Description of the geometric orientation in the horizontal plane of an object in the scene with respect to the illuminating wavefront.

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 along-track direction, since it is the relative along-track 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 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 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 one-way or two-way form, and in either voltage or power.

Beamwidth

(See also Side-Lobes in Radar and SAR glossary )

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

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, pseudo-shadowing, 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 shape-from-shading 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

Foreshortening is the spatial distortion whereby terrain slopes facing a side-looking 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

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 across-track dimension perpendicular to the flight direction, while azimuth (E) refers to the along-track dimension parallel to the flight direction. This side-looking 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.

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. )

The Lambert Conformal Conic (LCC) map projection is a State Plane Co-ordinate 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 north-south direction, the map scale will be reasonably accurate no matter how far the map is extended in an east-west direction. The Lambert projection is useful for mapping states that are relatively wide in an east-west direction. This projection is said to be conformal because the scales in all directions are equivalent.

Lambert Conformal Conic (LCC) map projection

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 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 dip-slopes. The effect is more pronounced for radars having smaller incidence angle. ( See also Shadow in Products glossary.

Layover

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)]

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

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 like-polarised When the polarisation of received radiation is the opposite of the transmitted radiation, the image is said to be cross-polarised cross polarisation requires multiple-scattering by the target and therefore results in weaker backscatter than like-polarisation Satellite radars generally use like-polarisation because the cross-polarised 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 ).

The Transverse Mercator (TM) map projection is a State Plane Co-ordinate 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 north-south lines of intersection. This projection is reasonably accurate within a narrow east-west zone, and may be extended indefinably in a north-south direction. It is therefore useful for states that are narrow in the east-west direction.

Tranverse Mercator (TM) 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

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 north-south strip.

Universal Transverse Mercator (UTM) Zone