Introduction to Microwave Remote sensing. Questions asked Module - III. Explain air borne and space borne sensors Describe advantages and disadvantages of air borne sensors Describe imaging with microwave radar Explain Synthetic Aperture Radar(SAR)
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RADAR is the most commonly used space-based active sensing system. It is an acronym for RAdio Detection And Ranging.
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
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 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)
University of Kansas
University of Kansas
Geometry of a side-looking airborne RADAR system.
Resolution depended on the size of the antenna
Adapted from Lillesand and Kiefer (1987)http://forsys.cfr.washington.edu/JFSP06/radar_overview.htm
(or pulses of microwave) at regular intervals
(common wavelengths Wavelength () Frequency ()
shown in parentheses)in cm in GHz
Ka (0.86 cm) 0.75 - 1.18 40.0 to 26.5
K 1.18 - 1.67 26.5 to 18.0
Ku 1.67 - 2.4 18.0 to 12.5
X (3.0 and 3.2 cm) 2.4 - 3.8 12.5 - 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
•nadir: point on the ground vertically beneath the center of camera lens
•azimuth(or flight) direction
•look (or range) direction: direction in which pulses of microwave energy are transmitted
•range (near, middle, and far)
• depression angle ()
• incidence angle ()
• altitude above-ground-level, H
Azimuth flight direction
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.
C.C.Tscherning, Nov. 2007
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.
Synthetic Aperture Radars were developed as a means of overcoming the limitations of real aperture radars.
SAR PrincipleA synthetic aperture is produced by using the forward motion of the radar. As it passes a given scatterer, many pulses are reflected in sequence. By recording and then combining these individuals signals, a "synthetic aperture" is created in the computer providing a much improved azimuth resolution