Microwave Remote Sensing Overview
Introduction
Microwave remote sensing uses and detects electromagnetic radiation from wavelengths of approximately 1mm to 1m. Microwave sensors can be operated day or night and in all weather conditions. Microwave radiation has long wavelengths that are able to penetrate through cloud cover, haze, dust, and even rain. Longer wavelengths are not susceptible to atmospheric scattering which primarily affects shorter wavelengths (visible, NIR). Some microwave remote sensing applications include monitoring snow and ice cover, sea and land surface temperatures, precipitation and a variety of radar applications.
Microwave Bands
The microwave spectrum is divided up into bands depending on the wavelength of energy. Each of the bands has different uses and scientific applications. For example the medium wavelength microwaves (C-band) can penetrate through clouds rain and dust to the Earth's surface. Shorter wavelength bands like the L-band are commonly used by Global Positioning Systems (GPS) but can also be used to measure soil moisture. Other bands like the X and Ku bands are primarily used for satellite communications.
| Band Designator | Frequency (GHz) | Wavelength (centimeters) |
|---|---|---|
| L band | 1 to 2 | 30.0 to 15.0 |
| S band | 2 to 4 | 15 to 7.5 |
| C band | 4 to 8 | 7.5 to 3.8 |
| X band | 8 to 12 | 3.8 to 2.5 |
| Ku band | 12 to 18 | 2.5 to 1.7 |
| K band | 18 to 27 | 1.7 to 1.1 |
| Ka band | 27 to 40 | 1.1 to 0.75 |
| V band | 40 to 75 | 0.75 to 0.40 |
| W band | 75 to 110 | 0.40 to 0.27 |
Passive Remote Microwave Sensing
Passive microwave sensors are similar to passive sensors in the visible and near-infrared portions in the spectrum. Passive microwave sensors detect naturally emitted microwave radiation and record this energy. The Earth naturally emits energy in the microwave portion of the spectrum, although in it is a very low level of energy. Passive radiometers are used to study weather, sea surface temperature and ice.
The Tropical Rainfall Measuring Mission (TRMM) passive microwave imager captured data underneath storm clouds to reveal the structure of Hurricane Katrina.
Image Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio
Active Microwave Remote Sensing
Active microwave sensors send pulses of microwave energy to the Earth's surface and record the time it takes for the pulse to return. These are sometimes referred to as active microwave radiometers and are more commonly known as radar system. The same radar technology that is used in remote sensing is used to track aircraft and the speeds of vehicles. There are three main types of active microwave sensors employed in remote sensing applications: Imaging Radar, Scatterometers and Radar Altimeters.
Synthetic aperture radar (SAR) of Nepal acquired by Sentinel-1A Image Credit: European Space Agency (ESA)