cost 723 utls summerschool cargese, corsica, oct. 3-15, 2005 stefan a. buehler institute of...

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

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


Geometry

Antenna

Radiometry



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



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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



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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



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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



Spectroscopy

Limb Spectra

Clouds


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




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




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?

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cost 723 utls summerschool cargese, corsica, oct. 3-15, 2005 stefan a. buehler institute of...

Documents

stefan buehler

utls summerschool cargese

oliver lemke slide

premier mission proposal

instruments basics

intensity of radiation

wavelength antenna size

scan angle h t