MICROW~2.PPT

Canada Centre for Remote Sensing, Natural Resources Canada Natural Resources Ressources naturelles Canada Canada CONSEQUENCES OF SYSTEM PROPERTIES Course Outline Consequences of Microwave System Properties  Frequency (Wavelength)  Polarisation  Image configuration  Beam mode  Beam position  Orbit (look direction, acquisition time and surface) Canada Centre for Remote Sensing, Natural Resources Canada Frequency (Wavelength) Most remote sensing radars operate at wavelengths between.5 cm to 75 cm. The microwave frequencies have been arbitrarily assigned to bands identified by letter. The most popular of these bands for use by imaging radars include: X-band: from 2.4 to 3.75 cm (12.5 to 8 GHz). Widely used for military reconnaissance and commercially for terrain surveys. Used on CV-580 SAR (Environment Canada). C-band: from 3.75 to 7.5 cm (8 to 4 GHz). Used in many spaceborne SARs, such as ERS-1 and RADARSAT. S-band: from 7.5 to 15 cm (4 to 2 GHz). Used in Almaz. L-band: from 15 to 30 cm (2 to 1 GHz). Used on SEASAT and JERS1. P-band: from 30 to 100 cm (1 to 0.3 GHz). Used on NASA/JPL AIRSAR. The capability to penetrate through precipitation or into a surface layer is increased with longer wavelengths. Radars operating at wavelengths greater than 2 cm are not significantly affected by cloud cover, however, rain does become a factor at wavelengths shorter than 4 cm. Canada Centre for Remote Sensing, Natural Resources Canada Choice of Radar Frequency: Application factors Radar wavelength should be matched to the size of the surface features that we wish to discriminate – Ice discrimination, small features, use X-band – Geology mapping, large features, use L-band – Foliage penetration, better at low frequencies, use P-band In general, C-band is a good compromise Canada Centre for Remote Sensing, Natural Resources Canada Frequency Comparison: C-, L-, and P-Bands FREQUENCY COMPARISON Flevoland, Netherlands Agricultural Scene C-Band L-Band Canada Centre for Remote Sensing, Natural Resources Canada P-Band Multipolarization colour composites courtesy of JPL Choice of Radar Frequency: System factors  System factors: – Low frequencies: · More difficult processing · Need larger antennas and feeds · Simpler electronics – High frequencies: · Need more power · More difficult electronics · Good component availability at X-band  Note that many research SARs have multiple frequency bands – e.g. JPL AIRSAR, SIR-C, Convair-580 Canada Centre for Remote Sensing, Natural Resources Canada Polarization Polarization refers to the orientation of the electric vector of an electromagnetic wave. Radar system antennas can be configured to transmit and receive either horizontally or vertically polarized electromagnetic radiation. When polarization of the transmitted and received waves is in the same direction, it is referred to as like-polarized. HH refers to horizontally transmitted and received waves; VV refers to vertically transmitted and received waves. When polarization of the transmitted waves is orthogonal to the polarization of the received radiation, it is referred to as cross-polarized; e.g. HV refers to horizontal transmission and vertical reception; VH for vertical transmission and horizontal reception. When the radar wave interacts with a surface and is scattered from it, the polarization can be modified, depending upon the properties of the surface. This modification affects the way the scene appears in polarimetric radar imagery, and the type of surface can often be deduced from the image. EM Wave Polarization Electrical Field VERTICAL POLARIZATION HORIZONTAL POLARIZATION Canada Centre for Remote Sensing, Natural Resources Canada Choice of Polarization  Basic or operational SARs usually have only one polarization for economy, e.g. HH or VV  Research systems tend to have multiple polarizations, e.g. all of: HH, HV, VV, VH (quad pol)  Multiple polarizations help to distinguish the physical structure of the scattering surfaces: – the alignment with respect to the radar (HH vs. VV) – the randomness of scattering (e.g. vegetation - HV) – the corner structures (e.g. HH VV phase Canada Centreangle) for Remote Sensing, Natural Resources Canada – Bragg scattering (e.g. oceans - VV) Weddell Sea Ice, Antarctica Shuttle SIR-C/X Image C-band, HH L-band, HV Canada Centre for Remote Sensing, Natural Resources Canada L-band, HH Victoria & Saanich Peninsula, Canada Urban Suburban Forest Agriculture / Clear-cut Shuttle SIR-C/X Image C-band, HH L-band, HV Canada Centre for Remote Sensing, Natural Resources Canada L-band, HH Incident Angle Refers to the angle between the radar illumination and the normal to the ground surface. Depending on the height of the radar above the Earth’s surface, the incident angle will change from the near range to the far range which in turn affects the viewing geometry. Local Incident Angle The term local incident angle takes into account the local slope of the terrain at any location within the image. It is the local incident angle which in part determines the image brightness or tone for each pixel. Canada Centre for Remote Sensing, Natural Resources Canada Canada Centre for Remote Sensing, Natural Resources Canada Natural Resources Ressources naturelles Canada Canada CONSEQUENCES OF TARGET PROPERTIES Course Outline Consequences of Microwave Target Properties  Surface roughness  Moisture content (di-electric properties)  Imaging geometry  Incident angle  Structure  Orientation Canada Centre for Remote Sensing, Natural Resources Canada Types of Backscattering: Diffuse and Specular Surface roughness influences the backscattering of microwave energy and thus the brightness of features on the radar imagery. Horizontal smooth surfaces scatter nearly all incident energy away from the radar and are called specular (from the Latin word speculum, meaning mirror). Specular surfaces, such as calm water or paved highways, appear dark on radar imagery. Microwaves incident upon a rough surface are scattered in many directions. This is known as diffuse or distributed scattering. Vegetation surfaces will cause diffuse reflectance, and result in a brighter tone on the radar imagery. Diffuse and Specular Backscattering Corner Scattere Diffuse Scattering Specular Scatterer Canada Centre for Remote Sensing, Natural Resources Canada Types of Diffuse Scatters In general, scenes observed by a SAR consist of two kinds of surfaces; distributed scatterers and discrete scatterers. Discrete scatterers are characterized by a relatively simple geometric shape, such as a building. The classic element used to represent discrete scattering is a corner reflector, a shape as is formed when all sides intersect at (nearly) right angles (such as the intersection of a paved road and tall building). Distributed scatterers consist of multiple small areas or surfaces from which the incident microwaves scatter in many different directions. Distributed scattering is produced from a forest canopy or cultivated fields A radar measures that component of the scattered energy which returns along the same path of the incident beam. Canada Centre for Remote Sensing, Natural Resources Canada Canada Centre for Remote Sensing, Natural Resources Canada Surface Roughness The roughness of a surface is determined relative to radar frequency and incident angle. Generally, a surface is considered smooth if its height variations are considerably smaller than the radar wavelength. In terms of a single wavelength, a given surface appears rougher as incident angle increases. Rough surfaces will usually appear brighter on radar imagery than smoother surfaces composed of the same material. In general a rough surface is defined as having a height variation of about half the radar wavelength. Canada Centre for Remote Sensing, Natural Resources Canada Surface Roughness Surface Scattering Patterns Incident Wave Scattering Pattern Smooth Incident Wave Incident Wave Scattering Pattern Medium Rough Canada Centre for Remote Sensing, Natural Resources Canada Scattering Pattern Very Rough Corner Scatterers Small objects may appear extremely bright on radar imagery. This is dependent on the geometric configuration of the object. The side of a building or a bridge, combined with scattering of microwave radiation from the ground is an example of a corner scatterer. When two surfaces are at right angles and open to the radar, a dihedral corner scatterer is formed. The return from type of scatterer is strong only when the surfaces are very nearly perpendicular to the illumination direction. Strong backscattering are caused by a trihedral corner scatterer. These are formed by the intersection of three mutually perpendicular plane surfaces open to the radar. Researchers often place corner scatterers at various ground locations to act as reference points on the radar imagery. Canada Centre for Remote Sensing, Natural Resources Canada Corner Reflectors Dihedral Canada Centre for Remote Sensing, Natural Resources Canada Trihedral Volume Scattering Volume scattering is related to multiple scattering processes within a medium, such as the vegetation canopy of a corn field or a forest. This type of scattering can also occur in layers of very dry soil, sand, or ice. Volume scattering is important as it influences the backscatter observed by the radar. Radar will receive backscatter from both the surface and the volume. The intensity of volume scattering depends on the physical properties of the volume (variations in dielectric constant, in particular) and the characteristics of the radar (frequency, polarization and incident angle). Canada Centre for Remote Sensing, Natural Resources Canada Volume Backscattering Canopy Backscattering Soil - Trunk Reflection Soil Backscattering (Corner Reflector) Canopy Soil Reflection Canada Centre for Remote Sensing, Natural Resources Canada Moisture Content The presence of moisture increases a material’s complex dielectric constant. The dielectric constant influences the ability of a material to absorb, reflect and transmit microwave energy. The moisture content of a material can change its electrical properties. This affects how a material appears on the radar image. Identical materials can vary in appearance at different times or different locations according to the amount of moisture they contain. The backscatter and hence image brightness, of most natural vegetation and surfaces is increased with increasing moisture content. Microwaves may penetrate very dry materials, such as desert sand. The scattering which results, is affected by both surface and subsurface properties. In general, the longer the radar wavelength, the deeper into the material the energy will penetrate. Imaging Geometry of SAR The figure on slide 28 shows the geometric characteristics of a SAR. Incident angle ( °) is the angle between the radar line-of-site and the local vertical with respect to the geoid. Incident angle is the most important parameter describing the relative geometry between the radar and the observed scene. System altitude alters incident angle and thus viewing geometry. Azimuth direction is the flight direction, or along-track direction. Range direction is the across-track direction. Slant range is the distance measured along a line between the antenna and the target. Ground range is the distance from the ground track to an object. Near range is the part of the radar image closest to the flight path or nadir, whereas far range is the part of the radar image farthest from the flight path. Canada Centre for Remote Sensing, Natural Resources Canada Geometry of SAR n tio c ire td gh Fli Swath width Altitude Sla nt ran g e ar e N e ng a r Incident Angle a rr Ground Range zim h ut A Canada Centre for Remote Sensing, Natural Resources Canada Fa e ng Imaging Geometries Canada Centre for Remote Sensing, Natural Resources Canada Comparison of Imaging Geometries System altitude has a large effect on the imaging geometry of the SAR. Spaceborne systems operate between 600-800 km, whereas airborne systems between 3-12 km. Slide 30 shows airborne systems would cover a larger range of incident angles (15°-60°) than spaceborne systems (37°- 40°). Higher altitude of spaceborne systems means incident angles are usually steeper. Canada Centre for Remote Sensing, Natural Resources Canada Comparison of Imaging Geometries SPACEBORNE SAR AIRBORNE SAR airborne 10 – 100 km spaceborne 25 – >500 km IMAGE SWATH Canada Centre for Remote Sensing, Natural Resources Canada Comparison of Satellite SARs & Aircraft SARs  Advantages of satellite SARs – More coverage per second (Km2/s) – Lower operating costs ($/Km2) – Not constrained by flying conditions or airport proximity – Wider area views – Somewhat simpler signal processing (no motion compensation)  Disadvantages – More expensive to design, build and launch – More difficult to provide multiple polarizations & frequencies – Cannot be flown anywhere on demand – Lower resolution in general Canada Centre for Remote Sensing, Natural Resources Canada Choice of Swath Width  Limited by range ambiguities and data handling capacity  A trade-off between azimuth resolution, number of looks, processing capability  For satellites: 30 - 150 Km typical  For aircraft: 10 - 100 Km typical  RADARSAT gets large swath widths per beam by reducing the resolution, and using careful antenna weighting to control range ambiguities  RADARSAT and future Envisat use ScanSAR to get extra wide swaths  See slide 35 Canada Centre for Remote Sensing, Natural Resources Canada RADARSAT-1 SAR Imaging Modes Extended Low Satellite Ground Track ScanSAR Wide Standard Fine Canada Centre for Remote Sensing, Natural Resources Canada Extended High RADARSAT-1 SAR Imaging Modes MODE BEAM & IN POSITION APPROXIMATE NOMINAL APPROXIMATE NUMBER OF a CIDENCE ANGLES GROUND AREA PROCESSED (DEGREES) RESOLUTION (M) (KM) LOOKS Wide W1 20 - 31 (3 positions) W2 165 x 165 1x4 31 - 39 150 x 150 SGF or SGX ScanSAR Narrow W3 SCNA 39 - 45 20 - 40 50 130 x 130 300 x 300 2x2 see slide 6 ScanSAR Wide SCNB SCWA 31 - 46 20 - 49 100 500 x 500 SCN 2x4 see slide 6 Extended High SCWB EH1 20 - 46 49 - 52 25 450 x 450 75 x 75 SCW 1x4 EH2 EH3 50 - 53 52 - 55 EH4 54 - 57 EH5 EH6 EL1 56 - 58 57 - 59 10 - 23 (6 beams) Extended Low 30 SGF or SGX 30 170 x 170 1x4 SGF or SGX SGF = SAR Georeferenced Fine Resolution Product (Path Image) SGX = SAR Georeferenced Extra Fine Resolution Product (Path Image Plus) SCN = ScanSAR Narrow Beam Product (Path Image) SCW = ScanSAR Wide Beam Product (Path Image) Canada Centre for Remote Sensing, Natural Resources Canada Ground range resolution varies across the swath. a RADARSAT-1 Image Products Sizes and Scales MODE PROCESSING LEVEL CEOS IMAGE APPROXIMAGE DIGITAL DIGITAL FILE SIZE FILM IMAGE SIZE APPROX. FILM (MB) (CM) SCALE 512 128 64 313 128 64 450 N/A 20 x 20 20 x 20 N/A 20 x 20 20 x 20 N/A N/A 1:250,000 1:312,500 N/A 1:500,000 1:625,000 N/A a PRODUCT Fine Path Image Plus Path Image Map Image Path Image Plus Path Image Map Image Path Image Plus Standard Wide ScanSAR Narrow ScanSAR Wide Extended High Extended Low SGX SGF SSG/SPG SGX SGF SSG/SPG SGX IMAGE SIZE (PIXELS x LINES) 16,000 x 16,000 8,000 x 8,000 8,000 x 8,000 12,500 x 12,500 8,000 x 8,000 8,000 x 8,000 15,000 x 15,000 b Path Image SGF 12,000 x 12,000 288 15 x 15 1:250,000 Map Image SSG/SPG 12,000 x 12,000 144 20 x 20 1:625,000 Path Image Path Image Path Image Plus Path Image Map Image Path Image Plus Path Image Map Image SCN SCW SGX SGF SSG/SPG SGX SGF SSG/SPG 12,000 x 12,000 10,000 x 10,000 9,375 x 9,375 6,000 x 6,000 6,000 x 6,000 17,000 x 17,000 13,600 x 13,600 13,600 x 13,600 144 100 176 72 36 578 370 185 15 x 15 20 x 20 N/A 1:500,000 1:625,000 N/A N/A N/A c b SPG products (Precision Map Image) have the same sizes and scales as SSG products (Map Image). a b c Film size is 24 x 24 cm. The digital product is divided into quarters and imaged onto four 24 x 22 cm film transparencies. The 8,000 x 8,000 line image is noted to be NORTH UP which requires approximately 40% additional image area. Canada Centre for Remote Sensing, Natural Resources Canada RADARSAT-1 Coverage RADARSAT can provide complete global coverage with the flexibility to support specific requirements. The satellite's ground track is repeated every 24 days. RADARSAT can provide daily coverage of the Arctic, view any part of Canada within three days, and achieve complete coverage at equatorial latitudes every six days using a 500 kilometre wide swath. Canada Centre for Remote Sensing, Natural Resources Canada RADARSAT-2 System Concepts StandardMode ModeImage ImageQuality QualityParameters Parameters Standard The sum of the azimuth and The sum of the azimuth and rangeambiguity ambiguityratios ratios range Global Dynamic Range Global Dynamic Range dB Relative radiometric Accuracy within Relative radiometric 100 km bywithin 100 km scene Accuracy 100 dBby 100 km scene km One orbit One orbit

Document Details

Related

Full Transcript