Lecture 7: SAR remote sensing

Question 1

RADAR

Answer 1

= Radio Detection And Ranging
→ based on microwaves having a centimetre- to metre-scale wavelength

Question 2

Imaging RADAR

Answer 2

= radar technique used to create a 2D image from the measurement of echoes coming from emitted radar pulses (→ active imaging sensor technique)

2 dimensions:
➢ Range direction (cross-path)
➢ Azimuth direction (along path)

What is measured:
➢ Echo (« strength » of the returned signal)
➢ Time between pulse and echo

Spatial resolution in range:
➢ as a function of the width of the pulse

Range as a function of time:
t = 2d/c → d = c t / 2

Spatial resolution in azimuth:
➢ = width of the illumination pattern
➢ Determined by angular width of the beam and slant-range distance
➢ Ra = slant range × wavelength / antenna length

Question 3

Range resolution

Answer 3

= Ability to separate two closely spaced objects in range
As a function of the pulse width used
Range resolution ≃ half of the pulse width

Question 4

Synthetic Aperture Radar

Answer 4

In Real Aperture Radar (RAR), a high azimuth resolution would require a non-realistic antenna length → Synthetic Aperture Radar
→ Taking advantage that scatterers (objects at the ground surface) remain within the real-aperture radar beam for many radar pulses.
SAR sensor = sensor having a side looking angle, which uses the motion of the radar antenna over a target region to provide a 2D image with a finer spatial resolution than a beam-scanning radar.

Question 5

Advantages of imaging radar

Answer 5

✓ All weather capability
✓ Can operate day and night
✓ Sensitivity to surface roughness
✓ Sensitivity to dielectric properties (water content, biomass, ice)
✓ Sensitivity to object structure (→ polarimetry)
✓ Subsurface penetration

Question 6

Amplitude image

Answer 6

“Strength” of the backscattered signal
Depends on the physical and electric properties of the surface

Question 7

Phase

Answer 7

Angle representing the # of periods spanned
Depends on the distance to the ground

Question 8

Backscattering

Answer 8

= reflexion of a signal that is back to the direction from where it comes
Function of the surface hit by the signal
➢ Surface roughness
➢ Dielectric constant (physical property influencing the reflectivity of an object)
➢ ! Double bounce

Function of the wavelength

Question 9

Speckle

Answer 9

Pixel brightness = mix of signals coming from different scatterers → « salt & pepper » effect
→ Degrades the image quality
Can be filtered (examples):
➢ Multi-looking
➢ Moving-window spatial filtering
➢ Multi-image averaging

Question 10

4 main backscattering mechanisms

Answer 10

1. Specular reflection
2. Corner reflection
3. Diffuse scattering
4. Volume scattering

Question 11

SAR-based flood mapping

Answer 11

GOAL = Taking advantage of the backscattering properties of water to detect flooded areas
Technique 1: Thresholding
Technique 2: Comparison
→ Reference image (no flood) versus Crisis image (during flood event)
More evolved techniques:
→ Decision tree
→ Active contour model
→ Deep learning approach

Limitations: Urban areas, forested areas and small, shallow and dynamic floods