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GES DISC DAAC Data Guide: MLS Instrument Document


MLS is the first satellite experiment to use microwave limb sounding techniques. A 1.6 m scanning antenna views the atmosphere in the spectral bands centered at 63, 183, and 205 GHz. The MLS instrument is designed to investigate the chemical composition of the upper atmosphere on a global scale, both day and night, over several years. MLS data are available from the Goddard Space Flight Center (GSFC) Distributed Active Archive Center (DAAC).


B Table of Contents:

1.Document Information:

Revision Date:Fri May 10 11:49:22 EDT 2002
03 April 1995

Review Date: 03 April 1995

2. Instrument Information:

Instrument Long Name, Acronym:
Microwave Limb Sounder, MLS
Instrument Introduction:
The Microwave Limb Sounder (MLS) is one of ten instruments on NASA's Upper Atmosphere Research Satellite (UARS). MLS is the first experiment to use microwave limb sounding techniques.
Instrument Mission Objectives:
The measurement objectives of the MLS instrument are chlorine monoxide (ClO), ozone (O3), water vapor (H2O), temperature and pressure in the stratosphere and mesosphere. A major emphasis is to investigate the relationship between chlorine monoxide and ozone destruction. Simultaneous measurements of ozone and wator vapor will also add to the understanding of stratospheric ozone chemistry. Measurements of water vapor and ozone at higher altitudes than previously studied will give information about chemistry in the mesosphere. Secondary measurement objectives include H2O2, HNO3, and volcanic injections of SO2.
Key Variables:
The measured parameters are temperature, and volume mixing ratios of chlorine monoxide, ozone (at the 183 GHz and 205 GHz bands), and water vapor. The units of measurement are parts per million by volume (ppmv) for chemical species concentrations, and degrees Kelvin for temperature.
Scanning or Data Collection Concept/Principles of Operation:
The MLS instrument consists of a 1.6-m scanning antenna system, three radiometers in spectral bands centered at 63, 183 and 205 GHz, optical multiplexers, and an internal calibration system. The antenna scans in the vertical over the altitude range of 5 to 90 km every 65.536 seconds. A switching mirror inside the radiometer box selects either the atmospheric signal from the antenna system, the calibration from an internal source, or a zero-reference space view. The signals from the switching mirror are then separated to the 63, 183 and 205 GHz radiometers. Next, the radiation is downconverted to intermediate frequency (IF) bands in the 0-3 GHz range. The IF signals are amplified and further converted to six 510 MHz wide spectral bands, each centered at 400 MHz. The bands are then input to six filter banks (O2 from 63 GHz; ClO, H2O2, and O3 from 183 GHz; H2O, and O3 from 205 GHz) where the signals are split into 15 contiguous spectral channels. The power received in each channel is integrated, and the signals are then transmitted to Earth. The resolution of individual channels varies from 128 MHz on the edge to 2 MHz near the center of the spectral line being measured.

3. Instrument Layout, Design, and Measurement Geometry:

MLS Instrument design diagram
List of Sensors:
The MLS instrument has three assembly units: spectrometer, power supply, and the main sensor. The sensor includes the scanning antenna, the radiometer box, multiplexing optics, and the calibration system.
Sensor Descriptions:
Antenna: The antenna subsystem is a dual-offset Cassegrain consisting of three elliptical reflectors. The primary reflector is an offset paraboloid, 1.6 x 0.8 meters, with a 1 meter focal length. The secondary reflector is hyperboloid with dimensions 0.45 x 0.24 meters, and the flat tertiary reflector sends the radiation into the radiometer module. The antenna is scanned in the vertical in discreet steps from about 5 km to 95 km, every measurement cycle of 65.536 seconds.

Radiometers: There are three radiometers in spectral bands centered at 63, 183 and 205 GHz. The 205 GHz radiometer is a single-ended fundamental mount based on the design by Archer and Mattauch [1981] with a GaAs Schottky diode. The 205 GHz radiometer measures the signal from the chlorine monoxide/hydrogen peroxide band and ozone band. The 183 GHz radiometer is basically the same as the 205 GHz radiometer. The 183 GHz radiometer measures the signal from the water vapor and ozone bands. The 63 GHz radiometer is a space-qualified commercial mixer- amplifier module using a pair of Schottky barrier diodes. The 63-GHz radiometer measures oxygen to determine atmospheric temperature.

Optical Multiplexer: The optical multiplexer separates the radiation from the switching mirror into the three radiometers. The 63 GHz beam is separated with a dichroic plate consisting of equally spaced circular holes in a metal plate. The 183 and 205 GHz beams are separated by a polarization grid of finely spaced wires.

Internal Calibration: The MLS onboard calibrations are performed by allowing the switching mirror to direct the radiometer FOVs to view both cold space and an internal ambient blackbody calibration target during each limb scan.

Sensor/Detector Specifications - Optics and Spacing:
      Primary mirror		1.6 m
      Vertical FOV		0.05 degrees
      Spectral frequencies	63 GHz, 183 GHz, 205 GHz

4. Manufacturer of Instrument:

Instrument: Jet Propulsion Laboratory, Pasadena, California

183 GHz radiometer subsystem: Heriot-Watt University and Rutherford Appleton Laboratory, U.K.

5. Calibration:

The MLS onboard calibrations are performed by allowing the switching mirror to direct the radiometer FOVs to view both cold space and an internal ambient blackbody calibration target during each limb scan (every 65.536 seconds).

6. Glossary of Terms:

An integrated collection of hardware containing one or more sensors and associated controls designed to produce data on an environment. MLS is a single sensor instrument.
A device which transmits an output signal in response to a physical input stimulus (as radiance, sound, etc.). The MLS sensor measures atmospheric radiation in the microwave region.

7. List of Acronyms:

ClO             chlorine monoxide
FOV             field of view
GaAs            gallium arsenide
GHz             Giga-hertz
H2O             water vapor
H2O2            hydrogen peroxide
HNO3            nitric acid
km              kilometer
m               meter
MHz             Mega-hertz
MLS             Microwave Limb Sounder
O3              ozone
ppmv            parts per million by volume
SO2             sulfur dioxide
UARS            Upper Atmosphere Research Satellite

Change History

Version 2.0
Version baselined on addition to the GES Controlled Documents List, April 3, 1995.
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Last updated: Aug 31, 2010 03:57 PM ET