RADAR Remote Sensing

Introduction A Quick Reminder Passive RS Systems: Active RS Systems Record energy reflected (visible, NIR) and emitted (TIR) from the Earth’s surface Active RS Systems Transmit their own EM energy to Earth and record backscattered energy

Introduction Most widely used active RS systems RADAR LIDAR SONAR Radio Detection and Ranging Microwaves (long waves) LIDAR Light Detection and Ranging Laser light waves (short waves) Topographic mapping, etc. SONAR Sound Navigation and Ranging Sound waves (long waves) Bathymetric mapping, etc.

RADAR Basics Two types of RADAR data collection systems Active: Record ‘artificially generated’ microwave energy Passive: Record microwave ‘naturally emitted’ microwave energy

History of RADAR RS James Clerk Maxwell (1831-1879) Provided mathematical descriptions of magnetic and electric fields associated with EM radiation Heinrich R. Hertz (1857-1894) Provided fundamental knowledge of creation and propagation of EM energy in microwave and radio portions of EMS His studies on interaction of radio waves with metallic surfaces eventually facilitated the invention of radios and radars Guglielmo M. Marconi (1874-1937) Used above fundamental physics principles to construct the first antenna to transmit and receive radio waves 1901: Radio waves across the Atlantic 1909: Novel Prize in physics

History of RADAR RS A. H. Taylor and L.C. Young 1922 Realized that radio signals might be useful for detecting the distance to ships at sea, etc. and both at night and in bad weather Today, we use almost only microwaves but the term RADAR was never changed 1935 Combined antenna transmitter and receiver in one instrument Groundwork for RADAR development during WW II for navigation and target location (back then: no RADAR imagery from air- or spacecraft)

History of RADAR RS RADAR Imagery SLAR: Side-Looking Air-borne Radar Used since 1950s by military Some systems declassified since the 1960s Continuous-strip mapping capability Reconnaissance over vast regions to the left or right of aircraft: Long-range standoff data collection Radargrammetric measurement Science of extracting quantitative geometric object information from RADAR imagery

History of RADAR RS Two SLAR Types Real Aperture Radar (AKA “brute force radar”) Use antenna of fixed length (e.g., 1 – 2 m) Synthetic Aperture Radar (SAR) (aperture = antenna) Also use ‘fixed-length” antenna but are able to synthesize a much larger antenna (by sensing a greater number of beams) Example: An 11-m antenna on an orbital platform can be synthesized electronically to have a synthetic length of 15 km The longer the antenna, the better the resolving power

History of RADAR RS Since 1960s and 1970s Various SARs launched for Earth resource reconnaissance Particularly useful in areas that have a more or less perennial cloud cover (e.g., tropical areas) 1978: SEASAT (USA) 1981; 1984; 1994: SIR-A; SIR-B; SIR-C/X-SAR (USA) 1991: ALMAZ-1(USSR) 1991; 1994: ERS-1; ERS-2 (ESA) 1992: JERS-1(Japan) 1995; 2006: RADARSAT; RADARSAT2 (Canada) 2000: SRTM (USA) 2002: Envisat; ASAR (ESA)

Advantages of RADAR RS All-weather RS system Certain microwave frequencies penetrate cloud cover Provides synoptic views of large areas for mapping 1:10,000 – 1:400,000 Coverage can be obtained at user-specific times Day and night Provides different perspectives that cannot always be obtained using aerial photography Permits imaging at shallow look angles Angle of illumination can be controlled (has its own illumination) Provides information on surface roughness, dielectric properties, and moisture content Senses in wavelengths outside the optical regions of the EMS,

Advantages of RADAR RS May penetrate vegetation, sand, and snow surface layers Enables resolutions to be independent of distance to object Size of a resolution cell may be as small as 1 m May produce images from different types of polarized energy HH, HV, VV, VH Has multi-frequency potential May operate simultaneously in several wavelengths (frequencies) Can measure ocean wave properties, even from orbital altitudes Can produce overlapping images suitable for stereoscopic viewing Can produce overlapping images suitable for radargrammetry Supports interferometric operation using two antennas 3-D mapping Analysis of incident-angle signatures of objects

Disadvantages of RADAR RS Can be very noisy and difficult to classify Large amounts of geometric relief displacement Geometric relief displacement inconsistent across the image

RADAR: How does it work? Microwave energy is transmitted toward an area from an antenna in very short bursts or pulses Energy is reflected off of the area (backscattered) and recorded at the same antenna after a period of time Propagation of one radar pulse

RADAR: How does it work? Strength (detection) and time delay (ranging) of return signals is measured Range or distance between the transmitter and the area are inferred from time elapsed between transmission and reception Resulting antenna return

Typical SLAR System Components Canberra IFSAR System Layout (Source: Original figure by H.A.Malliot, High Altitude Mapping Missions, Inc.)

Wavelength, Frequency, Pulse Length Pulse of EM radiation transmitted through antenna Has a specific wavelength Much longer than TIR Measured in centimeters (cm) Names of radar wavelengths: Alphabetic descriptor, not actual wavelength or frequency Relicts of secret work on radar RS in WW II Has a specific duration Pulse length Measured in microseconds (msec)

Wavelength, Frequency, Pulse Length RADAR Band l (cm) n (GHz) Ka 0.75 – 1.18 40 – 26.5 K 1.19 – 1.67 26.5 – 18 Ku 1.67 – 2.4 18 – 12.5 X 2.4 – 3.8 12.5 – 8 C 3.9 – 7.5 8 – 4 S 7.5 – 15 4 – 2 L 15 – 30 2 – 1 P 30 – 100 1 – 0

Wavelength, Frequency, Pulse Length Ka, K, and Ku bands: Shortest wavelengths Partially absorbed by water vapor, thus limiting cloud penetration Used by ground-based weather radars to track cloud cover and precipitation X-band (SIR-C; SRTM) Shortest wavelength used by any orbital/ sub-orbital imaging radar C-band (SIR-C; ERS 1 & 2; RADARSAT 1 & 2; SRTM; Envisat, ASAR) S-band (ALMAZ-1) L-band (SEASAT, SIR-A, B, C; JERS-1) P-band Longest radar wavelengths

Azimuth, Range Direction, etc. Ka, K, and Ku bands: Shortest wavelengths Partially absorbed by water vapor, thus limiting cloud penetration Used by ground-based weather radars to track cloud cover and precipitation X-band (SIR-C; SRTM) Shortest wavelength used by any orbital/ sub-orbital imaging radar C-band (SIR-C; ERS 1 & 2; RADARSAT 1 & 2; SRTM; Envisat, ASAR) S-band (ALMAZ-1) L-band (SEASAT, SIR-A, B, C; JERS-1) P-band Longest radar wavelengths

Other Important Parameters Azimuth Direction; Range Direction; Depression Angle; Look Angle; Incident Angle; Polarization Φ Look angle θ Incident angle Slant range Nadir Swath

Other Important Parameters Pulse length determines range resolution Beam width determines azimuth resolution System Resolution Nadir Swath Φ Look angle θ Incident angle Slant range

Other Important Parameters Azimuth Direction Straight line in which the aircraft travels Range / Look Direction Direction in which active microwave energy illuminates strips of the terrain (orthogonal to the aircraft’s azimuth direction) Near-range: terrain illuminated nearest the aircraft Far-range: terrain illuminated farthest away from the aircraft Significantly affects feature interpretation Objects that trend (or strike) in a direction perpendicular to the range direction are more enhanced/emphasized than objects that lie parallel to it

Other Important Parameters Effect of Range Direction on Radar Imagery Example: X-band image of the Kaduna State in Nigeria East-west azimuth direction with RADAR looking north East-west azimuth direction with RADAR looking south Note: Orient image so that the look direction is toward you. That way, the shadows fall toward you, keeping you from experiencing pseudoscopic illusion.

Other Important Parameters Depression Angle (γ) Angle between a horizontal plane extending out from the aircraft fuselage and the EM pulse of energy from the antenna along the radar line-of-sight to a specific point on the ground Varies from a near-range to a far-range depression angle Look Angle (Φ) Angle between the vertical from the antenna to the ground and the radar line-of-sight Varies from a near-range to a far-range look angle Complement of the depression angle

Other Important Parameters Incident Angle (θ) Angle between the radar pulse of energy and a line perpendicular to the Earth’s surface where it makes contact Complement of the depression angle where the terrain is flat (often assumed in radar studies) Incident angle in terms of the relationship between radar beam and surface slope: Vertical Normal to surface Radar wave Incident angle, θ Local slope angle, α Local incident angle Scattering surface

Other Important Parameters Polarization Unpolarized energy vibrates in all possible directions perpendicular to the direction of travel Radar antennas send and receive polarized energy ( Pulse of energy is filtered) Four Polarizations Like-polarized (co-polarized): Send and receive horizontally polarized energy (HH) Send and receive vertically polarized energy (VV) Cross-polarized: Send horizontal and receive vertically polarized energy (HV) Send vertical and receive horizontally polarized energy (VH)

RADAR Polarization Cinder cone and basalt lava flow, AZ

Slant- vs. Ground Range Geometry Uncorrected imagery is displayed in slant-range geometry Based on actual distance from the radar to features in the scene Near-range objects are more compressed than far-range objects May be converted to ground-range geometry so that features in the scene are in their proper planimetric position (See book.)

RADAR Issues Geometric distortions exist in almost all RADAR imagery No problem in flat terrain Simple algorithms for converting slant-range to ground-range geometry Problem in variable terrain RADAR image is formed in the range (cross-track) direction The higher the object, the closer it is to the antenna, and the sooner it is detected on the radar image  Displacement toward the antenna (vs. air photos where relief displacement is away from the PP) Foreshortening & Layover = Elevation-induced distortions Shadows Speckle

RADAR & Topography Foreshortening In high relief areas, slopes facing the sensor will appear compressed and their lengths will be incorrect

RADAR & Topography Layover Extreme case of foreshortening When the return signal from the top of a feature is received before that of the base Object will appear to lean toward the sensor

RADAR & Topography Radar Shadow The down range dimension of a tall object is not illuminated by the sensor so that no energy is available to be backscattered

RADAR & Topography Speckle Grainy salt-and-pepper pattern in RADAR imagery Due to coherent nature of the RADAR wave, which causes random constructive and destructive interference, hence random bright and dark areas in an image Can be reduced but at the cost of degraded resolution (LAB) 1-look radar image 4-look radar image 16-look radar image ------------ Radar speckle reduction using multiple-look techniques ------------

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