Remote Sensing: Passive Microwave

AMSR2 image of the Arctic Ocean
This image of Antarctica was captured by the Advanced Microwave Scanning Radiometer-2 (AMSR2) sensor aboard the Global Change Observation Mission 1st - Water "SHIZUKU" (GCOM-W1) satellite on 10 February 10, 2020. Ice concentration is color coded, with higher concentrations in white, and lower concentrations in dark purple.
—Credit: University of Bremen Sea Ice Today

Objects at the Earth's surface emit not only infrared radiation; they also emit microwaves at relatively low energy levels. When a sensor detects microwave radiation naturally emitted by the Earth, that radiation is called passive microwave. Clouds do not emit much microwave radiation, compared to sea ice. Thus, microwaves can penetrate clouds and be used to detect sea ice during the day and night, regardless of cloud cover.

Microwave emission is not as strongly tied to the temperature of an object, compared to infrared. Instead, the object's physical properties, such as atomic composition and crystalline structure, determine the amount of microwave radiation it emits. The crystalline structure of ice typically emits more microwave energy than the liquid water in the ocean. Thus, sensors that detect passive microwave radiation can easily distinguish sea ice from ocean.

A major drawback to measuring passive microwave radiation is that the energy level is quite low. As a result, the radiation must be collected over a larger region. Details of the sea ice, such as leads, are not easily detected.

Because of their ability to detect sea ice through clouds during the day and night, passive microwave sensors provide nearly complete images of all sea ice-covered regions every day. These sensors have provided the most complete, long-term observations of sea ice, allowing scientists to detect notable changes in Arctic 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 1978, NASA's Scanning Multichannel Microwave Radiometer (SMMR) provided detailed, reliable information about sea ice. In 1987, a series of DMSP Special Sensor Microwave/Imager (SSM/I) sensors continued the time series, or long-term record, of sea ice data through present. In 2002, NASA launched the Aqua satellite, which carried the Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E) sensor. JAXA followed AMSR-E with the AMSR-2 mission launched in 2012. The system's improved technology complemented the time series of sea ice data. ESMR, SMMR, SSM/I, AMSR-E, and AMSR-2 sea ice data are available from NSIDC.

Last updated: 3 April 2020