cost 723 utls summerschool cargese, corsica, oct. 3-15, 2005 stefan a. buehler institute of...
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![Page 1: COST 723 UTLS Summerschool Cargese, Corsica, Oct. 3-15, 2005 Stefan A. Buehler Institute of Environmental Physics University of Bremen](https://reader034.vdocuments.us/reader034/viewer/2022051618/56649d375503460f94a10861/html5/thumbnails/1.jpg)
COST 723 UTLS Summerschool
Cargese, Corsica, Oct. 3-15, 2005
Stefan A. Buehler
Institute of Environmental Physics
University of Bremen
www.sat.uni-bremen.de
OBS 1: Microwave Limb Sounding
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Stefan Buehler, COST 723 UTLS Summerschool, Cargese, Oct. 3-15, 2005
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Overview
Basics of limb sounding instruments
Basics of the measurement
Past, present, and future instruments
Summary
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Overview
Basics of limb sounding instruments
Geometry
Antenna
Radiometry
Basics of the measurement
Past, present, and future instruments
Summary
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ho: Platform altitude
θ: Scan angle
ht: Tangent altitude
typically:
RE = 6000 km
ho = 600 km
ht = 6-60 km
Geometry
Measure thermal radiation from the atmosphere (passive!)
Good altitude resolution, because we can scan vertically
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Small ∆θ ↔ large ∆ht.
Accurate Pointing necessary.
Narrow Field of View necessary.
(Figure: Oliver Lemke)
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Antenna
Field of view diameter = “beam width”, even for passive instrument
Given by angle of half power of received (or transmitted) radiation
Diffraction theory:θHPBW ~ Wavelength / Antenna size
(Figure: Oliver Lemke)
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Antenna Technology
θHPBW ~ Wavelength / Antenna size
Needs large antenna, particularly for low frequencies
Scan angle small
Needs accurate scanning mechanism (or wobble the whole satellite)
The Odin reflector mounted on the spacecraft body. (Source: PREMIER mission proposal)
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The Radiometer Challenge
The absolute power of thermal radiation in the mm-wave spectral range is low.
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(Kraus, J. D., Radio Astronomy, McGraw-Hill Book Company, 1966)
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The Radiometer Challenge
The absolute power of thermal radiation in the mm-wave spectral range is low. (The peak of the Planck function is in the infrared.)
Need to amplify the signal by many orders of magnitude for detection.
No good amplifiers for frequencies above approximately 100 GHz (technology constantly moving)
State of the art: Heterodyne Receivers
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A Typical Heterodyne Radiometer
RF = Radio frequency
LO = Local oscillator
IF = Intermediate frequency
(Figure: Oliver Lemke)
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The Heterodyne Principle
Mixer generates signal with νIF = | νRF - νLO |
This can then be amplified and analyzed with a spectrometer
Intensity unit: Brightness temperature = The temperature a black body would need to generate the same intensity of radiation
(Figure: Oliver Lemke)
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The Radiometer Formula
TNET = Noise equivalent temperature (noisiness of individual measurement)
TSys = System noise temperature (characteristic noise of measurement system)
ΔB = Frequency bandwidth
Δt = Integration time
tB
TT
Sys NET
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The Radiometer Challenge (2)
We cannot make ΔB and Δt as large as we want. (We want spectral resolution, and the satellite flies by fast.)
Need low noise receivers.
Mixer and first amplifier most critical, because their noise is amplified by subsequent stages.
Cool mixer to low operation temperature. Best: Superconducting (SIS) mixers at 4 K.
Cooled HEMT amplifiers.
tB
TT
Sys NET
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Submillimeter-wave Sensor at –269 C
640 GHz SIS Mixer
4 K Mechanical Cooler
4 K
20 K
100 K
0.4 mm
SIS: Superconductor-Insulator-Superconductor
Superconductive Device:
Nb/AlOx/Nb
SMILES
(Figures: SMILES Team)
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Overview
Basics of limb sounding instruments
Basics of the measurement
Spectroscopy
Limb Spectra
Clouds
Past, present, and future instruments
Summary
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Microwave Spectroscopy
Absorption by a gas (Lambert-Beer’s Law):
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Absorption coefficient α determined by
Continua
Electronic transitions (1015 Hz, UV visible)
Vibrational transitions (1013 Hz, Infrared)
Rotational transitions (1011 Hz, mm / sub-mm)
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Model: ARTS (www.sat.uni-bremen.de/arts)
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Absorption coefficient α given by
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Limb Spectra
H2OO3
(Emde et al., J. Geophys. Res., 109(D24), D24207, 2004)
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Comparison to IR Limb Sounder
Annual global mean probability of limb transmittance >3%, as estimated from ECMWF fields of temperature, water vapour, water and ice clouds sampled globally one day in ten over a year (Kerridge et al., ESA UTLS study final report).
Clouds significant, but less critical than for IR
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Overview
Basics of limb sounding instruments
Basics of the measurement
Past, present, and future instruments
UARS-MLS and MAS (past)
EOS-MLS and Odin (present)
SMILES and PREMIER (future)
Summary
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The first two Microwave Limb Sounders
O2 63 GHz
H2O 183 GHz
O3 184 GHz
ClO 205 GHz
MAS MLS
‘Millimeterwave Atmospheric Sounder’ ‘Microwave Limb Sounder’
On the Space Shuttle On the UARS satellite
Atlas 1, March 1992 1991-1998
Atlas 2, April 1993
Atlas 3, November 1994
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A sample MAS H2O Measurement
MAS Water Vapor Band
MMC 20075,
27.3.1992,
16.59 GMT
59° N 021° W,
Tangent altitude range:
6-54 km
Retrieval = Calculate trace gas profile from measured spectra
Requires radiative transfer model = forward model
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Retrieval by Optimal Estimation
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H2O Retrieval Example
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UARS/MLS Highlights
Stratospheric ozone and chlorine chemistry research
Impact of volcano eruptions on the stratosphere (SO2 loading)
Tropical dynamics (‘tape recorder’ effect in the tropical lower stratosphere).
Publication overview at: http://mls.jpl.nasa.gov/joe/um_pubs.html
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(Source: MLS Website)
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(Source: MLS Website. See also: Read et al., Geophys. Res. Lett. 20, 1299-1302, 1993.)
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The Tropical LS Taperecorder
Transit time from 100 to 46 hPa > 6 months
(Data created by H. Pumphrey. See also: Mote et al. J. Geophys. Res., vol. 101, 3989-4006 [1996])Deviation from mean VMR [ppm]
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EOS MLS
The next Generation of the Microwave Limb Sounder MLS
Launched July 15, 2004, on the Aura satellite
(Figure: MLS Webpage)
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EOS MLS Observed Species
(WATERS, et al.: EOS MLS ON AURA, IEEE GRS submitted 2005)
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Odin
Small satellite with just two limb sounders, one sub-mm, one UV-visible
The whole platform is moved, not just the antenna
Launched February 20, 2001
Frequencies:118.25 - 119.25 GHz 486.10 - 503.90 GHz 541.00 - 580.40 GHz
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JEM / SMILES
SIS mixer, cooled to < 4 Kelvin
Proof of concept for later instruments of this type
Very accurate ozone and chlorine species data
Measurements at 625 and 650 GHz
‘Superconducting Sub-Millimeter Wave Limb Emission Sounder’
Launch 2008 on the ‘Japanese Experimental Module’ (JEM) of the International Space Station
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PREMIER
Combined IR and sub-mm limb imagers
Proposed to current ESA call, possible launch 2013
Focus on chemistry climate interaction in the UTLS
Sub-mm measurements at 320-360 GHz
Array detectors
Main Products: O3, H2O, CO, N2O, HNO3, ClO, CH4, CFC11, CFC12, C2H6, SF6
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Overview
Basics of limb sounding instruments
Basics of the measurement
Past, present, and future instruments
Summary
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Summary (1)
Limb sounding is well suited for the measurement of trace gases in the stratosphere and upper troposphere.
Particularly good for chlorine species and humidity. (Also many other trace gases.)
Good altitude resolution (1–2 km).
Horizontal resolution is ≈ 100 km.
We look at emission lines of trace gases.
Emission means continuous measurement, day & night.
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Summary (2)
Less affected by clouds than UV and IR techniques.
Cirrus clouds must be taken into account in the UT.
Past: UARS-MLS and MAS.
Present: EOS-MLS and Odin.
Future: SMILES? PREMIER? Third generation of MLS?
Technological challenges are low noise receivers with large bandwidth. Antenna size is also always a cost factor.
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Thanks for your attention.Questions?
...