NASA ARSET: Basics of Synthetic Aperture Radar (SAR), Session 1/4

Main Ideas and Concepts

• Introduction to Synthetic Aperture Radar (SAR):

  Presented by Erika Podest, a scientist at NASA's Jet Propulsion Laboratory. This is the first session of a four-part webinar series on SAR.

• Electromagnetic Spectrum:

  The electromagnetic spectrum ranges from radio waves to gamma rays, with visible light being a small portion. Microwave Sensors, which operate in a lower frequency range, are less affected by weather and can penetrate through various mediums (vegetation, snow, soil).

• Types of Remote Sensing:
  • Passive Remote Sensing: Measures energy emitted or reflected by the Earth's surface (e.g., optical sensors).
  • Active Remote Sensing: Provides its own illumination source (e.g., Radar and Lidar).
• Advantages and Disadvantages of Radar Remote Sensing:
  • Advantages:
    • Operates in day/night and various weather conditions.
    • Can penetrate through vegetation, snow, and soil.
    • Minimal atmospheric corrections required.
  • Disadvantages:
    • Information content can be difficult to interpret.
    • Presence of speckle (grainy effect) complicates image interpretation.
    • Distortions due to topography need to be accounted for.
• Basic Concepts of Radar:

  Radar operates by sending out pulses of microwave energy and measuring the reflected signals. Two categories of Radar: imaging and non-imaging. Synthetic Aperture Radar (SAR) synthesizes a long antenna using the movement of the sensor platform for high-resolution images.

• Key Parameters Influencing Radar Functionality:
  • Wavelength: Longer wavelengths penetrate deeper into surfaces.
  • Polarization: Refers to the orientation of the Radar signal (horizontal or vertical).
  • Incidence Angle: The angle between the Radar signal and the surface, affecting backscatter.
• Ground Parameters Affecting Radar Signals:
  • Dielectric Properties: Influence Radar reflectivity; wetter surfaces appear brighter due to higher dielectric constants.
  • Surface Roughness: Determines how Radar energy interacts with surfaces, affecting the brightness in Radar images.
• Backscattering Mechanisms:

  Different scattering mechanisms include specular reflection, rough surface scattering, double bounce, and volume scattering. The intensity of backscatter is related to surface characteristics and Radar parameters.

• Distortions in Radar Images:

  Geometric distortions (e.g., foreshortening, layover, shadowing) must be corrected for accurate interpretation. Radiometric corrections are necessary to account for variations in backscatter due to terrain relief.

• Speckle Reduction Techniques:

  Speckle can be reduced through multi-looking (averaging multiple looks) or spatial filtering (smoothing pixel values).

• Applications of SAR:

  SAR is utilized for various applications including land cover classification, oil spill detection, and monitoring environmental changes.

• Future of SAR:

  Upcoming missions and advancements in SAR technology will enhance capabilities in monitoring environmental changes and natural resources.

Methodology/Instructions

• Understanding Radar Parameters: Focus on frequency and wavelength for specific applications.
• Image Interpretation: Consider dielectric properties and surface structure when analyzing Radar images.
• Distortion Corrections: Apply corrections for geometric and radiometric distortions before analysis.
• Speckle Reduction: Choose between multi-looking and spatial filtering based on the required resolution.

Speakers/Sources Featured

• Erika Podest: Scientist at NASA's Jet Propulsion Laboratory and instructor for the NASA ARSET program.