Microwave introduction

• Microwave sensing


  Encompasses both active and passive forms of remote sensing
• Microwave portion of the spectrum


  • Covers the range from approximately 1cm to 1m in wavelength
  • Longer wavelength microwave radiation can penetrate through cloud cover, haze, dust, and all but the heaviest rainfall
  • Longer wavelengths are not susceptible to atmospheric scattering which affects shorter optical wavelengths
• Passive microwave sensing


  • Similar in concept to thermal remote sensing
  • All objects emit microwave energy of some magnitude, but the amounts are generally very small
  • A passive microwave sensor detects the naturally emitted microwave energy within its field of view
  • Emitted energy is related to the temperature and moisture properties of the emitting object or surface
• Passive microwave sensor


  1. Antenna is used to detect and record the microwave energy

  2. Microwave energy can be emitted by the atmosphere

  3. Reflected from the surface

  4. Emitted from the surface

  5. Transmitted from the subsurface
• Passive microwave sensors


  • Characterized by low spatial resolution
• Applications of passive microwave remote sensing

  • Meteorology
  • Hydrology
  • Oceanography
• Active microwave sensors


  • Provide their own source of microwave radiation to illuminate the target
  • Generally divided into two distinct categories: imaging and non-imaging
• RADAR


  • RAdio Detection And Ranging
  • Sensor transmits a microwave (radio) signal towards the target and detects the backscattered portion of the signal
  • Strength of the backscattered signal is measured to discriminate between different targets
  • Time delay between the transmitted and reflected signals determines the distance (or range) to the target
• Non-imaging microwave sensors


  • Include altimeters and scatterometers
  • Profiling devices which take measurements in one linear dimension, as opposed to the two-dimensional representation of imaging sensors
• Radar altimetry


  • Transmit short microwave pulses and measure the round trip time delay to targets to determine their distance from the sensor
  • Generally altimeters look straight down at nadir below the platform and thus measure height or elevation
• Scatterometers


  • Used to make precise quantitative measurements of the amount of energy backscattered from targets
  • Amount of energy backscattered is dependent on the surface properties (roughness) and the angle at which the microwave energy strikes the target
• Advantages of radar


  • Capability of the radiation to penetrate through cloud cover and most weather conditions
  • Can be used to image the surface at any time, day or night
• Radar image is quite different from and has special properties unlike images acquired in the visible and infrared portions of the spectrum
• First demonstration of the transmission of radio microwaves and reflection from various objects achieved by Hertz


  1886
• First rudimentary radar developed for ship detection


  Early 1900s
• Experimental ground-based pulsed radars developed for detecting objects at a distance

  1920s and 1930s
• First imaging radars used with rotating sweep displays for detection and positioning of aircrafts and ships

  World War II
• Side-looking airborne radar (SLAR) developed for military terrain reconnaissance and surveillance


  After World War II
• Advances in SLAR and development of higher resolution synthetic aperture radar (SAR) for military purposes

  1950s
• Radars declassified and began to be used for civilian mapping applications


  1960s
• Canada became involved in radar remote sensing

  Mid-1970s
• Canada's SURSAT (Surveillance Satellite) project


  1977 to 1979
• Launch of ESA's ERS-1 satellite


  1991
• Launch of Japan's J-ERS satellite


  1992
• Launch of ERS-2 and Canada's RADARSAT satellite


  1995