The electrically scanning microwave radiometer (ESMR) was an instrument carried by the Nimbus-5 satellite

Nimbus program

The Nimbus satellites were second-generation U.S. robotic spacecraft used for meteorological research and development. The spacecraft were designed to serve as stabilized, Earth-oriented platforms for the testing of advanced systems to sense and collect atmospheric science data...

, precursor to the scanning multichannel microwave radiometer

Scanning Multichannel Microwave Radiometer

The scanning multichannel microwave radiometer [pronounced simmer] was a five-frequency microwave radiometer flown on the Seasat and Nimbus 7 satellites. Both were launched in 1978, with the Seasat mission lasting less than six months until failure of the primary bus. The Nimbus 7 SMMR lasted from...

 (SSMR) and special sensor microwave/imager

Special Sensor Microwave/Imager

The special sensor microwave/imager is a seven-channel, four-frequency, linearly polarized passive microwave radiometer system. It is flown on board the United States Air Force Defense Meteorological Satellite Program Block 5D-2 satellites. The instrument measures surface/atmospheric microwave...

 (SSMI) instruments.

The ESMR instrument only senses horizontally polarized radiation at a frequency of 19 GHz, and can be used to calculate sea ice concentration

Sea ice concentration

Sea ice concentration is a useful variable for climatescientists and nautical navigators. It is defined as the area ofsea ice relative to the total at a given point in the ocean....

. However, results are difficult to intercompare to SSMR / SSMI. The ESMR scanned along the satellite track, leading to a wide range of incident angles; SSMR scanned with a constant angle of 50 degrees, allowing both horizontally and vertically polarised data to be received; SSMR also had 5 instead of one channels, leading to improved sea ice retrievals.