Advanced Microwave Sounding Unit

Advanced Microwave Sounding Unit
Name	Advanced Microwave Sounding Unit
Abbreviation	AMSU
Operator	National Oceanic and Atmospheric Administration / National Aeronautics and Space Administration
Type	Microwave radiometer
Launched	1998 (AMSU-A on NOAA-15)
Country	United States

Advanced Microwave Sounding Unit

The Advanced Microwave Sounding Unit is a series of spaceborne microwave radiometers used for atmospheric sounding and remote sensing of temperature, humidity, and surface properties, developed for polar-orbiting meteorological satellites. It supports operational forecasting and climate monitoring across programs such as National Weather Service, European Centre for Medium-Range Weather Forecasts, Japan Meteorological Agency, and research efforts at institutions including Jet Propulsion Laboratory and University of Wisconsin–Madison. The instrument family bridges datasets from earlier sensors like Microwave Sounding Unit to later systems such as Atmospheric Infrared Sounder and Cross-track Infrared Sounder.

Introduction

AMSU instruments provide passive microwave observations in multiple frequency bands tailored to atmospheric absorption lines and surface emission; channels near the oxygen complex and water vapor lines enable vertical profiling comparable to radiosonde networks like the Global Climate Observing System profiles. The sensors operate from platforms including NOAA-15, NOAA-18, and MetOp-A, contributing to reanalysis projects such as ERA-Interim, MERRA-2, and JRA-55. Data stewardship is performed by agencies including National Centers for Environmental Prediction and the European Organisation for the Exploitation of Meteorological Satellites in coordination with climate centers like the Intergovernmental Panel on Climate Change reporting activities.

Design and Instrumentation

The AMSU family typically comprises two subinstruments with distinct channel sets: a higher-frequency module for temperature sounding and a lower-frequency module for humidity and surface/precipitation sensing, building on engineering heritage from the Microwave Humidity Sounder concept and technology validated at Raytheon and Ball Aerospace. The antenna and radiometer chain incorporate a scanning reflector, feedhorn array, superheterodyne receivers, and cryogenic calibration references influenced by designs from Jet Propulsion Laboratory and NASA Goddard Space Flight Center. Spectral channels are centered on microwave absorption features near 23.8 GHz and the oxygen complex around 50–60 GHz, enabling vertical weighting functions comparable to data assimilation inputs used by European Centre for Medium-Range Weather Forecasts and NOAA/NWS models. Thermal control and pressure stabilization systems derive from platform bus designs used on Polar Operational Environmental Satellites and MetOp spacecraft.

Data Products and Processing

Raw radiances from AMSU are processed into geolocated brightness temperatures, swath-calibrated radiance datasets, and derived retrievals such as temperature profiles, atmospheric water vapor columns, and surface emissivity estimates integrated into products at centers like National Environmental Satellite, Data, and Information Service and EUMETSAT. Level 1 processing includes calibration using cold-space views and onboard blackbody references, while Level 2 and Level 3 products employ inversion schemes, variational retrievals, and optimal estimation methods common to processing at NOAA NESDIS and European Centre for Medium-Range Weather Forecasts. Data formats follow metadata standards adopted by Committee on Earth Observation Satellites with distribution through systems such as Data Assimilation Research Testbed and global archives maintained by NASA Langley Research Center and National Snow and Ice Data Center.

Applications in Meteorology and Climate

AMSU-derived products underpin numerical weather prediction at centers including European Centre for Medium-Range Weather Forecasts, NOAA/NCEP, and Met Office, providing constraints for global models like GFS and contributing to reanalyses such as ERA5 and MERRA. Climate monitoring uses long-term AMSU records to detect tropospheric temperature trends assessed in reports from the Intergovernmental Panel on Climate Change and climate studies led by institutions like NOAA and University of Oxford. Additional applications include tropical cyclone intensity estimation employed by the National Hurricane Center, precipitation retrieval comparisons with Tropical Rainfall Measuring Mission, and polar surface studies coordinated with National Snow and Ice Data Center and Arctic Monitoring and Assessment Programme efforts.

Calibration, Validation, and Error Characterization

Calibration strategies rely on cold-space observations, onboard calibration loads, and cross-calibration with instruments such as Advanced Very High Resolution Radiometer and Radiometer Atmospheric Sounder; validation campaigns compare AMSU retrievals with radiosonde networks like Global Climate Observing System stations, dropsonde missions supported by NOAA Aircraft Operations Center, and field campaigns coordinated by NASA Earth Science. Error characterization addresses systematic biases, inter-satellite drift, antenna pattern correction, and radio-frequency interference, with techniques developed in collaboration with EUMETSAT, National Institute of Standards and Technology, and research groups at Massachusetts Institute of Technology and University of Colorado Boulder.

History and Platforms

The AMSU concept evolved from microwave sounding instruments flown on TIROS and later NOAA-15 onward, with AMSU-A and AMSU-B units carried on successive polar-orbiting satellites including NOAA-15, NOAA-18, and MetOp-A. Operational deployment tied into programs by NOAA, EUMETSAT, and NASA and interfaced with ground segments developed by contractors such as Raytheon and Ball Aerospace. The dataset continuity strategy involved intercalibration with predecessor instruments like Microwave Sounding Unit and successor systems on missions such as Suomi NPP.

Future Developments and Successor Instruments

Successor instruments integrate lessons from AMSU into advanced microwave sounders and combined microwave/infrared suites exemplified by Microwave Sounding Unit successors and instruments aboard Suomi National Polar-orbiting Partnership, MetOp-SG, and planned missions by European Space Agency and National Aeronautics and Space Administration. Future directions emphasize higher spectral resolution, improved radiometric stability, radio-frequency mitigation coordinated with International Telecommunication Union, and synergistic data fusion with hyperspectral sounders developed at Jet Propulsion Laboratory and research efforts at California Institute of Technology and University Corporation for Atmospheric Research.

Category:Remote sensing instruments