Active Microwave Remote Sensing . Lecture 8 Oct 22, 2007. Recap: passive and active RS. Passive : uses natural energy, either reflected sunlight (solar energy) or emitted thermal or microwave radiation. Active : sensor creates its own energy Transmitted toward Earth or other targets
Oct 22, 2007
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)
(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
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. SLAR is usually a real aperture radar. The longer the antenna (but there is limitation), the better the spatial resolution
• azimuth (or flight) direction
• look (or range) direction
• 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.
Most radar systems and data providers now provide the data in ground-range geometry
Pulse duration (t)
= 0.1 x 10 -6 sec
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.
Response of A Pine Forest Stand to X-, C- and L-band Microwave Energy
to heavy rainfall
SIR-C/X-SAR Images of a Portion of Rondonia, Brazil, Obtained on April 10, 1994
Shuttle Imaging Radar (SIR-C) Image of Maui