Recap: passive and active RS Passive: uses natural energy, either reflected sunlight (solar energy) or emitted thermal or microwave radiation. Emitted microwave radiation can be all weathers capability Active: sensor creates its own energy Transmitted toward Earth or other targets Interacts with atmosphere and/or surface Reflects back toward sensor (or backscatter) Radar can be all weathers capability, Lidar for land surface still be affected by cloud and rain.
Widely used active RS systems RADAR: RAdio Detection And Ranging Long-wavelength microwaves (1 – 100 cm) LIDAR: LIght Detection And Ranging Short-wavelength laser light (UV, visible, near IR) SONAR: SOund Navigation And Ranging: (very long wave, low Hz) Sound can not travel through vacuum, so acoustic energy is not EMR energy Earth and water absorb acoustic energy far less than EMR energy Seismic survey use small explosions, record the reflected sound Medical imaging using ultrasound Sound waves can pass through a water column. Sound waves are extremely slow (300 m/s in air, 1,530 m/s in sea-water) Bathymetric sonar (measure water depths and changes in bottom topography ) Imaging sonar or sidescan imaging sonar (imaging the bottom topography and bottom roughness)
Types of radar Nonimaging radar Traffic police use handheld Doppler radar system determine the speed by measuring frequency shift between transmitted and return microwave signal Plan position indicator (PPI) radars use a rotating antenna to detect targets over a circular area, such as NEXRDA Satellite-based radar altimeters (low spatial resolution but high vertical resolution) Imaging radar Usually high spatial resolution, Consists of a transmitter, a receiver, one or more antennas, GPS, computers
Microwaves Band Designations (common wavelengths Wavelength ( ) Frequency ( ) shown in parentheses) in cm in GHz _______________________________________________ Ka (0.86 cm)0.75 - 1.1840.0 to 26.5 K1.18 - 1.6726.5 to 18.0 K u 1.67 - 2.418.0 to 12.5 X (3.0 and 3.2 cm)2.4 - 3.812.5 - 8.0 C (7.5, 6.0 cm)3.8 - 7.5 8.0 - 4.0 S (8.0, 9.6, 12.6 cm)7.5 - 15.0 4.0 - 2.0 L (23.5, 24.0, 25.0 cm)15.0 - 30.0 2.0 - 1.0 P (68.0 cm)30.0 - 100 1.0 - 0.3 Band Designations (common wavelengths Wavelength ( ) Frequency ( ) shown in parentheses) in cm in GHz _______________________________________________ Ka (0.86 cm)0.75 - 1.1840.0 to 26.5 K1.18 - 1.6726.5 to 18.0 K u 1.67 - 2.418.0 to 12.5 X (3.0 and 3.2 cm)2.4 - 3.812.5 - 8.0 C (7.5, 6.0 cm)3.8 - 7.5 8.0 - 4.0 S (8.0, 9.6, 12.6 cm)7.5 - 15.0 4.0 - 2.0 L (23.5, 24.0, 25.0 cm)15.0 - 30.0 2.0 - 1.0 P (68.0 cm)30.0 - 100 1.0 - 0.3
Two imaging radar systems In World War II, ground based radar was used to detect incoming planes and ships (non-imaging radar). Imaging RADAR was not developed until the 1950s (after World War II). Since then, side-looking airborne radar (SLAR) has been used to get detailed images of enemy sites along the edge of the flight field. The longer the antenna (but there is limitation), the better the spatial resolution. SLAR can be either a RAR or a SAR. Real aperture radar (RAR) Aperture means antenna A fixed length (for example: 1 - 15m) Synthetic aperture radar (SAR) 1m (11m) antenna can be synthesized electronically into a 600m (15 km) synthetic length. Most (air-, space-borne) radar systems now use SAR.
Radar Nomenclature and Geometry Radar Nomenclature nadir nadir azimuth (or flight) direction azimuth (or flight) direction look (or range) direction look (or range) direction range (near, middle, and far) range (near, middle, and far) depression angle ( ) depression angle ( ) incidence angle ( ) incidence angle ( ) altitude above-ground-level, H altitude above-ground-level, H polarization polarization Radar Nomenclature nadir nadir azimuth (or flight) direction azimuth (or flight) direction look (or range) direction look (or range) direction range (near, middle, and far) range (near, middle, and far) depression angle ( ) depression angle ( ) incidence angle ( ) incidence angle ( ) altitude above-ground-level, H altitude above-ground-level, H polarization polarization Azimuth flight direction Flightline groundtrack Look/Range direction Far range Near range
Slant-range vs. Ground-range geometry Radar imagery has a different geometry than that produced by most conventional remote sensor systems, such as cameras, multispectral scanners or area-array detectors. Therefore, one must be very careful when attempting to make radargrammetric measurements. Uncorrected radar imagery is displayed in what is called slant-range geometry, i.e., it is based on the actual distance from the radar to each of the respective features in the scene. It is possible to convert the slant-range display into the true ground-range display on the x-axis so that features in the scene are in their proper planimetric (x,y) position relative to one another in the final radar image. Radar imagery has a different geometry than that produced by most conventional remote sensor systems, such as cameras, multispectral scanners or area-array detectors. Therefore, one must be very careful when attempting to make radargrammetric measurements. Uncorrected radar imagery is displayed in what is called slant-range geometry, i.e., it is based on the actual distance from the radar to each of the respective features in the scene. It is possible to convert the slant-range display into the true ground-range display on the x-axis so that features in the scene are in their proper planimetric (x,y) position relative to one another in the final radar image.
Most radar systems and data providers now provide the data in ground-range geometry
Range (or across-track) Resolution Pulse duration (t) = 0.1 x 10 -6 sec t.c called pulse length. The short pulse length will lead fine range resolution. However, the shorter the pulse length, the less the total amount of energy that illuminates the target. t.c/2
Azimuth (or along-track) Resolution The shorter wavelength (λ) and longer antenna (D) will improve azimuth resolution. The shorter the wavelength, the poorer the atmospheric and vegetation penetration capability There is practical limitation to the antenna length, while SAR will solve this problem.
SAR A major advance in radar remote sensing has been the improvement in azimuth resolution through the development of synthetic aperture radar (SAR) systems. Great improvement in azimuth resolution could be realized if a longer antenna were used. Engineers have developed procedures to synthesize a very long antenna electronically. Like a brute force or real aperture radar, a synthetic aperture radar also uses a relatively small antenna (e.g., 1 m) that sends out a relatively broad beam perpendicular to the aircraft. The major difference is that a greater number of additional beams are sent toward the object. Doppler principles are then used to monitor the returns from all these additional microwave pulses to synthesize the azimuth resolution to become one very narrow beam.
Azimuth resolution is constant = D/2, it is independent of the slant range distance,, and the platform altitude. So the same SAR system in a aircraft and in a spacecraft should have the same resolution. There is no other remote sensing system with this capability.
At time n+3, the shortest distance and area of zero Doppler shift
Speckle noise Using SAR, we can get high spatial resolution in the azimuth dimension (direction). But the coherently recording returned echoes (SAR) also causes speckle noise. For one-single channel SAR system, the speckle noise has a multiplicative nature for the amplitude and an additive nature for the phase. For multi-dimensional (or polarimetric) SAR (or PolSAR) system, speckle noise is even complicated. There are two ways to remove speckle noise: Using several looks, i.e., averaging takes place, usually 4 or 16 looks (N). But lose resolution: Azimuth resolution = N(D/2) Modeling the noise, then remove them.
Backscatter The portion of the outgoing radar signal that the target redirects directly back towards the radar antenna. When a radar system transmits a pulse of energy to the ground (A), it scatters off the ground in all directions (C). A portion of the scattered energy is directed back toward the radar receiver (B), and this portion is referred to as "backscatter".
Frame of the RADARSAT-1 SAR image highlighted white showing the four regions selected to compare typical NRCS values of multiyear (blue), first-year (magenta), marginal ice zone (orange), and lead (red) ice as observed by Envisat, RADARSAT, and QuikSCAT on Oct. 12, 2007. The insert in the top right shows the SIMBA drift track during Oct. 12, 2007. Superposed are also the ship positions during the in- and outbound legs (compare Fig. 1). The total number of selected 12.5 km x 12.5 km grid cells is 200. Burcu et al. 2009
Mean Envisat (diamonds) and RADARSAT-1 (crosses) SAR NRCS values obtained for approximated ASPeCt observation boxes (see Figure 2) for Oct. 26, 2007, as a function of ice type (mixtures) according to the ASPeCt observations (compare Figure 7) grouped from thickest to thinnest ice. ASPeCt observations with ice concentration less than 80 % are not included. The ice type (mixtures) are: 1: Thick first year, First year; 2: Thick first year, First year, Nilas; 3: Thick first year, Young grey-white; 4: Thick first year, Nilas; 5: Thin ice types (Young grey, Pancake, Nilas, and Grease) ice. Note that RADARSAT (09:33 UTC), and Envisat (06:26 UTC) SAR image acquisition times differ by about 3 hours. The NRCS values have been separated from each other horizontally for better discrimination. Error bars annotated to each NRCS value denote one standard deviation based on 6400 and 256 values for RADARSAT (input pixel size: 25 m) and Envisat (input pixel size: 125 m) data, respectively. In the top part of the figure ASPeCt observations based snow depth is given for the primary (triangles) and secondary (squares) ice types (right y- axis). Burcu et al. 2009
Response of A Pine Forest Stand to X-, C- and L-band Microwave Energy Penetration ability to forest
Polarization Unpolarized energy vibrates in all possible directions perpendicular to the direction of travel. The pulse of electromagnetic energy is filtered and sent out by the antenna may be vertically or horizontally polarized. The pulse of energy received by the antenna may be vertically or horizontally polarized VV, HH – like-polarized imagery VH, HV- cross-polarized imagery
Radar Shadow Shadows in radar images can enhance the geomorphology and texture of the terrain. Shadows can also obscure the most important features in a radar image, such as the information behind tall buildings or land use in deep valleys. If certain conditions are met, any feature protruding above the local datum can cause the incident pulse of microwave energy to reflect all of its energy on the foreslope of the object and produce a black shadow for the backslope Unlike airphotos, where light may be scattered into the shadow area and then recorded on film, there is no information within the radar shadow area. It is black. Two terrain features (e.g., mountains) with identical heights and fore- and backslopes may be recorded with entirely different shadows, depending upon where they are in the across-track. A feature that casts an extensive shadow in the far-range might have its backslope completely illuminated in the near-range. Radar shadows occur only in the cross-track dimension. Therefore, the orientation of shadows in a radar image provides information about the look direction and the location of the near- and far-range
Shuttle Imaging Radar (SIR-C) Image of Maui Shadows and look direction
Major Active Radar Systems Seasat, June 1978, 105 days mission, L-HH band, 25 m resolution SIR-A, Nov. 1981, 2.5 days mission, L-HH band, 40 m resolution SIR-B, Oct. 1984, 8 days mission, L-HH band, about 25 m resolution SIR-C, April and Sept. 1994, 10 days each. X-, C-, L- bands multipolarization (HH, VV, HV, VH), 10-30 m resolution JERS-1, 1992-1998, L-band, 15-30 m resolution (Japan) RADARSAT, Jan. 1995-now, C-HH band, 10, 50, and 100 m (Canada) ERS-1, 2, July 1991-now, C-VV band, 20-30 m (ESA) ASAR on EnviSat, 2002-now, C band (ESA) AIRSAR/TOPSAR, 1998-now, C,L,P bands with full polarization, 10m NEXRAD, 1988-now, S-band, 1-4 km, TRMM precipitation radar, 1997, Ku-band, 4km, vertical 250m (USA and Japan)
Active Radar Systems for Mars MARSIS (Mars advanced radar for subsurface and ionosphere sounding) of Mras Express, 2003, 1.8-5 MHz, up to 5 KM deep (ESA) http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=34826&fbodylongid=1601 SHARAD (shallow subsurface radar) of MRO, 2005, 15-25 MHz, up to 1Km deep (ISA-NASA) http://mars.jpl.nasa.gov/mro/mission/sc_instru_sharad.html