Microwave Limb Sounder

Microwave Limb Sounder
Name	Microwave Limb Sounder
Type	Atmospheric remote sensing instrument
Applications	Stratospheric and mesospheric composition, temperature profiling, trace gas retrievals
Launched	Various dates
Operator	NASA, ESA, NOAA

Microwave Limb Sounder is a remote sensing instrument concept used to measure atmospheric composition and temperature by observing microwave emissions tangent to Earth's limb. Developed for stratospheric and mesospheric studies, it has been flown on multiple satellite platforms to provide vertical profiles of trace gases, temperature, and dynamics relevant to climate, ozone chemistry, and air quality. Instruments based on this concept have been central to collaborations among agencies and research institutions worldwide.

Introduction

The limb sounding technique implemented by the instrument concept was motivated by early atmospheric research programs and satellite projects including Nimbus 4, Nimbus 6, Upper Atmosphere Research Satellite, ERS-1, and Envisat where passive microwave observations complemented infrared and ultraviolet sounding from missions such as TIROS, NOAA-15, and Aqua. Development involved teams at organizations including NASA Goddard Space Flight Center, Jet Propulsion Laboratory, European Space Agency, National Oceanic and Atmospheric Administration, and academic groups at Massachusetts Institute of Technology, California Institute of Technology, University of Colorado Boulder, and University of Oxford. Scientific drivers trace to concerns raised by initiatives like the World Climate Research Programme, the Intergovernmental Panel on Climate Change, and the Montreal Protocol for monitoring ozone-depleting substances.

Instrument Design and Measurement Principles

Design centers on a microwave heterodyne radiometer that observes thermal emission along tangential lines of sight, enabling high vertical resolution similar to instruments developed for missions such as UARS, ACE, and ODIN. Core components include a precision antenna assembly often with a conical scan mechanism similar in heritage to designs by Hughes Aircraft Company, low-noise receivers using mixers and local oscillators developed by contractors such as Raytheon, cryogenic front ends inspired by detectors used on COBE and Planck, and backend spectrometers (filterbank, FFT) with heritage in projects at Caltech, JPL, and MIT Lincoln Laboratory. Measurement principles draw on radiative transfer theory formalized in frameworks used at Harvard University, University of Cambridge, and Max Planck Institute for Chemistry to retrieve profiles of rotational transitions of molecules like hydrogen cyanide, hydrogen chloride, chlorine monoxide, nitrogen dioxide, ozone, water vapor, and isotopologues measured in bands similar to those exploited by Microwave Sounding Unit and Advanced Microwave Sounding Unit instruments. Limb geometry offers sensitivity to layered structures and permits retrieval algorithms akin to ozone profile inversion methods developed at NOAA Aeronomy Laboratory and National Center for Atmospheric Research.

Satellite Missions and Platforms

Implementations have flown on platforms managed by agencies and consortia such as NASA, ESA, JAXA, Canadian Space Agency, and NOAA. Notable flight projects include instruments on spacecraft operated by NASA Goddard Space Flight Center and collaborations with missions like Aura, MetOp, SMILES, SCISAT, and experimental payloads on platforms from Ball Aerospace and Boeing. Launch vehicles and programs associated with deployments include rockets and launch services from entities such as United Launch Alliance, Arianespace, and national programs including Indian Space Research Organisation and Roscosmos. Ground segment support has involved facilities at Wallops Flight Facility, Kennedy Space Center, Esrange, and international data centers including archives at NASA Distributed Active Archive Center and European Space Agency Mission Operations Centre.

Data Products and Processing

Data pipelines produce geophysical products—vertical profiles, limb radiance spectra, and gridded reanalyses—managed in formats and standards defined by Committee on Earth Observation Satellites, World Meteorological Organization, and archival systems at NASA Goddard Earth Sciences Data and Information Services Center. Processing chains incorporate level 0 to level 3 production with radiometric calibration, spectral line fitting, inversion using optimal estimation frameworks developed at Rutherford Appleton Laboratory, NCAR, and SRI International, and assimilation into models maintained by centers such as European Centre for Medium-Range Weather Forecasts, Goddard Modeling and Assimilation Office, and UK Met Office. Data products are compared and cross-validated with observations from ground-based networks including Network for the Detection of Atmospheric Composition Change, balloon campaigns organized by National Center for Atmospheric Research and European Centre for Medium-Range Weather Forecasts, and aircraft campaigns coordinated by NASA Armstrong Flight Research Center and DLR.

Scientific Applications

The instrument concept supports studies of stratospheric ozone depletion investigated by research groups at Scripps Institution of Oceanography, University of Bristol, and University of Leeds and informs assessments by the World Meteorological Organization and the Intergovernmental Panel on Climate Change. It provides measurements critical to profiling trace gases implicated in radiative forcing in reports from IPCC working groups, studying polar processes observed in campaigns such as SOLVE and Arctic Research of the Composition of the Troposphere from Aircraft and Satellites, and constraining chemical transport models developed at Jet Propulsion Laboratory, Harvard University, and Max Planck Institute for Meteorology. Applications extend to volcanic plume monitoring relevant to operations at International Civil Aviation Organization, air quality studies in collaboration with Environmental Protection Agency, and long-term climate trend detection used by numerical centers like ECMWF and NOAA National Centers for Environmental Prediction.

Calibration, Validation, and Error Sources

Calibration relies on onboard references and vicarious methods drawing on standards from National Institute of Standards and Technology, intercomparisons with reference instruments at observatories like Mauna Loa Observatory and Ny-Ålesund, and campaigns using aircraft platforms operated by NASA Armstrong Flight Research Center and DLR. Validation sources include correlative datasets from Fourier Transform Spectrometer networks, ozonesonde profiles from campaigns coordinated by WMO and GAW, and comparisons with limb-sounding instruments on Envisat and AURA. Error sources arise from spectroscopic parameter uncertainties cataloged in databases such as HITRAN maintained by groups at Harvard Smithsonian Center for Astrophysics and retrieval assumptions in optimal estimation methods developed at Rodgers, C. D.-influenced teams; additional errors stem from instrument pointing uncertainties, calibration drifts, and cloud-contamination assessed in intercomparison studies by NCAR and Rutherford Appleton Laboratory.

Historical Development and Future Directions

The lineage of limb-sounding microwave radiometry traces to early microwave radiometer experiments on platforms associated with research at MIT, UCLA, and Caltech, progressed through operational implementations on projects led by NASA Goddard, ESA, and industrial partners such as Ball Aerospace and Honeywell. Future directions emphasize constellation concepts promoted by Committee on Earth Observation Satellites and assimilation into next-generation Earth system models developed at IPCC-affiliated institutions, with technology advances in superconducting mixers, compact cryocoolers from vendors collaborating with JPL and NASA Jet Propulsion Laboratory, and synergy with active instruments from projects led by ESA and JAXA for improved spatiotemporal coverage. Emerging mission concepts are being considered by funding bodies including NASA Earth Science Division, European Space Agency Directorate of Earth Observation, and national agencies such as UK Space Agency and Canadian Space Agency.

Category:Remote sensing instruments