15 Microwave Remote Sensing II

Introduction to Microwave Remote Sensing II

Key Concepts

  • Microwave remote sensing utilizes radar signals for various applications in geoscience and environmental monitoring.

Reflectance Mechanisms

Incidence Angle

  • Defined as the angle between the incident radar signal and a perpendicular to the surface.

Types of Reflection

  1. Specular Reflection

    • Occurs when the surface is smooth relative to the wavelength.

    • The angle of incidence equals the angle of reflection.

  2. Diffuse Reflection

    • Happens when the surface is rough relative to the wavelength.

    • The signal scatters uniformly in all directions.

  3. Corner Reflection

    • Involves double reflection from adjacent smooth surfaces.

    • High reflectance appears as bright sparkles in images and tends to be larger in proportion than its actual size.

Radar Image Brightness

Geometric Characteristics

  • The slope facing the sensor impacts signal returns.

  • Surface roughness affects the quantity and type of returns.

Electrical Characteristics

  • The Complex Dielectric Constant plays a role in reflectance with water having a value of 80, and most materials ranging from 3-8 when dry.

  • Moisture content significantly influences reflectance, often more than the material itself.

  • Metal objects like bridges and silos reflect radar signals strongly.

Natural Features Response

Soil Responses

  • Top soil moisture provides high returns and affects radar wave penetration.

Vegetation Responses

  • Returns are high when plant size is similar to radar wavelength or when vegetation is dense.

  • High moisture content in vegetation yields greater energy returns.

  • HH polarized sensing is more effective in penetrating dense vegetation compared to HV polarized sensing.

Water and Ice Responses

  • Smooth water surfaces produce specular reflectance with minimal returns.

  • Rough water generates variable strength returns.

  • Factors like ice age, roughness, and snow cover influence radar returns.

Radar Band Designation

  • Radar bands classified by wavelength:

    • Ka: 0.75-1.1 cm

    • K: 1.1-1.67 cm

    • Ku: 1.67-2.4 cm

    • X: 2.4-3.75 cm

    • C: 3.75-7.5 cm

    • S: 7.5-15 cm

    • L: 15-30 cm

    • P: 30-100 cm

Interferometric Radar

  • Based on the phase difference of radar signals from antennas at different spatial locations.

  • Single-Pass Interferometry: Uses two antennas on one aircraft.

  • Repeat-Pass Interferometry: Involves one antenna making multiple passes to gather data.

Notable Radar Satellites

  • SEASAT SAR

  • Shuttle Image Radar (SIR-A, SIR-B, SIR-C)

  • ERS-1 and ERS-2: European Space Agency with a 785 km orbit and a 30m resolution system.

  • RADARSAT: Canadian Space Agency designed for observing various features like sea ice and agriculture.

  • Shuttle Radar Topography Mission (SRTM): Used single-pass interferometry for high-resolution elevation data.

Summary of SRTM

  • Acquired elevation data for nearly the entire globe, creating a comprehensive digital topographic database.

  • Conducted in February 2000, required specialized radar on the Space Shuttle Endeavour.

References

  • Lillesand, T. M., Kiefer, R.W., & Chipman, J.W. (2015). Remote Sensing and Image Interpretation (7th ed.). John Wiley & Sons, Inc.

  • Campbell, J.B. & Wynne, R.H. (2011). Introduction to Remote Sensing (5th ed.). The Guilford Press.