Along Track 
See Azimuth below. 
Aspect Angle 
Description of the
geometric orientation in the
horizontal plane of an object in
the scene with respect to the
illuminating wavefront.

Azimuth 
The term azimuth is
used to indicate linear distance or
image scale in the
direction parallel to the radar flight path.
In an image, azimuth is
also known as alongtrack
direction, since it is the relative
alongtrack position of an object
within the antenna'"s
field of view following the
radar's line of flight.
Azimuth is predominately used in
radar terminology. The azimuth
direction is perpendicular to the range direction.
The resolution of an image in the
azimuth directions for a SAR image is
constant and is independent of the
range. For two objects to be
resolved, they must be separated in
the azimuth direction by a distance
greater than the beamwidth on the ground.
Azimuth
(See also Imaging
Geometry )

Azimuth Direction 
Direction parallel to
the line of flight, also referred to
as the alongtrack direction.
(See Azimuth ) 
Azimuth Line

A line of constant
range. Each azimuth line is
parallel to the flight path. Note
that this usage is not universal;
some researchers use
'azimuth line' to refer to
what is here called 'range
line' (and vice versa).
Azimuth Line

Beamwidth 
Beamwidth is a
measure of the width of the
radiation pattern of an antenna. For SAR
applications, both the vertical
beamwidth, affecting the width of
the illuminated swath, and the
horizontal or azimuth pattern,
which determines,
indirectly, the azimuth resolution,
are frequently used. Beamwidth may
be measured in the oneway or
twoway form, and in
either voltage or power.
Beamwidth
(See also SideLobes in
Radar and SAR glossary )

Depression Angle 
Depression angle
usually refers to the line of sight
from the radar to an
illuminated object as measured from
the horizontal plane at the radar. For
image interpretation, use of the
term is not recommended because it
does not account for the effects of
Earth curvature, and it does
not conveniently include effects of
local slope in the scene. It is more
appropriate for an engineering
description of the vertical
antenna pattern at
the radar itself.
Depression Angle

Digital Elevation Model (
DEM ) 
A
quantitative model of a landform in
digital form, normally given as metres
above sea level (including the height of
the vegetation), and referenced to a
geographic coordinate system. 
Elevation Angle 
The elevation angle is
that which is located between the slant range and
the nadir. It
closely approximates the incidence angle,
but there are differences as a
result of the curvature of the Earth. (
See also Depression Angle
and Imaging Geometry) 
Elevation Displacement 
Elevation
displacement, also referred to as
geometric distortion, is
the image displacement in a remote sensing
image toward the nadir point in radar
imagery due to sensor/target imaging
geometries. In a radar image
the displacement is toward
the sensor and can become quite
large when the sensor is nearly
overhead. The displacement increases
with decreasing incidence
angle. The four characteristics
resulting from the
geometric relationship between the
sensor and the terrain that are
unique to radar imagery are foreshortening,
pseudoshadowing, layover, and shadowing. Topographic
features like mountains, as well as
artificial targets like tall
buildings, will be displaced from
their desired orthographic position
in an image. The effect may be used
to create stereo images.
It may be removed from an image
through independent knowledge of the
terrain profile. In many
applications, an approximate
correction may be derived through
shapefromshading techniques.
Elevation displacement will be
greater in slant range
than ground range
due to the fact that the image is
more compressed in a slant range
presentation. Elevation
displacement is also most pronounced
at near range.
Elevation displacement

Ellipsoid 
A model used to describe
the shape of the planet Earth, which is
not a true sphere but an oblate spheroid
compressed along the polar axis and
bulging slightly around the equator. 
Ellipticity Angle 
Defined as the magnitude
of the arctangent of the ratio of
the polarisation ellipse'"s
minor and major axes. If negative, the
ellipse rotation is righthanded; if
positive, is the ellipse
rotation is lefthanded. 
Far Range 
Portion of the radar
image farthest from the flight path. (
See also "Imaging
Geometry" ) 
Foreshortening 
Foreshortening is
the spatial distortion whereby
terrain slopes facing a sidelooking
radar's (SLAR)
illumination are mapped as having a
compressed scale relative
to its appearance, as if the same
terrain were level. Foreshortening
is a special case of elevation
displacement. The effect is more
pronounced for steeper slopes
and for radars that use steeper incidence angles.
( See also Shadow in Products glossary.
Foreshortening

Ground Range 
Ground Range is the
perpendicular distance from the ground
track to a given object on the
Earth'"s surface. Also defined
as the range direction of a sidelooking
radar image as projected onto
the nominally horizontal reference
plane, similar to the spatial display of
conventional maps. Ground range
projection requires a geometric
transformation from slant range to
ground range; for spacecraft data, a
geoid model of the Earth is used,
whereas for airborne radar data,
a planar approximation is sufficient.
This can lead to relief or elevation
displacement, foreshortening, and
layover on radar images. However, if
terrain elevation information is used,
the effect on viewing geometry can be
minimised. ( See also "Imaging
Geometry" , "Depression
Angle" and "ASAR Level
1B Algorithm Physical
Justification"  Radar
Geometry 2.6.1.1.2. , as well as ) 
Imaging Geometry 
The imaging geometry
of a radar system is
different from the framing and
scanning systems commonly employed
for optical remote sensing.
Similar to optical systems, the
platform travels forward in the
flight direction (A) with the nadir (B) directly
beneath the platform. The microwave
beam is transmitted obliquely at
right angles to the direction of
flight illuminating a swath (C) which is
offset from nadir. Range (D) refers
to the acrosstrack
dimension perpendicular to the
flight direction, while azimuth (E) refers
to the alongtrack
dimension parallel to the flight
direction. This sidelooking
viewing geometry is typical of
imaging radar systems (airborne or spaceborne).
Imaging Geometry
The portion of the image
swath closest to the nadir track of
the radar platform is called the near range (A)
while the portion of the swath
farthest from the nadir is
called the far range (B).
The incidence
angle is the angle between the
radar beam and
ground surface (A) which increases,
moving across the swath from near to
far range. The look angle (B) is the
angle at which the radar
looks at the
surface. In the near range, the
viewing geometry may be
referred to as being steep, relative
to the far range, where the viewing
geometry is shallow. At all ranges
the radar antenna measures
the radial line of sight distance
between the radar and each
target on the surface. This is the
slant range
distance (C). The ground range
distance (D) is the true horizontal
distance along the ground
corresponding to each point measured
in slant range.

Incidence Angle 
The incidence angle
is the angle defined by the incident
radar beam and
the vertical (normal) to the
intercepting surface. In general,
reflectivity from distributed
scatterers decreases with increasing
incidence angle. The incidence angle
is commonly used to describe
the angular relationship between the
radar beam and the ground, surface
layer or a target. A change of the
radar illumination angle
often affects the radar backscattering
behaviour of a surface or
target. The incidence angle changes
across the radar image swath; it
increases from near range to
far
range. In the case of satellite
radar imagery,
the change of incidence
angle for flat terrain across the
imaging swath tends to be
rather small, usually on
the order of several degrees. In the
case of an inclined surface (slope),
the local incidence angle (L) is
defined as the angle
between the incident radar beam and
a line that is normal to that
surface. The local incidence angle
determining, in part, the
brightness, or
image tone, for each
picture element (pixel) and slope
facet, is a key element in the
prominent rendition of terrain
features in radar imagery.
Incidence Angle
Microwave
interactions with the surface are
complex and different
scattering mechanisms may occur in
different angular regions. Returns
due to surface scattering are
normally strong at low incidence
angles and decrease with increasing
incidence angle, with a slower rate
of decrease for rougher surfaces.
Returns due to volume scattering
from an heterogeneous medium with
low dielectric constant tend
to be more uniform for all incidence
angles. Thus, radar backscatter has
an angular dependence, and there is
potential for choosing
optimum configurations for different applications.
( See also "Imaging
Geometry" and the section
"Incidence
Angles" in the User
Guide 1.1.5.2. )

Lambert Conformal Conic
(LCC) map projection 
The Lambert
Conformal Conic (LCC) map projection
is a State Plane
Coordinate System that
consists of 120 zones designed to
optimally represent sections of the
individual states. Points on the
earth are projected onto a cone that
intersects the earth's surface
at two parallels of latitude. Along
these two circles the scale will be
exact. If the parallels are close
in a northsouth direction, the map
scale will be reasonably accurate no
matter how far the map is extended
in an eastwest direction.
The Lambert projection is useful for
mapping states that are relatively
wide in an eastwest direction. This
projection is said to be
conformal because the scales in all
directions are equivalent.
Lambert Conformal
Conic (LCC) map projection

Latitude 
Latitude is the
number of degrees north or south of
the equator, an imaginary circle on
the Earth's surface everywhere
equidistant between the two poles.
Geocentric and
Geodetic Latitudes

Layover 
Layover is an
extreme form of elevation
displacement or foreshortening in
which the top of a
reflecting object, such as mountain,
is closer to the radar (in slant range)
than are the lower parts of the
object. The image of such a
feature appears to have fallen over
towards the radar. Also defined as
the displacement of the top of an
elevated feature with
respect to its base on the radar
image. The peaks look like
dipslopes. The effect is more
pronounced for radars having smaller
incidence angle. ( See also Shadow in Products glossary.
Layover

Location 
Coordinates (geodetic
latitude, longitude) of a point
on the geoid, expressed in the
Earthfixed coordinate system. 
Longitude 
The angular distance on
the Earth, or on a globe or map, east or
west of the prime meridian at
Greenwich, England to the point on the
Earth's surface for which the
longitude is being ascertained,
expressed in degrees, or in hours,
minutes and seconds. 
Map Projections 
ASAR processing
supports 6 different map
projections: Mercator
(MERC), Transverse
Mercator (TM), Universal
Transverse Mercator ( UTM),
Polar
Stereographic Mercator (PS),
Universal Polar
Stereographic (UPS) , and
the Lambert Conformal
Conic (LCC).
Cylindical
projections are based upon the
various methods of
projecting the Earth upon a cylinder
that is either tangent to the
equator (normal or equatorial form),
a meridian (transverse) or
obliquely aligned. Any of these
classes are available in both
conformal and equal area form. These
projections are best used in
mapping applications involving a
zone near the line of tangency.[e.g.
Mercator, Transverse Mercator (TM),
Universal Transverse
Mercator (UTM)]. Applications should
be limited to equatorial regions,
but it is frequently used for
navigational charts with
latitude of true scale specified
within or near the chart's boundaries.
Conic projections
involve the transformations to a
cone either secant or tangent to
the Earth's surface.[e.g.
Lambert Conformal Conic (LCC)]
Stereograhic
projections are those in which
perspective is a point at the
opposite end of the globe.
In other words, the light is a point
source shown from a point on the
globe through to the other end of
the globe (e.g., a South
Pole point of projection would shine
light through to the North Pole). An
example of a stereographic
projection is the North
Polar Stereographic Projection shown below:
North Polar
Stereographic Projection
( image courtesy of
Peter H. Dana California State
University )
[e.g. Universal Polar Stereographic
(UPS) Projection, Polar
Stereographic Mercator (PS)]

Mercator (MERC) Map Projection 
The Mercator (MERC)
map projection, sometimes referred
to as the Plain Mercator,
is made from the centre of the Earth
onto a cylinder surrounding and
touching it at the Equator.
Therefore, the meridians
are equally spaced, parallel and
vertical lines, and the parallels of
latitude are
parallel, horizontal staight lines,
spaced farther and farther apart as
their distance from the Equator increases.
Mercator map projection

Nadir 
Nadir is a single point,
or locus of points on the surface of the
Earth directly below a sensor as
it progresses along its line of flight.
Nadir can be both a point and a line. In
other words, when a straight
line is drawn between the sensor and the
centre of the Earth, the nadir is the
point where that line intersects the
surface of the Earth. When the
contiguous nadir points are joined along
the ground, they form the nadir line.
For radar, the nadir line
corresponds to the beginning of the range. ( See also "Imaging
Geometry" ) 
National Systems Projection (NSP) 
A
National Systems map Projection (NSP) is
any of 4 map projections used in
ASAR processing, other that the UTM or UPS projections. These
are the LCC, TM, MERC and PS projections. 
Near Range 
Refers to the portion of
a radar image closest to the satellite
flight path. ( See also "Imaging
Geometry" ) 
OneWay 
Numbers relating to only
one direction of propagation are denoted
as oneway, and the
corresponding numbers that include the
round trip are called twoway. The radar illuminates the
scene through the transmit pattern of
the antenna. It receives
the backscattered energy
through the receive pattern of
the antenna. Thus the received pulse must travel two
ways, out to each object at range , and back again
the same distance. The
difference between oneway and twoway
is important in measuring effective
antenna pattern widths, in signal phase, and in the
relationship between twoway delay time
and range distance. 
Orbit 
The path of a satellite
as it revolves around the Earth is its
orbit. (For the ASAR Orbit State
Vectors see the section entitled
"Auxiliary Data Sets
for Level 1B Processing" in
chapter 2 ). 
Pixel Geometry 
A set of angles (sun
zenith angle, viewing zenith angle
and azimuth difference angle) specifying
how the pixel is seen from the
instrument and from the sun. 
Polarisation Signature 
A threedimensional plot
of the received backscattered power as
a function of the ellipticity and
orientation angles of a
polarimetric antenna. 
Polar
Stereographic Mercator (PS) Map projection 
(
See Map
Projections and Mercator Map
Projection ) 
Radar Polarisation 
Radar
polarisation is the orientation of
the electric (E) vector in an
electromagnetic wave, frequently
horizontal or vertical , in
conventional imaging radar systems.
Polarisation refers to the
orientation of the plane of the
electric field (E), as opposed to
the magnetic field (M)
Radar polarisation
Remote sensing
radars are usually designed to
transmit either vertically
polarised or horizontally polarised
radiation. This means that the
electric field of the wave is in a
vertical plane or a
horizontal plane. Likewise, the
radar can receive either vertically
or horizontally polarised radiation,
and sometimes both. The
planes of transmitted and received
polarisation are designated by the
letters H for Horizontal and V for
Vertical. Thus the
polarisation of a radar image can be
HH, for horizontal
transmit, horizontal
receive, VV for vertical
transmit, vertical receive, HV for horizontal
transmit vertical receive, and vice
versa (VH).
When the polarisation of received
radiation is the same as the
transmitted radiation, the image is
said to be likepolarised When
the polarisation of received
radiation is the opposite of the
transmitted radiation, the image is
said to be crosspolarised cross
polarisation requires
multiplescattering by the target
and therefore results in weaker backscatter than
likepolarisation Satellite radars
generally use likepolarisation
because the crosspolarised
signals are too weak to produce a
good image.
Polarisation is established by the antenna, which may
be adjusted to be
different on transmit and on
receive. Reflectivity of microwaves from an
object depends on the relationship
between the polarisation state and
the geometric structure of the
object. Possible states of
polarisation, in addition to
vertical and horizontal, include all
angular orientations of the E
vector, and time varying
orientations leading to
elliptical and circular polarisations.
( See also the section entitled "Dual
Polarisation" 1.1.5.1. in
chapter 1 ).

Range 
Range is the line of
sight distance between the radar and each
illuminated scatterer (target). In SAR usage, the term is
applied to the dimension of an image
perpendicular to the line of
flight of the radar. Slant range is the
distance from the radar, toward
each target and measured perpendicular
to the line of flight. Ground
range is the same distance,
projected using a geometrical
transformation onto a reference surface
such as a map. Radar data are
collected in the slant range domain, but
usually are projected onto the ground
range plane when these data are
processed into an image. The
resolution of the image in the range
direction is dependent on the length of
the emitted pulse; shorter pulses
result in finer resolution. ( see also
"ASAR Level 1B
Algorithm Physical
Justification"  Radar
Geometry 2.6.1.1.2. ) 
Range Curvature 
Describes the changing
distance between the radar and an
object during the time that the object
is illuminated by the antenna. Range curvature is
more important for long range systems
such as satellite SARs, and must be
compensated in the processor as a part
of image focusing. 
Range Gate Bias 

Range Migration 
The changing range delay
to a point target as the target
passes through the antenna beam. See the
discussion "Range Cell
Migration Correction
(RCMC)" 2.6.1.2.3.1.4. in the section entitled
"Range Doppler" in chapter 2. 
SideLooking 
Where the radar antenna
beam is pointed sideways, typically
nearly perpendicular to the flight
direction of the spacecraft. (See also
SLAR in the
"Radar and SAR glossary" and
"ASAR Level 1B
Algorithm Physical
Justification"  Radar
Geometry 2.6.1.1.2. ) 
Slant Range 
Represents the distance
measured along a line between the
radar antenna and the
target. Image direction as measured
along the sequence of
lineofsight rays from the radar to each and
every reflecting point in the
illuminated scene. Since a SAR looks down and to
the side, the slant range to
ground range transformation has an
inherent geometric scale which changes
across the image swath. ( See also "Imaging
Geometry" and "ASAR Level 1B
Algorithm Physical
Justification"  Radar
Geometry 2.6.1.1.2. , as well as ) 
State Plane Coordinate
System (SPCS) 
The State Plane
Coordinate System (SPCS), was
established in 1935 in the
United States. Each of the States have
defined, by State legislative action,
one or more spcs zones in terms of
datum, geographical extent and
cartographic projection parameters
relating geographic coordinates to the
cartesian coordinates used in land surveying. 
Swath Angle 
An angle subtending the
arc between swath centre and a point 
Transverse Mercator (TM) map projection 
The Transverse
Mercator (TM) map projection is a State Plane
Coordinate system that
consists of 120 zones designed to
optimally represent sections of the
individual states. It uses a
cylindrical surface that intersects
the earth along two lines parallel
to a meridian of longitude, called
the central meridian. The scale will
be exact along the two
northsouth lines of intersection.
This projection is reasonably
accurate within a narrow eastwest
zone, and may be extended
indefinably in a northsouth
direction. It is therefore useful
for states that are narrow in the
eastwest direction.
Tranverse Mercator
(TM) map projection

Trihedral Reflector 
Corner reflector
formed from three mutually orthogonal surfaces. 
Universal Polar
Stereographic (UPS) Map Projection 
The Universal Polar
Stereographic (UPS) map projection
is a special case polar aspect of
the azimuthal stereographic
projection, which is based upon
projections to a plane tangent to
the Earth's surface.
Universal Polar
Stereographic (UPS) Map Projection

Universal Transverse
Mercator (UTM) 
The Universal
Transverse Mercator (UTM) projection
is a special ellipsoidal form of the
general Transverse
Mercator (TM) projection .
It is a planar map projection, that
provides a specific geographic
coordinate system to which data can
be referenced. It is based on a
series of 60 zones worldwide, each
covering 6 degrees of longitude in a
northsouth strip.
Universal Transverse
Mercator (UTM) Zone

Width  Radar 
3dB. Width of a
distribution equal to the distance
between the outer two points on
the distribution having power level half
of that at the peak. 
Width, Equivalent Rectangle 
A standard definition to
measure the effective width of a
distribution. The width is that of a
rectangular distribution with the same
amplitude as the maximum of the
distribution, and having the
same area in the rectangle as is in the
measured distribution. 
Zero Doppler Direction 
The angle from the
satellite to the target,
relative to the broadside 
Zero Doppler Range 
The closest approach
range from the satellite to the target. 