
I ( see Complex Number and
) |
| In-Phase/Quadrature Channels (I/Q) |
In-phase component of a
( See Complex Number and
) |
| Integrated Side Lobe Ratio (ISLR) |
Given a single point
scatterer on the ground, the processed
SAR image consists of
a narrow, strong peak at the location of
the scatterer (the mainlobe),
surrounded by smaller peaks called sidelobes. The sum of
energy in the sidelobes, divided
by the sum of energy in the mainlobe, is
the Integrated Side Lobe Ratio (ISLR). |
| Interferometer |
Device such as an
imaging radar that uses two different
paths for imaging, and deduces
information from the coherent
interference between the two signals. In
SAR applications, spatial interferometry has
been demonstrated to measure terrain
height, and time delay
interferometry is used to measure
movement in the scene such as oceanic currents. |
| Interferometric Synthetic
Aperture Radar (InSAR) |
SAR interferometry
is a technique involving
phase measurements
from successive satellite SAR images
to infer differential range and range
changes for the purpose of detecting
very subtle changes on, or
of, the earth'"s surface
with unprecedented scale, accuracy
and reliability.
SAR
interferometry has
been demonstrated successfully in a
number of applications,
including topographic mapping,
measurement of terrain displacement
as a result of earthquakes, and
measurement of flow rates
of glaciers or large ice sheets. The
term InSAR, is most commonly
associated with repeat-pass
interferometry, as discussed
in the section entitled "Interferometrey" 1.1.5.4.
in the User Guide. In
contrast, D-InSAR is used to
described differential interferometery.
( see also the section entiled "Processing
Algorithms For AP SLC" 2.6.2.1.1.2.1.4.
in chapter 2).
|
| Interferometry |
A technique that uses
the measured differences in the
phase of the return
signal between two satellite passes to
detect slight changes on the
Earth's surface. ( see Interferometre ) The
combination of two radar measurements of
the same point on the ground, taken at
the same time, but from slightly
different angles, to produce stereo
images. Using the cosine rule from
trigonometry to calculate the
distance between the radar and the
Earth's surface, these measurements
can produce very accurate height maps,
or maps of height changes.
Mapping height changes provides
information on earthquake damage,
volcanic activity, landslides, and
glacier movement. ( see also the
section entitled "Interferometrey"
in the User Guide 1.1.5.4. ) |
| Inverse Synthetic Aperture
Radar (ISAR) |
A SAR system that makes
use of the motion of the target to
synthesise a large aperture antenna. |
| Linear Frequency Modulation (FM) |
A linear FM signal has a
quadratic phase variation with time, so
the instantaneous frequency varies
linearly with time. |
| Linear Polarisation |
A polarisation state in
which one of the perpendicular
components of the electric field has
zero magnitude. In this case, the
polarisation ellipse collapses to a
straight line; the tip of the
electric field vector traces a straight
line on a plane that is transverse to
the wave propagation direction. |
| Linear Range Cell Migration
Correction (RCMC) |
A shift and
interpolation operation is performed in
the range direction to
correct for linear Range Cell Migration (RCM). |
| Linearity |
Property according to
which an operation on a sum of signals
is equivalent to the same
operation applied to each of the signals
individually, and the resulting numbers
added together. Linearity, over
the dynamic range of the system, is an
essential attribute of most measurement
devices such as an imaging radar. |
| Look Direction |
The radar look direction defines
the angle in the horizontal plane in
which the radar antenna is pointing when
transmitting a pulse and
receiving the return signal from the
ground or from an object. The look
direction is an angular measurement (in
degrees) and is usually made
with respect to true North. In side-looking imaging
radar (SLAR), the look
direction is often orthogonal (normal)
to the flight trajectory (azimuth) of the
platform carrying the radar and is thus
synonymous with the range direction. The
radar look direction is an important
parameter when analysing features with a
preferred orientation, for
example fracture patterns in rock
formations, regular street patterns, or
ocean waves, as these may be enhanced
through choice of appropriate radar
illumination direction. ( See also "Imaging
Geometry" in the geometry
glossary ). |
| Looks |
Radar
terminology refers to individual looks
as groups of signal samples in a SAR processor
that splits the full synthetic aperture
into several sub-apertures, each
representing an independent look of the
identical scene. The resulting
image formed by incoherent summing of
these looks is characterised by reduced
speckle and degraded
spatial resolution. The SAR signal
processor can use the full
synthetic aperture and the complete
signal data history in order to produce
the highest possible resolution, albeit
very speckled, single-look complex
(SLC) SAR image product. Multiple looks may
be generated by averaging over range and/or azimuth
resolution cells. For an improvement in
radiometric resolution using multiple
looks there is an associated degradation
in spatial resolution. Note that
there is a difference between the number
of looks physically implemented in a
processor, and the effective number
of looks as determined by the statistics
of the image data. ( See also ). |
| Matched Filtering |
The matched filter
can be thought of as a filter with a
different time delay for
each frequency component of the
signal passing through the filter.
That is, the different frequency
components of the linear
FM signal are each
delayed so that they all arrive at
the same time at the
output of the filter. This way, all
the signal energy is gathered into a
narrow peak in the compressed pulse.
( see also "ASAR Level
1B Algorithm Physical
Justification" - Pulse
Compression 2.6.1.1.3. )
Real Part of Point
Target Signal
Real Part of Matched Filter
Compressed Target Signal
|
| Microwave |
A very short
electromagnetic wave. The portion of the
electromagnetic
spectrum lying between the far
infrared (IR) and the
conventional radio frequency portion.
While not bounded by definition, it
is commonly regarded as extending from 1
mm to 1 m in wavelength (300 GHz
to 0.3 GHz frequency). Passive systems
operating at these wavelengths sometimes
are called microwave systems. Active systems are
called radar, although the literal
definition of radar requires a
distance measuring capability not always
included in active systems. |
| Motion Compensation |
Adjustment of a sensing
system and/or the recorded data to
remove effects of platform motion,
including rotation and translation, and
variations in along-track velocity.
Motion compensation is essential
for aircraft SARs, but usually is not
needed for spacecraft SARs. |
| Multifrequency Radar |
Broadband systems that
transmit pulses in a range of
frequencies and wavelengths. |
| Multipolarisation Radar |
A radar capable of
simultaneously and coherently
acquiring several independent complex
polarisation measurements for every
pixel in the image. |
| Night Sensing |
The collection of
non-photographic data during the period
of time between sunset and sunrise. |
| Noise Equivalent Sigma Nought |
A measure of the
sensitivity of a given SAR. It describes the
strength of the (additive) system noise
in terms of the equivalent
(average) power in the image domain that
would result from an idealised
distributed scatterer of the stated
reflectivity. Smaller noise
equivalent sigma nought values are
better. Within physical limitations,
smaller may be achieved by increasing
the power of the radar
transmitter, or by decreasing the noise figure of
the electronics. |
| Noise Figure |
Factor that describes
the noise level in a radar receiver
relative to the that in a theoretically
perfect receiver. The noise figure,
which is always larger than one, is
typically two or more, and is
usually expressed in decibels. |
| Optical Correlator |
A device that enhances
weak signals in noise by performing
an optical operation approximating the
computation of a correlation function. A
radar optical correlator uses the
original synthetic aperture
radar signal film recording of doppler
phase histories to make the radar image
by methods that are similar to those
used in optical Fourier transformation. |
| Passive System |
Systems that sense
naturally available energy. |
| P-Band |
A frequency range from
0.999 to 0.2998 GHz (30 to 100 cm
wavelength) within the microwave
(radar) portion of the electromagnetic
spectrum. P-band is an experimental SAR frequency that has
only been used to-date for research and
development purposes. It is part of the
NASA JPL AIRSAR
multi-frequency (C-, L- & P-band)
SAR system designed for Earth
observation experiments. P-band is not
hindered by atmospheric effects and is
capable of seeing through heavy rain
showers. P-band SAR penetration
capabilities are very significant with
regard to vegetation canopies, glacier
or sea ice, and soil. Its vegetation
canopy imaging capability is
considered a key element in estimating
vegetation biomass by means of remote sensing. |
| Phase |
See Radar Phase
|
| Phase Preserving |
When the phase at the
peak is correct, the processing
algorithm is referred to as phase
preserving, regardless of the phase
variation across the impulse response. |
| Phase Unwrap |
In SAR
interferometry (InSAR) , the phase delay of the
carrier signal at a certain
point in the interferogram is a function
of the terrain height at that point.
However, the phase of the carrier signal
can only be measured to within
one cycle, or 360 degrees. Phase
unwrapping refers to converting the
measured phase to the absolute phase, by
adding the appropriate number of
cycles, or multiple of 360 degrees, to
the measured phase. |
| Polarimetric Active Radar
Calibrator (PARC) |
Device used to receive
and retransmit radar pulses. These devices
usually consist of a polarisation sensitive
receive and transmit antenna and a
stable amplifier which boosts
the signal level so that the device
being calibrated receives a
high signal of a given polarisation. |
| Polarimetric Radar |
A radar which permits
measurement of the full polarisation signature
of every resolution element. |
| Polarisation |
The process of confining
the vibrations of the magnetic, or
electric field, vector of light or other
radiation to one plane. Orientation of
the plane of the electric field relative
to the Earth's surface. See
also Radar Polarisation
in Geometry glossary. |
| Polarisation Beamwidth |
For a pair of antennas
in a transmit-receive
configuration, the polarisation
beamwidth is defined as the
angle between the two directions from
the transmitter at which the
polarisation efficiency is one-half of
its maximum value. |
| Polarisation Efficiency |
For an electromagnetic
wave that is incident upon a
receiving antenna, the polarisation
efficiency is defined as the ratio of
power actually received in an
impedance-matched load, to the
maximum power that could be received if
the antenna had optimum polarisation for
the received wave. |
| Polarisation Ellipse |
For an elliptically
polarised wave, the tip of the
electric field vector traces an ellipse
on a plane that is transverse to the
wave propagation direction. This
polarisation ellipse describes
the polarisation properties of the
electromagnetic waves, including the
ratio of the perpendicular electric
field components and their
relative phases. |
| Polarisation Loss |
The inverse of polarisation efficiency. |
| Pulse |
A short burst of
electromagnetic radiation transmitted by
the radar. Also described
as a group of waves with a distribution
confined to a short interval of
time. Such a distribution is described
in the time domain, or in spatial
dimensions, by its width and its
amplitude or magnitude, from
which its energy may be found. In radar,
use is made of modulated or coded pulses
which must be processed to decode
or compress the original pulse to
achieve the impulse response
observed in the image. Coded pulses have
a time-bandwidth product that is much
larger than unity. The resolution that
may be achieved after processing
is determined by the bandwidth of the
original pulse. |
| Pulse Compression |
In collecting the SAR data, a
long-duration linear FM
pulse is transmitted.
This allows the pulse energy to be
transmitted with a lower peak
power. The linear FM pulse has the
property that, when filtered with a matched filter,
the result is a narrow pulse in which
all the pulse energy has been collected
to the peak value. Thus, when a
matched filter is applied to the
received echo, it is as if a narrow
pulse were transmitted, with is
corresponding range resolution
and signal-to-noise ratio (SNR). ( see
also "ASAR
Level 1B Algorithm Physical
Justification" - Pulse Compression 2.6.1.1.3. ) |
| Pulse Repetition Frequency (PRF) |
Rate of recurrence of
the pulses transmitted by
a radar. |
| Quadrature (Q) Channel |
The
signal component that is 90° out of
phase with respect to the reference
frequency. It is represented by the
letter Q. It is the imaginary part,
which indicates the magnitude of the
signal, of the complex number.
See Complex Number. |
| Quadrature Polarisation
Radar (Quad Pol Radar) |
A Radar system designed
to simultaneously collect imaging data
of a scene in two orthogonal polarisation states on
transmit and the same two polarisation
states on receive. From such a
data set a complete scattering matrix of
the reflectivity of the scene may be
synthesised, leading to the concept
of polarisation signature. |
| Radar |
See "Radio Detection
and Ranging"
|
| Radar Angles |
A radar echo from a
region where there are no visible
targets; may be caused by
insects, birds, or refractive index
variations in the atmosphere. |
| Radar Antenna |
The radar antenna
is a structure for transmitting and
receiving radiated energy; it is an
important subsystem that defines, to a
great extent, a radar's
operational capabilities and cost. In
radar remote sensing the main function
of the antenna is to concentrate a
radiated microwave energy into
a beam of required shape, referred to as
the antenna pattern, to transmit
it into the desired direction (look direction),
and to receive the returned energy from
surfaces or objects. Radar remote
sensing antennas provide scene
illumination ( See Active Remote Sensing
System ). The main parameters of
radar antennas are operating frequency band,
antenna pattern shape (directivity),
power (or antenna-) gain,
beam-width, side-lobe level, polarisation, and
power handling capability. Moving
antennas can be used to form a
synthetic
aperture, where the physical antenna
is small compared to the
synthesised antenna, and has a
sufficiently wide radiation pattern to
illuminate the observed surface over a
significant period of platform motion. |
| Radar Beam |
The vertical fan-shaped
beam of electromagnetic energy produced
by the radar transmitter. |
| Radar Cross Section (RCS) |
Measure of radar
reflectivity. The Radar Cross Section
(RCS) is expressed in terms of
the physical size of an hypothetical
uniformly scattering sphere that would
give rise to the same level of
reflection as that observed from
the sample target. |
| Radar Height Indicator (RHI) |
Also known as Range
Height Indicator. |
| Radar Parallax |
Apparent change in the
position of an object due to an
actual change in the point of view of
observation. For a SAR, true parallax
occurs only with viewpoint changes that
are away from the nominal flight path of
the radar. In contrast to aerial
photography, parallax cannot be
created by forward and aft looking
exposures. Parallax may be used to
create stereo viewing of radar images. |
| Radar Phase |
Phase is a property of a
periodic phenomenon, for example a wave,
referring to its starting point or
advancement (fraction) relative to an
arbitrary origin. In radar remote
sensing, the concept of phase is
usually applied to the oscillation of
electromagnetic waves. When viewed as a
cyclical phenomenon, like wave motion or
the crankshaft motion of a
bicycle pedal, phase can be expressed in
degrees. One-quarter cycle represents a
phase rotation of 90 degrees;
completion of one complete cycle
corresponds to a phase rotation of 360
degrees. Waves are considered in-phase,
if their origins of phase 0
degrees are perfectly aligned;
out-of-phase conditions are met when
phase 0 and 180 degrees are aligned.
Precise knowledge of phase
properties in radar signal data is a key
element in interferometric as well as in
polarimetric SAR. |
| Radar Transmission |
Energy sent by the
radar, normally in the form of a
sequence of pulses, to
illuminate a scene of interest. |
| Radar Wave Energy |
For a waveform of
time-limited duration such as a radar
pulse reflected by an object,
the pulse energy is given by the power
of the signal integrated over the
duration of the signal It is expressed
in units of watt-seconds = joules = j |
| RADARSAT-1 |
RADARSAT-1 is an
advanced Earth observation satellite
project developed by Canada to
monitor environmental change and to
support resource sustainability. It is
Canada's first Earth observation
satellite and the world's first
operationally-oriented radar sensor.
With a planned lifetime of five years,
RADARSAT-1 is equipped with a Synthetic Aperture
Radar (SAR). Launched in
November 1995, this C-band SAR satellite
includes a steerable beam, which offers
a wide selection of image scales
and resolutions. It operates at 5.3 GHz.
RADARSAT-2 is scheduled for launch in
late 2003. |
| Radio Altimetry |
The science and
techniques involved in using a radar
altimeter for precise
measurements of distance between the
sensor and the surface or feature. The
basic concept of radar altimetry
involves a short radar pulse
transmitted toward the surface or
feature and accurate measurement of the
round trip time to the reflecting
surface, which yields precise
distance. The accuracy, which is
generally on the order of several
centimetres or tens of centimetres,
depends mainly on the sharpness
of the pulse, or signal bandwidth, and
the footprint of the radar beam.
Measurement techniques involve
beam-limited and pulse-limited radar
altimeters that provide two dimensional,
high-resolution height profiles along
the flight line. The synthetic
aperture approach is also used by SAR
altimeters to reduce the size of the
footprint along the flight line. .
Prominent geoscientific
applications of radar altimetry include
measurement of ocean surface dynamics as
well as solid surface topographic mapping. |
| Radio Band |
The range of wavelengths or
frequencies of electromagnetic radiation
designated as radio waves; approximately
10(4) to 10(9) Hz in frequency. |
| Radio Detection And Ranging
(RADAR ) |
A method, system or
technique, including equipment
components, for using beamed,
reflected, and timed electromagnetic
radiation to detect, locate, and (or)
track objects, to measure altitude and
to acquire a terrain image. The
radio detection instrument consists of a
transmitter that sends out
high-frequency radio waves and a
receiver that picks them up
after they have been reflected by an
object. Basic building blocks of a radar
are the transmitter, the antenna
(normally used for both
transmission and for reception), the
receiver, and the data handling
equipment. A synthetic aperture radar
system, by implication, includes
an image processor, even though it may
be remotely located in time or space
from the radar electronics. The
advantage radar sensors have over other
types of sensors, is that microwaves can
penetrate clouds, most rain storms, and
even dry snow. Therefore, for
those parts of the world where cloud and
rain present a problem in acquiring
images (tropics, coastal/maritime
regions), radar is highly
beneficial. ( see also , chapter 4) |
| Radio Echo |
The signal reflected by
a radar target, or the
trace produced by this signal on the
screen of the cathode-ray tube in a
radar receiver. |
| Radio Frequency (RF) |
A frequency that falls
within the radio band: 10(4)
to 10(9) Hz. |
| Radiometer |
An instrument for
quantitatively measuring the intensity
of electromagnetic radiation in
some band of wavelengths in any part of
the electromagnetic spectrum. Usually
used with a modifier, such as an
infrared radiometer or a microwave radiometer. |
| Radiometric Compensation |
The sensitivity of the
antenna is sensitive to angle and the
radiometric compensation adjusts for
variation in the received signal due to
this effect. See Side Lobes. |
| Range Time |
The fast time within a
received pulse, relative to the pulse
transmission time |
| Real Aperture Radar (RAR) |
A
radar system where the antenna beamwidth
is controlled by the physical
length of the antenna. Also known as
brute force or noncoherent radar. A SLAR system in
which azimuth resolution is determined
by the physical length of the antenna
and by the wavelength. The radar returns
are recorded directly to produce
images. The advantages of RAR is their
simple design and data processing.
However, its resolution is poor
so RAR are limited to short range, low
altitude missions, scanning short
wavelengths. The use of the data is
limited as shorter wavelengths
experience a large amount of atmospheric
effects, scattering and dispersion, for
example. Because the missions are flown
at low altitudes, the coverage
is small. The resolution is limited by
the length of the antenna. The antenna
needs to be many times longer
than the wavelength to produce narrow
bandwidths. However, it is impractical
to design an antenna long enough to
produce high-resolution data. (
see also FAQS(Chapter 4. ) ) |
| Remote Sensing (R/S) |
Group of techniques for
collecting image or other forms of data
about an object from
measurements made at a distance from the
object, and the processing and analysis
of the data. ( see also , chapter 4). |
| Repeat Pass Interferometry |
Method based on two
image acquisitions of the same
scene from slightly displaced orbits of
a satellite. Phase information of
the two image data files are
superimposed. The two phase values at
each pixel are then
subtracted, leading to an interferogram
that records only the differences in
phase between the two original images.
Phase differences can be related
to the altitude variation at each
position in the swath and enable the
production of a Digital Elevation
Model (DEM). For optimum
results, there should be no change in
the backscatter to
maintain coherence; vegetated
sites are therefore a problem.
For detection of feature movement (e.g.
tracking glaciers) orbits should be as
close as possible. Ground control
points (GCPs) are required to accurately
superimpose the two images. And
knowledge of the sensor location is
critical. With a good baseline
and coherence, this technique can be
better than stereo ( ~10 m vertical
accuracy). ( see Interferometry ) |
| Rotor-SAR (ROSAR) |
A synthetic aperture
radar concept whereby antennas are
mounted at the tips of the rotor
blades (i.e. helicopter) illuminating a
circular ring-shaped swath. |
| SAR Focusing |
In a long synthetic
aperture (array), SAR focusing involves
the removal and compensation of path
length differences from the
antenna to the target on the ground. The
main advantage of a focused synthetic
aperture is that it increases its array
length over those radar signals that can
be processed, and thus increases
potential SAR resolution at any
range. SAR focusing is a necessary
process when the length of a synthetic
array is a significant fraction of the
range to ground being
imaged, as the lines-of-sight (range)
from a particular point on the
ground to each individual element of the
array differ in distance. These range
differences, or path length differences,
of the radar signals can affect
image quality. In a focused SAR image
these phase errors
can be compensated for by applying a
phase correction to the return signal at
each synthetic aperture element.
Focusing errors may be
introduced by unknown or uncorrected
platform motion. In an unfocused SAR
image, the usable synthetic aperture
length is quite limited. |
| S-Band |
A nominal frequency
range from 4 to 2 GHz (7.5 to 15 cm
wavelength) within the microwave
(radar) portion of the electromagnetic
spectrum. S-band radars are used for
medium-range meteorological
applications, for example rainfall
measurements, as well as airport
surveillance and specialised tracking tasks. |
| Scanning Synthetic Aperture
Radar (ScanSAR) |
A having the capability to
illuminate several subswaths by
scanning its antenna off-nadir into
different positions.
ASAR ScanSAR geometry
|
| Sea Satellite (SEASAT) |
NASA ocean
research satellite that was in operation
July-September of 1978. Seasat was the
first (civilian) satellite to carry a SAR. It operated at
L-band, using horizontal polarisation at
22° incidence angle.
Data from Seasat is still important for
applications and processing technique development. |
| Sensitivity Time Control (STC) |
Pre-programmemed change
in radar amplitude due to
weaker backscatter from
greater ranges and varying incidence angles
across the imaged swath. |
| Short Pulse Radar |
Radar system capable of
generating nanosecond pulses of
energy at microwave frequencies. |
| Sidelobes |
Non-zero levels in a
distribution that are separated from
the desired central
response. Sidelobes arise naturally
in antenna patterns,
for example, although in
general they are a nuisance, and
must be suppressed as much as
possible. Large side-lobes may lead
to unwanted multiple
images of a single feature.
( See also Beamwidth in the
Geometry glossary )
|
| Side-Looking Aperture Radar (SLAR) |
A high-resolution real
aperture radar (RAR) having antennas
aimed to the right or left of the flight
path. Also referred to as side-looking
radar. An all-weather, day/night
remote sensor which is particularly
effective in imaging large areas of
terrain. It is an active sensor, as it
generates its own energy which is
transmitted and received to produce a
photo-like picture of the ground. It
provides high-resolution strip
maps with photograph-like detail. ( see also FAQS(Chapter 4. ) ) |
| Signal Noise |
Signal noise is any
unwanted or contaminating signal
competing with the desired signal. In a
SAR, two common kinds of noise are
additive (receiver) noise and signal
dependent noise, usually either
additive or multiplicative. The relative
amount of additive noise is described by
the signal-to-noise ratio (SNR).
Signal dependent noises, such as azimuth
ambiguities or quantization noise, arise
from system imperfections, and are
dependent on the strength of the
signal itself. Speckle is sometimes
considered to be a kind of
signal dependent multiplicative noise in
a SAR system. |
| Spectral Window |
A band of the electromagnetic
spectrum which offers maximum
transmission and minimal attenuation
through a particular medium. |
| Spectrum |
See Electromagnetic Spectrum
|
| Stereo Sensing |
Science and techniques
used in obtaining stereo imagery (other
than aerial photography). |
| Synthetic Aperture |
A synthetic aperture, or
virtual antenna, consists of a
long array of successive
and coherent radar signals
that are transmitted and
received by a physically short (real)
antenna as it moves along a
predetermined flight or orbital path.
The synthetic aperture is formed
by pointing the real radar antenna of
relatively small dimensions, which are
restricted in size by the satellite
platform, broadside to the direction of
forward motion of that platform. The
points at which successive pulses are transmitted
can be thought of as the elements of a
long synthetic array, which a
signal processor will then use and
process to generate a SAR image. This
detailed array of radar signal data is
the key to achieving high azimuth resolution.
This long virtual antenna concept is the
basis for synthetic aperture radar, or
SAR. ( see also "Scientific Background" 1.1.2. ) |
| Synthetic Aperture Radar (SAR) |
A synthetic aperture
radar, or SAR, is a coherent radar system
that generates high-resolution remote
sensing imagery. Signal processing uses
magnitude and phase of the received
signals over successive pulses from
elements of a synthetic aperture
to create an image. As the line of sight
direction changes along the
radar platform trajectory, a synthetic
aperture is produced by signal
processing that has the effect of
lengthening the antenna. The
achievable azimuth resolution of
a SAR is approximately equal to one-half
the length of the actual (real) antenna
and does not depend on platform
altitude (distance). High range
resolution is achieved through pulse compression
techniques. In order to map the ground
surface the radar beam is directed to
the side of the platform
trajectory; with a sufficiently wide
antenna beam width in the along-track direction,
an identical target or area may be
illuminated a number of times without a
change in the antenna look angle. ( see also
"Scientific Background" 1.1.2.3. ) |
| Synthetic Aperture Time |
The time for a target to
cross the azimuth antenna beam. |
| Unpolarised Wave |
A polarisation state in
which the two perpendicular
components of the electric field have
equal magnitudes and a random relative
phase difference. |
| Vertical Polarisation |
Linear
polarisation with the lone electric
vector oriented in the vertical
direction in antenna co-ordinates. |
| Vertical Transmit-Horizontal
Receive Polarissation (VH) |
A
mode of radar
polarisation where the
microwaves of the
electric field are oriented in the
vertical plane for signal transmission,
and where the horizontally
polarised electric field of the backscattered energy
is received by the radar antenna. |
| Vertical Transmit-Vertical
Receive Polarisation ( VV ) |
A
mode of radar
polarisation where the
microwaves of the
electric field are oriented in the
vertical plane for both signal
transmission and reception by
means of a radar antenna. In this case,
the plane of the electric field of
the microwave energy is designated by
the letter V (vertical) for both
transmit and receive events, i.e. VV;
this transmit-receive polarity
is also called like-polarised as opposed
to cross-polarised (horizontal transmit
- vertical receive, HV). The amount of
radar backscatter received
at a particular linear
polarisation state from a particular
ground surface or object depends, in
part, on the scattering mechanism and depolarisation effects
involved. The transmit-receive acronym
is often used in conjunction
with the frequency band (wavelength)
designation of a particular radar
system, e.g. C-VV for C-band. Several
satellite SAR designs have used
single-band, horizontally
like-polarised systems, for example the
European ERS-1 and ERS-2 (C-VV). |
| Wavelength |
In a periodic wave, the
distance between two points of
corresponding phase in consecutive cycles |
| Wavelet |
Filter for enhancing and
compressing radar images. Wavelets are
operators that act both in space
(azimuth and range) and in frequency
(signal content). The classification and
identification of texture and
features in images incorporates fractal
and multi-fractal models that can
parameterise geophysical processes.
Wavelets may be tailored to
retain desired signal behaviour or to
preserve smoothness. They are also used
to characterise complex signals. |
| X-Band |
A nominal frequency
range from 12.5 to 8 GHz (2.4 to 3.75 cm
wavelength) within the microwave
(radar) portion of the electromagnetic
spectrum. X-band is a suitable frequency
for several high-resolution radar
applications and has often been used for
both experimental and operational
airborne SAR systems, designed for
military as well as civilian
remote sensing applications. The
corresponding wavelength for these
systems is on the order of 3 cm, which
has been found useful for
mapping and surveillance tasks. |
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