GMAT 9600 Principles of Remote Sensing Week 4 Radar Background & Surface Interactions Acknowledgment Mike Chang Natural Resources Canada Process of Atmospheric Radiation Dr. Linlin Ge and Prof Bruce Forster Atmospheric Transmittance Microwaves used by Radar Band K X C L P Wavelength (cm) 1.1 ~ 1.7 2.4 ~ 3.8 3.8 ~ 7.5 15.0 ~ 30.0 30.0 ~ 100.0 Frequency (MHz) 26,500 ~ 18,500 12,500 ~ 8,000 8,000 ~ 4,000 2,000 ~ 1,000 1,000 ~ 300 Microwaves are not micro. They are in the microwave portion of the spectrum. (approximately from 1 mm to 1 m)
RADAR RAdio Detection And Ranging. A radar system is a ranging device that measures distances as a function of round trip travel times of a directed beam of pulses spread out over specific distances. Predicted in the early part of the 20th century, the first important system was built in England in 1938. Why Radar It is an active system. Capability of penetrating the atmosphere under virtually all conditions. Microwave reflections or emissions from earth materials bear no direct relationship to their counterparts in the visible or thermal portions of the spectrum. Radar Background Airborne or spaceborne radar remote sensing. All weather, active and 24 hour imaging system. Used as side-looking radar (SLR) or side-looking airborne radar (SLAR). Real aperture radars (RAR). Synthetic aperture radar (SAR). Basic Configuration of SLR
Basic Configuration of SLR cont. Beam Width of the Antenna Horizontal beam width in radians, β c H ; β c H = λ / L Vertical beam width in radians, β c V ; β c V = λ / D where, λ is the wavelength of the signal. L is the antenna length. D is the antenna width. ERS Radar Geometry Right-hand side looking radar Radar Antenna Radar antenna of RADARSAT 1, during construction. A radar antenna is an electromechanical structure for transmitting and receiving electromagnetic energy in the microwave spectrum. Real Aperture Radar Azimuth Resolution r az = R β c H ; where R is the slant range, and R = h / cosө ; (h is the vertical height of antenna) or R = c T / 2 ( where c is speed of light and T is the time from the signal left the antenna to when it returned.) also, β c H = λ / L so that r az = h λ / ( L * cos Ө ) Vertical Beam width Horizontal Beam width Slant Range, R
Real Aperture Radar Range Resolution Slant range resolution, r r, is solved by the transmission of short pulses of time, τ, so that r r = c τ/2 The equivalent ground range resolution is therefore, r g = c τ/(2sinө) Summary Resolutions of RAR Azimuth resolution is improved for smaller λ and Ө (near range), and for lower altitude and larger antenna length. Ground range resolution is improved for larger Ө as τ becomes smaller. Summary Resolutions of RAR cont. The look angle Ө has the opposite effect on azimuth and ground range resolutions. The minimum size of τ is limited by the signal power. Smaller λ is made the greater the interference from rain or the objects of the same order of size as λ. Synthetic Aperture Radar - SAR First introduced by Carl Wiley in 1951. A long antenna is synthesized as a long linear array of short real antenna apertures. Using the sensor motion along track to transform a single physically short antenna into an array of such antennas that can be linked together mathematically as part of the data recording and processing procedures. Having a very narrow effective antenna beamwidth. Synthetic Aperture Radar SAR cont. Synthetic aperture image generation: The target (in red) is illuminated with many successive radar pulses, and the image is formed by a coherent combination of all the received echoes. By processing the return signals according to their Doppler shifts, a very small effective beamwidth can be generated.
Azimuth Resolution of SAR Points on the ground at near range are viewed by proportionately fewer antenna elements than those at far range (effective antenna length increases with range). Essentially constant azimuth resolution irrespective of range. Azimuth resolution for SAR: r a = L / 2 ; where (L( is antenna length) Azimuth Resolution of SAR cont. Note: There are technical limit to how small an antenna can be for effective use. The radar complexity, accuracy, storage and processor requirements will increase with range, R, or altitude, h, when the wavelength is constant. Radar complexity and accuracy requirements also increase with increasing λ while the radiated power requirements increase sharply as the length of the antenna is decreased. Range Resolution of SAR A linearly frequency-modulated signal pulse (chirp) is sent instead of a constant frequency to improve the range resolution. Common Characteristics of RAR and SAR Grazing and normal angles should not be used. The range resolution is poorer in near range. The signal power from a small target decreases inversely as the fourth power of the range. High frequencies are discouraged by power and weather difficulties. Common Characteristics of RAR and SAR cont. The range of the max to min signal strength returning to the antenna is from 100 to 1 or 20 db (10log 100 = 20 db) for diffuse natural surfaces; and 10 6 to 1 (10 log 10 6 = 60 db) for corner reflectors.
Speckles in Radar Images Corner Reflectors Radar signal is monochromatic -> > the return signals will be coherent -> > speckles. Average a number of resolution elements in the azimuth direction (azimuth looks) to reduce the speckles. Many spaceborne SAR systems average four azimuth looks (SIR-B/C, ERS-1/2), and some use three (JERS-1). The ground range resolution is engineered to be about the same as azimuth resolution so that the image element is square. Ground Range Distortions Both RAR and SAR images, a square raster based output, will suffer a number of geometric distortions. Ground Range Distortions Relief Displacement Relief displacement in SLR images is one dimensional and perpendicular to the flight line. Radar images display ranges, or distances, from terrain features to the antenna. In contrast to scanner imagery and photography, the direction of relief displacement is reversed.
Relief Displacement cont. Relief Displacement cont. Effects of relief displacement: Foreshortening Layover Shadowing Relief Displacement - Foreshortening It occurs when the slope facing the antenna is less steep than the line perpendicular to the look direction. The slopes facing toward the radar antenna will be foreshortened. The slopes facing away will be elongated. This effect becomes more severe as the slope s steepness approaches normal to the look direction. Relief Displacement - Layover It is the extreme case when the slope facing the antenna is steeper than the line perpendicular to the look direction. It causes an inversion of the image geometry. The tops of the slopes will be imaged before their bases. Most severe at near range. Relief Displacement - Shadowing For the slopes facing away from the radar antenna, if they are: Less steep than the look direction, then will be illuminated by the radar pulse and very weak return signals (dark). Equal or steeper than the look direction, therefore, not be illuminated and appear black on the image.
Relief Displacement - Summary Example ERS1 Image ( ESA)