Passive Microwave Remote Sensing

Introduction

All objects greater than absolute zero emit electromagnetic radiation. Wien's Law tells us that the Earth's peak radiation occurs in the thermal infrared portion of the spectrum, at around 9-10 micrometers. The Earth also emits a small amount of microwave radiation, although at relatively low levels. Passive microwave sensors detect and record the microwave radiation that is naturally emitted by the Earth. Clouds do not emit much microwave energy and do not interfere with the naturally emitted microwave energy. Therefore passive microwave sensors can penetrate or "see through" cloud cover and can be operated during the day or the night.

Passive Sensors

Passive microwave sensors are referred to as microwave radiometers. Microwave radiometers can be on-board spacecraft, satellites, as well as aircraft. Microwave radiometers can be used for many climate application including estimating sea ice, salinity, water vapor, and sea surface temperatures.

Microwave radiometers measure the emitted radiance from surfaces. This is often referred to as brightness temperature which is related to the physical temperature and composition of the surface. Not all materials emit microwave energy equally. The object's physical properties, such as the atomic composition and crystalline structure also determine the amount of microwave radiation emitted. For example, the crystalline structure of ice emits more microwave energy than the liquid water in the ocean. The amount of microwave energy emitted is largely a function of an object's dielectric constant. Most materials on Earth have a dielectric constant between 1 and 8, while liquid water has a dielectric constant of around 80. Moisture content is strongly tied to microwave emissions. Open water surfaces are relatively poor emitters (and good reflectors) of microwave energy because of water’s large dielectric constant. Sea ice on the other hand is a relatively good emitter (poor reflector) of microwave energy. The difference in microwave emissions allows for the detection and measuring of sea ice.

A major advantages of passive microwave remote sensing is that microwave radiation can penetrate through cloud cover, rain and dust. It can also acquired during the day or the night. A major disadvantage to passive microwave remote sensing is that the energy level being emitted is quite low. The blackbody curve at the top of the page shows that the energy level emitted by the Earth is relatively low, especially in the microwave region. Therefore it is difficult to collect detailed data and the spatial resolution of passive microwave data sets is generally coarse. The data is well suited to measure and track large scale and global changes.

Applications

Sea Ice Monitoring

Passive microwave sensors have produced a relatively long, continuous record of sea ice. Because of the ability to detect through clouds and during the night passive remote sensing is ideal for measuring polar sea ice. Sea ice observations from passive microwave sensors began in 1972 with the Electrically Scanning Microwave Radiometer (ESMR) aboard NOAA's Nimbus-5 satellite. In 2002 the Aqua Satellite was launched with the advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E) sensor and produced data until 2011. A variety of physical properties could be derived from AMS-E data, including water vapor, cloud liquid water, precipitation, sea surface temperature, sea surface wind speed, sea ice concentration, snow water equivalent, and soil moisture.

Antarctic sea ice concentration, ranging from 0 percent (purple) to 100 percent (white) on captured by AMSR-E on August 7th 2004.

The National Snow and Ice Data Center (NSIDC) researches and provides data on snow, ice, glaciers and climate interactions. A variety of NSIDC Data is available for download through their website. The NSIDC has also recently released visualizations on Google Earth. The time series data display changes in sea ice extent, snow cover and glacier changes. The goal is to make data and visualization more available to the public to increase understanding of the cyrospheres, the frozen water component of the Earth. The data can be downloaded at http://nsidc.org/data/google_earth

.

Soil Moisture

The Soil Moisture Active Passive (SMAP) satellite was launched by NASA on Jan 31, 2015 with an objective of mapping global soil moisture. SMAP also is designed to detect whether soils are frozen or thawed. SMAP contains both an active and passive microwave sensor. The passive microwave radiometer measures Earth's natural microwave emissions. SMAP can map the entire globe with high-resolution radar data every two to three days. The instruments sense conditions in the top 5 cm of soil through moderate vegetation cover to yield globally mapped estimates of soil moisture and its freeze-thaw state.

← Back

Next →

Module Home