Imaging radar Imaging radars provide map-like coverage to one or both sides of the aircraft.

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1 CEE 6100 / CSS 6600 Remote Sensing Fundamentals 1 Imaging radar Imaging radars provide map-like coverage to one or both sides of the aircraft. Acronyms: RAR real aperture radar ("brute force", "incoherent") SLR slide-looking radar SLAR side-looking airborne radar SAR synthetic aperture radar ("coherent") SLAR geometry θ = look angle γ = depression angle H = altitude R s = slant range R g = ground range ρ g = ground range resolution ρ s = slant range resolution β = beamwidth RADAR: Resolution of real aperture radar (RAR) Resolution: For real aperture systems, "Angular, "Azimuthal", or "along track" resolution is defined by the beamwidth and range distance. points are resolvable in azimuth in near range. points are not resolvable in azimuth in far range The azimuthal resolution degrades with range. For small angles (β) rr ββ = width of illuminated area rr ββ ββrr ss Angular resolution of a real aperture radar is a function of the distance from the aircraft, the slant range (RR ss ), and the beamwidth (β).

2 CEE 6100 / CSS 6600 Remote Sensing Fundamentals 2 Resolution of real aperture systems: azimuthal resolution β = Beamwidth ~ λ/d, where D = "diameter of aperture" i.e., the length of the antenna of a side-looking radar system (β is measured in radians). If ρ β = angular resolution: ρρ ββ ββrr ss ρρ ββ λββrr ss DD e.g., for:λ = 1 cm, D = 3 m and R = 15 km ρρ ββ = λ 1 cccc RR = 15 kkkk = 50 mm DD 3 mm In general, for real aperture (brute force) systems, higher resolution is achieved with longer antennas (increase D) or shorter λ (i.e., decrease β). Distance to object and range resolution define parameters: 2 RRss = HH RR gg cos γγ = RR gg RR ss distance is determined by measuring time: (dddddddd = vv tt) if the radar transmits a very short pulse which is reflected (re-radiated) by an object at distance RS from radar, the total distance traveled is 2RS; the time required for "round trip" is found from the relation: RR ss = cccc 2 ; tt = 2RR ss /cc the reflections from objects at different distances from radar will arrive at receiver (antenna) with different time delays. RAR achieves range (across-track) resolution through pulsing and time-delay sorting. tt 1 = 2RR ss cc ; tt 2 = 2RR ss + RR ss cc tt = tt 2 tt 1 = 2(RR ss + RR ss ) cc 2RR ss cc = 2 RR ss cc for non-overlapping returns: Δτ ~ τ duration of transmitter pulse, i.e., 2 RR ss ττ RR cc ss cccc 2 where Rs = minimum resolvable separation. Rs is a useful estimate of slant range resolution. Since ρs cτ/2. The corresponding ground-range resolution is: cτ ρ g = ρ s sec(φ) = 2cos(φ) > ττ = pulse width

3 CEE 6100 / CSS 6600 Remote Sensing Fundamentals 3 Why radar (and sonar) are side-looking Note: The resolution improves as the depression angle decreases and worsens as the depression angle increases. AAAA φφ 0, ρρ gg (φφ) cccc 2 AAAA φφ ππ 2, ρρ gg(φφ) RADAR:Slant range vs. Ground range Ground range image Slant range image Ground range image H Angles to the target a. look angle the angle between the nadir direction and the radar beam. b. depression angle the angle between the horizontal (at the platform) and the radar look direction. c. incidence angle the angle between the local vertical and the incident radar beam (at the ground). d. local incidence angle (look angle) the angle between the local normal and the incident radar beam. e. azimuth angle the angle between the ground track and the look direction

4 CEE 6100 / CSS 6600 Remote Sensing Fundamentals 4 Incidence angle The incidence angle is the angle between the radar beam and the local vertical. A change in incidence angle causes variations in pixel brightness. Due to their greater altitude, satellite incidence angles vary less than airborne incidence angles. This leads to more uniform illumination on satellite images than airborne radar images. The local incidence angle is the angle between the radar beam and the local normal to the target object. A change in the local incidence angle also causes variations in brightness of the return signal. RADAR cross-section: Orientation of the surface The orientation of the target surface relative to the look angle also affects the signal strength. For a given material, the power returned is greatest when the surface is perpendicular to the illuminating beam.

5 CEE 6100 / CSS 6600 Remote Sensing Fundamentals 5 Signal Strength: Corner Reflectors Natural or man-made objects may perform as corner reflectors, e.g., the side of a building combined with the reflection from the ground. Two surfaces at right angles form a dihedral corner reflector. The return from a dihedral reflector is strong only when the reflecting surfaces are nearly perpendicular to the illumination direction. From CCRS Three mutually perpendicular surfaces form a trihedral corner reflector. Return from a trihedral reflector is strong for all viewing/illumination angles. Radar Corner Reflectors for position calibration Surface roughness and incidence angle effects Volume scattering, however, is the result of multiple scattering events occurring inside a medium. Volume scattering causes partial depolarization of the scattered signal, reducing the strength of the like-polarized signal.

6 CEE 6100 / CSS 6600 Remote Sensing Fundamentals 6 C-Band HH, HV and VV: The grass and forest areas are bright in the HV image because volume scattering caused significant depolarization of the returning signal. Surface scattering by the rougher lava flows caused much less depolarization, so these flow areas are darker in the HV image, though not as dark as the smooth lava flows. Note that the rough lavas and vegetation have nearly the same brightness signatures in the like-polarized images. C-, L-, and P-Band VV: Ancient lava surfaces are vegetated, with forest at higher elevations and grass and shrubs at lower elevations. Two types of recent, unvegetated lava flows are present: fluid, smoothsurfaced flows and more viscous flows with a rough blocky surface. In the C-Band image, only the smooth recent lavas appear dark; the other surfaces are rough at this wavelength. At the longer wavelengths the grassland area becomes smooth and thus dark as well. The rough lavas and forest have similar brightness at all three of these radar wavelengths.

7 CEE 6100 / CSS 6600 Remote Sensing Fundamentals 7 SIR-C Image (20 km wide) L-Band (15-30 cm Optical image. The radar image area is outlined. Stern, R.J., Dixon, T.H., Nielson, K.C. and Sultan, M. (1996) SIR-C "Studies of the Precambrian Hamisana and Nakasib Structures, NE Sudan, in Arid Regions of Low Relief and in the Subsurface". JPL SIR-C/X project Final Report.

8 CEE 6100 / CSS 6600 Remote Sensing Fundamentals 8 Bragg scattering Since the incidence angle is oblique to the local mean angle of the ocean surface, there is almost no direct specular reflection except at very high sea states. Bragg resonance is the primary mechanism for backscattering radar pulses. The Bragg equation defines the ocean wavelengths for Bragg scattering as a function of radar wavelength and incidence angle: n λ L = 2sin θ Where λ is the radar wavelength, L is the sea surface wavelength and θ is the incidence angle. Short Bragg-scale waves are formed in response to wind stress. If the sea surface is rippled by a light breeze with no long waves present, the radar backscatter is due to the component of the wave spectrum which resonates with the radar wavelength. Bragg Wavelength as a function of radar wavelength & Incidence angle Incidence angle, θ Radar wavelength, λ θ = cm (X-band) L = 4.7 cm 3.5 cm 1.9 cm 5.7 cm (C-band) L = 8.3 cm 4.4 cm 3.3 cm 23 cm (L-band) L = 33.6 cm 17.9 cm 13.3 cm Maidanado, Uruguay Malvinas Current Brazil Current Clouds Incidence angle: RADARSAT (C-Band) 8:44 AM, 5 Sep 1998 AVHRR Channel 4 (Thermal) 9:56 AM, 5 Sep 1998 (dark = cold)

9 CEE 6100 / CSS 6600 Remote Sensing Fundamentals 9 RADAR: Satellite vs. Airborne systems Airborne Radar Great flexibility in deployment (wavelength, resolution, polarization, incidence angle, aspect angle, swath width.) Variable (selectable) illumination geometry. Shadowing, foreshortening and brightness will vary within a scene. Subject to turbulence and must be equipped with a motion compensation system. Satellite Radar More uniform illumination geometry than airborne systems Rapid coverage of large areas Frequency of coverage is limited by orbital constraints Flexibility (orientation, viewing angle) is limited Radar characteristics The four characteristics resulting from the geometric relationship between the sensor and the terrain that are unique to radar imagery are: foreshortening, layover pseudo-shadowing, shadowing. } relief displacement Foreshortening (A'B') is the effect by which the foreslopes of hills and mountains appear to be compressed. The image of foreslopes will appear brighter than other features on the same image. The greatest amount of foreshortening occurs where the slope is perpendicular to the incoming radar beam. The base, slope and top of the mountain will be imaged at the same time and will be superimposed on the image. Foreshortening can be minimized by using a less sharp incidence angle. However, lower incidence angles allow for more shadowing to occur on the image. While the hill slopes AB and BC are equal, the foreslope (AB) is compressed (A'B') much more than the backslope (BC) is compressed (B'C'), due to the radar imaging geometry.

10 CEE 6100 / CSS 6600 Remote Sensing Fundamentals 10 Layover is the effect where the image of an object appears to lean toward the direction of the radar antenna. It is the result of the tops of objects or slopes being imaged before their bases. Layover effects are most severe on the near range side of images. While the mountain slopes AB and BC are equal, the radar imaging geometry dictates that the radar-facing slope (AB) will be imaged (B'A') as leaning toward the radar. This is due to the mountain top (B) having been imaged before the base(a) due to RA > RB. Relief displacement is in opposite direction from airphotos. Brooks Range, Sadlerochit Mountains ERS-1 SAR Image On aerial photographs and other optical sensor images, relief displacement falls away from the nadir point because the top is imaged further from nadir than the base of a structure. In radar images the top of a structure may be imaged before the base. Thus, the relief displacement falls towards the nadir. Relief 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. Relief displacement is also most pronounced at near range.

11 CEE 6100 / CSS 6600 Remote Sensing Fundamentals 11 Pseudo-shadowing is an effect by which the backslope of hills and mountains appear to be expanded and darker relative to the foreslope. It is the result of return signals spread out over a larger distance (A', B') than the actual horizontal distance (A, B). This dispersed return is not always detectable (Leonardo, 1983). A', B' = return signal spread A, B = actual ground distance A' < A B' > B Radar Shadow When terrain slopes are greater than the depression angle, true radar shadows mask down range features. Slopes facing away from the radar antenna will return very weak signals if any. This results in dark or black areas on the image. In areas of high relief, as the depression angle becomes shallower, shadow length increases with range. The shallower the depression angle is on such terrain, the more information will be lost. Applications: Flooding of the Po River October, 2000 Approximately 12,000 people have been affected by rising flood waters in northwestern Italy during early October, This RADARSAT Fine 1 Far image was acquired on October, 18, 2000 at 6:00am local time. The imagery was received, processed, interpreted by CCRS scientists and provided to authorities in Europe within 24 hours of the acquisition time. The dark blue areas overlaid on the image indicate the normal water level of the Po River and light blue areas indicate the flood extent.