1. Active Microwave Remote Sensing

2. Radar=Radio Detection and Ranging
Radar system components

3. Radar: How it Works
A directed beam of microwave pulses are transmitted from an antenna
The energy interacts with the terrain and is scattered
The backscattered microwave energy is measured by the antenna
Radar determines the direction and distance of the target from the instrument as well as the backscattering properties of the target

4. Radar Parameters Azimuth Direction Range angle Depression angle
direction of travel of aircraft or orbital track of satellite
Range angle
direction of radar illumination, usually perpendicular to azimuth direction
Depression angle
angle between horizontal plane and microwave pulse (near range depression angle > far range depression angle)
Incident angle
angle between microwave pulse and a line perpendicular to the local surface slope
Polarization
linearly polarized microwave energy emitted/received by the sensor (HH, VV, HV, VH)

5. Radar Nomenclature • Nadir • azimuth flight direction • look direction
• range (near and far)
• depression angle ()
• incidence angle ()
• altitude above-ground-level, H
• polarization

7. Slant Range vs. Ground Range

8. Radar Pulse Length

9. Synthetic Aperture Radar
Antenna “length” is increased synthetically by building up a history of backscattered signals from the landscape along the track of the sensor
Implemented by keeping track of the Doppler shift of the reflected signal (frequency of the transmitted signal is known)

10. Layover
Layover occurs when the incidence angle (q) is smaller than the foreslope (a+)
i.e., q < a+.
This distortion cannot be corrected!

12. Radar Shadowing
Radar shadowing can be useful for interpreting geomorphological features

13. Radar Backscatter Power received = Power per unit area at target x
Effective scattering area of the target
Spreading loss of reradiated signal
Effective receiving area of antenna

14. Radar Backscatter Coefficient
The efficiency the terrain to reflect the radar pulse is termed the “radar cross-section”, 
The radar cross-section per unit area, (A) is called the “radar backscatter coefficient” (˚) and is computed as :
The radar backscatter coefficient determines the percentage of electro- magnetic energy reflected back to the radar from within a radar pixel
This is similar to the reflectance in optical remote sensing

15. Radar Backscattering

16. Radar Backscattering Depends on the properties of the target:
roughness
dielectric constant
Depends on characteristics of the radar:
depression angle
frequency/wavelength
polarization

17. Rayleigh Criterion for Roughness
A surface is considered smooth at or below a height, h, if:
[ cm ]
h = the vertical relief (average height of surface irregularities)
l = the radar wavelength (measured in cm)
g = the depression angle

18. Surface Roughness in RADAR Imagery

19. Nile River Sudan Space Shuttle Color-Infrared Photograph
SIR-C Color Composite:
• Red = C-band HV
• Green = L-band HV
• Blue = L-band HH
C-band, = 6cm
L-band, = 24cm

20. Radar and the Dielectric Constant
Dielectric constant depends on the type of material as well as its moisture state
it is analogous to the refractive index of the material
it is primarily a function of moisture content
also depends on chemical properties such as salinity
Dielectric constant is the ratio of the capacitance of a material to that of a vacuum. Also known as the “relative permittivity”

21. Dielectric Constant
dielectric constant of liquid water is 80; dry soil is 2-4.

22. Radar frequency and backscatter
Depth of radar penetration through the vegetation canopy varies directly with l

23. Types of Active Microwave Surface and Volume Scattering that Take Place in a Hypothetical Pine Forest Stand

24. Response of A Pine Forest Stand to X-, C- and L-band Microwave Energy