MICROW~3_For Test Questions.pptx

Canada Centre for Remote Sensing, Natural Resources Canada Natural Resources Ressources naturelles Canada Canada CONSEQUENCES OF VIEWING GEOMETRY Course Outline  Consequences of viewing geometry  Radar slant range and ground range  Look direction  Local incidence angle  Topographic/Relief displ...

Canada Centre for Remote Sensing, Natural Resources Canada Natural Resources Ressources naturelles Canada Canada CONSEQUENCES OF VIEWING GEOMETRY Course Outline  Consequences of viewing geometry  Radar slant range and ground range  Look direction  Local incidence angle  Topographic/Relief displacement  Resolution cell  Radar shadow  Foreshortening  Layover Canada Centre for Remote Sensing, Natural Resources Canada Radar Slant Range / Ground Range Radar can be presented in either slant or ground range, as shown in Slides 4 and 5 Slant range is the natural radar range observation coordinate, defined as the line-of-sight from the radar to each reflecting object. Ground range is slant range projected onto the geoid of the Earth. Slant range data can be converted to ground range by resampling. Canada Centre for Remote Sensing, Natural Resources Canada Radar Slant Range / Ground Range Sl an tR an ge (r ) R Ground Range (rGR) rGR Canada Centre for Remote Sensing, Natural Resources Canada Slant Range vs Ground Range Radar Imagery Canada Centre for Remote Sensing, Natural Resources Canada Look Direction Look direction is defined as 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. Unless perfectly symmetrical or perfectly random, targets have a preferred orientation. For example, opposing look directions for agricultural fields may produce different tones on the image due to row direction related to planting, tilling, or harvesting. In areas with high relief, opposing look directions are often necessary to fill in areas of radar shadow. On fixed-looking systems, such as RADARSAT, two look directions can be acquired using ascending (east-looking) and descending (west-looking) passes (see Slide 7). Canada Centre for Remote Sensing, Natural Resources Canada Look Direction Sarawak (Malaysia) Canada Centre for Remote Sensing, Natural Resources Canada Local Incident Angle Slide 9 shows that the local incident angle ( loc) is defined as the angle between the radar lineof- sight to the line normal (or orthogonal) to the local slope.  i is the flat-earth or ellipsoid incident angle. Local incident angle can have a large effect on image brightness per pixel. Local incident angle is the largest source of error in radiometric calibration. Canada Centre for Remote Sensing, Natural Resources Canada Local Incident Angle Source: Raney, 1998 Source: Raney, 1998 Canada Centre for Remote Sensing, Natural Resources Canada Local Incident Angle Effects LOCAL INCIDENT ANGLE EFFECTS Canada Centre for Remote Sensing, Natural Resources Canada Topographic/Relief Displacement Due to the different imaging geometries of radar and optical systems, as seen in Slide 14 to 15, topographic displacement differs between the systems. Horizontal displacement for a radar sensor is highest near nadir, and decreases with incident angle and can be severe at small incident angles (see Slide 14). In contrast, topographic displacement for optical systems (Slide 15) increases with incident angle. Since imaging radars usually view the scene from an oblique perspective (i.e. Side-looking), they are subject to one-dimensional relief displacement analogous to that inherent in aerial photography. Tall objects are displaced radially from nadir in air photos, whereas terrain distortion in radar imagery is perpendicular to the flight path (or satellite track) which results in tall objects being displaced toward the sensor. Canada Centre for Remote Sensing, Natural Resources Canada Topographic Displacement - Radar Sensor apparent viewing direction  mountain top reference surface orthographic projection of mountain top radar ground range projection of mountain top Horizontal displacement of a 100m mountain top (m) airborne  satellite Source: Toutin, Th. and Y. Carbonneau, 1992, “MOS and SEASAT Image Geometric Correction”, IEEE-TGARS, Vol. 30, No. 3, pp. 603-609. Canada Centre for Remote Sensing, Natural Resources Canada Topographic Displacement - Optical Sensor Optical Sensor by similar triangles   nadir reference surface Optical Sensor Horizontal displacement of a 100m mountain top (m)  Canada Centre for Remote Sensing, Natural Resources Canada Effect of Topography / Local Incident Angle on Image Brightness Local topographic slope (Slide 17) can have a significant effect on image brightness. Local topographic slope causes changes in local incident angles. A small local incident angle results in brighter radar returns. A larger local incident angle results in darker radar returns. Slope-induced radiometric effects are useful for some applications such as geomorphology and geology. Canada Centre for Remote Sensing, Natural Resources Canada Image Brightness as an Effect of Topography  loc loc Brighter smaller local incident angle Nominal Brightness Canada Centre for Remote Sensing, Natural Resources Canada w do Darker larger local incident angle ha rS da Ra  loc Resolution Cell Resolution is the minimum distance that describes how well the radar can discriminate closely spaced reflectors. Resolution cell is 3-dimensional in the illuminated space. The area of the rectangle in Slide 19 is called the resolution cell. rA is the azimuth resolution and rR is the range resolution. Source: Raney, 1998 Canada Centre for Remote Sensing, Natural Resources Canada Resolution Cell Source: Raney, 1998 rR = range resolution Canada Centre for Remote Sensing, Natural Resources Canada rA = azimuth resolution Slant / Ground Range Resolution Slide 21 shows the scale differences in slant range (rR) and ground range (rGR) images. Differences between slant and ground range resolution are highest at small incident angles. For example, Slide 21 shows that rR at 10° is 10 m, while the rGR at 10° is 29 m. At 70°, rR and rGR converge. Canada Centre for Remote Sensing, Natural Resources Canada Slant and Ground Range Resolution - Canada Centre for Remote Sensing, Natural Resources Canada Radar Shadow Radar shadows in imagery indicate those areas on the ground surface not illuminated by the radar because geometry and scene relief (slide 23). Since no return signal is received, radar shadows appear very dark in tone on the imagery (slide 24). In imagery, radar shadows occur in the down-range direction behind tall objects. They are a good indicator of radar illumination direction if annotation is missing or incomplete. Since incident angle increases from near to far-range, terrain illumination becomes more oblique. As a result, shadowing becomes more prominent toward far-range. Radar shadow is most common in steep terrain imaged at large incident angles. Information about the scene, such as an object’s height, can also be obtained from radar shadows. Shadowing in radar imagery is an important key for terrain relief interpretation. Radar Shadow ion wa vef ron t illum inat scene distortion Source: Raney, 1998 Canada Centre for Remote Sensing, Natural Resources Canada shadow Radar Shadow in Airborne SAR Image of Folded Sandstone Beds Canada Centre for Remote Sensing, Natural Resources Canada Foreshortening Foreshortening in a radar image is the appearance of compression of those features in the scene which are tilted towards the radar (slides 26 and 27). It leads to relatively brighter appearance of these slopes, and must be accounted for by the interpreter. Foreshortening is at a maximum when a steep slope is orthogonal to the radar beam. In this case, the local incident angle is zero, and as a result, the base, slope and top of a hill are imaged simultaneously and, therefore, occupy the same position in the image. The horizontal displacement resulting from the small incident angles causes foreshortening of the slope facing the radar. For a given slope or hillside, foreshortening effects are reduced with increasing incident angles. At the grazing angle, where incident angles approach 90°, foreshortening effects are eliminated, but severe shadowing may occur. Foreshortening In selecting incident angle, there is always a trade-off between the occurrence of foreshortening and the occurrence of shadowing in the image. m illu n io at in t on r f ve wa scene displa foreshortening cement Canada Centre for Remote Sensing, Natural Resources Canada Source: Raney, 1998 Foreshortening Source : DeSève, Toutin & Desjardins, IJRS, 17(1):131-142, 1996. Canada Centre for Remote Sensing, Natural Resources Canada Layover Layover occurs when the reflected energy from the upper portion of a feature is received before the return from the lower portion of the feature. In this case, the top of the feature will be displaced, or “laid over” relative to its base when it is processed into an image. In general, layover is more prevalent for viewing geometries with small incident angles, such as from satellites. Layover is an extreme case of foreshortening, and occurs when the incident angle is smaller than the local topographic slope (Slide 29). Extreme horizontal displacement causes the top of the mountain to be mapped “overlaying” the fore slope (Slide 30). In the layover case there is no radar shadow, but severe elevation displacement and layover of the fore slope. Difficult for interpretation since each pixel may contain scatter from more than one area. Canada Centre for Remote Sensing, Natural Resources Canada Layover ilum n tio ina  i ron f e v wa t scene distortion layover Source: Raney, 1998 Canada Centre for Remote Sensing, Natural Resources Canada Layover Effects on SAR Imagery (Lima, Peru) Relief Displacement (Radar Sensor) The type and degree of relief displacement in the radar image is a function of the angle at which the radar beam hits the ground, i.e. it depends upon the local slope of the ground. In summary, when angle of radar-facing slope is less than θi, foreshortening occurs; if it is greater than θi , layover occurs Layover Foreshortening 0° Local incident angle Shadow 90º If the 2 angles are equal, top and base will occupy same slant-range position, foreshortening reaches a maximum. Canada Centre for Remote Sensing, Natural Resources Canada