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1. Introduction

Public interest in atmospheric research has remainedhigh since the detection of the Antarctic ozone hole in19suconisanprhoerawaanma

will have an important effect on the ozone layer, sur-face ultraviolet levels, and the earths future climate.For the understanding of these and many other at-

WvalLenfurKaSeeMuO.serGe

R200

084. Meanwhile, the Montreal Protocol has beenccessful in reducing the emission and atmosphericcentration of chlorofluorocarbons CFCs. There

a big debate, however, on the question of whetherd, if at all, by when the ozone layer will return to itse-ozone-hole state. Climate is changing as green-use gas concentrations rise, and there is a consid-ble but unexplained increase in stratosphericter vapor, which could have important chemicald climate effects. The combined effect of theny ongoing changes to the atmospheric system

mospheric processes, a detailed knowledge of thecomposition and distribution of trace gas constituentsas well as their evolution with time is indispensable.

One of the techniques that is able to simulta-neously measure vertical distributions of ozone- andclimate-relevant atmospheric constituents, is limbsounding with a Fourier-transform spectrometer inthe infrared spectral region. Fourier-transform in-frared experiments for remote sensing of atmosphericspecies in the limb-viewing mode have been used nowfor almost three decades on stratospheric balloons,aircraft, and space platforms. Many of these exper-iments apply the solar occultation technique.16 Asmaller number of instruments has been used to an-alyze the emission of the atmosphere.710 From air-borne carriers balloons and aircraft, the latterapproach allows observations with a maximum de-gree of flexibility in terms of scanning strategy andobservation time. From spacecraft, limb-emissionspectroscopy offers the advantage of continuous ob-servation on a global scale with high samplingrates. Therefore the basic decision for all Michel-son Interferometer for Passive Atmospheric Sound-ing MIPAS instruments had been to design theinstruments for thermal limb-emission sounding

hen this research was performed, F. Friedl-Vallon felix.friedl-lon@imk.fzk.de, G. Maucher, O. Trieschmann, A. Kleinert, A.gel, C. Keim, H. Oelhaf, and H. Fischer were with the Institut

Meteorologie und Klimaforschung, Forschungszentrumrlsruhe, Postfach 3640, 76021 Karlsruhe, Germany. M.feldner was with the Meteorologisches Institut, Universitatnchen, Theresienstrasse 37, 80333 Munchen, Germany.Trieschmann is now with the Bundesanstalt fur Gewas-

kunde, Kaiserin-Augusta-Anlagen 15-17, 56068 Koblenz,rmany.eceived 8 August 2003; revised manuscript received 10 March4; accepted 24 March 2004.003-693504163335-21$15.0002004 Optical Society of America

1 June 2004 Vol. 43, No. 16 APPLIED OPTICS 3335esign and characterization oichelson Interferometer fortmospheric Sounding MIPA

lix Friedl-Vallon, Guido Maucher, Meinhard Seefne Kleinert, Anton Lengel, Corneli Keim, Herman

MIPAS-B2 is a balloon-borne limb-ement is a Fourier spectrometer thacryogenic temperatures. Essentialsystem, which is based on an inertreference system. The major scienplete trace gas families in the stratodirections. The specifications, thethe results of characterization measSociety of America

OCIS codes: 120.6200, 010.1280the balloon-bornessiveB2

er, Olaf Trieschmann,elhaf, and Herbert Fischer

ion sounder for atmospheric research. The heart of the instru-rs the mid-infrared spectral range 414 m and operates at

this application is the sophisticated line-of-sight stabilizationvigation system and is supplemented with an additional starenefit of the instrument is the simultaneous detection of com-re without restrictions concerning the time of day and viewingn considerations, the actual realization of the instrument, andents that have been performed are described. 2004 Optical

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Table 1. Milestones of the MIPAS-B Project

Date Milestone

S-B1AS-Bnt of

na, dna, st wi

latituTHElatitutical

sat vticalt vacond

333stead of solar occultation, even though a signifi-ntly higher instrumental effort is necessary totain sufficiently high signal-to-noise ratios11NRs.The MIPAS-B1 instrument was the first of a series

MIPAS balloon- and aircraft-borne emissionunders.1214 MIPAS-B2, described here, buildson experience gained with its precursor MIPAS-, which was operational for several years and con-buted to the European Arctic Stratospheric Ozoneperiment EASOE with two flights, providing,., the first vertical profiles of the key reservoirecies ClONO2 and N2O5 inside the arcticrtex.1518 MIPAS-B1 has served as proof of con-t for all succeeding MIPAS versions.

Based on the experience achieved with MIPAS-B1,e satellite version was proposed to and accepted bye European Space Agency in the late 1980s as ae payload of the environmental research satellitevisat.19 This satellite was launched on 1 March02, and MIPAS on Envisat is delivering data thate largely compliant with specifications.20The MIPAS-B2 instrument described here was de-loped primarily between 1992 and 1994 and had itsiden flight on 11 February 1995. Since then sev-l upgrades have enhanced its performance. It

rticipated successfully in all recent major Euro-an field campaigns, such as the Second Europeanratospheric Arctic and Mid-latitude ExperimentESAME, the validation campaign for the ILAS

proved Limb Atmospheric Spectrometer experi-nt on board ADEOS Advanced Earth Observingtellite, the Third European Stratospheric Experi-nt on Ozone THESEO, the Validation of Inter-tional Satellites and Study of Ozone LossINTERSOL experiment, and the Envisat valida-n program see Table 1. With data from thesempaigns, we have proved to be able to measurertical profiles of all relevant molecules belonging toe NOy family. Besides the NOy compounds, the

ost complete set of source gases could be quanti-ively evaluated, in particular N2O, CH4, CFCs,H6, and SF6, along with H2O and O3. Scientific

19861989 Development of MIPA19891992 Four flights with MIP19931994 Design and developmeFeb., March 1995; Flights 1, 2 Two flights from KiruFeb., March 1997; Flights 3, 4 Two flights from KiruJuly 1997, Flight 5 Gap, mid-latitude flighMay 1998, Flight 6 Aire sur lAdour, mid-Jan. 1999, Flight 7 Kiruna, weak vortex,Apr. 1999, Flight 8 Aire sur lAdour, mid-Jan. 2001, Flight 9 Kiruna, vortex, synopFeb. 2002, Flight 10 Kiruna, edge of vortexSept. 2002, Flight 11 Aire sur lAdour, EnviDec. 2002, Flight 12 Kiruna, vortex, synopMarch 2003, Flight 13 Kiruna, vortex, EnvisaJuly 2003, Flight 14 Kiruna, polar summer

6 APPLIED OPTICS Vol. 43, No. 16 1 June 2004dies based on these data contributed to importantues related to ozone research such as denitrifica-n21,22 and dehydration,23 nitrogen partitioning2427luding its diurnal variation,28 chlorine activationd ozone loss,29,30 polar stratospheric clouds,31 as-ssment of spectroscopic data,32 and satellitelidation.3337 Data generated by MIPAS-B2 wereo used in support of designing and validating re-eval algorithms for MIPAS-Envisat.38This paper will focus on the specifications of thetrument and the hardware solutions and the ra-nale behind them, and it will outline the charac-ization measurements that have been performedprove compliance with specifications.

Instrument Specifications

Fundamental Design Considerations

e decision to use limb emission Fourier spectros-y stems from the following reasoning:

Fourier spectroscopy offers high resolution,oad spectral coverage, and complete simultaneitythe measurement of all species. The detection of emitted radiance obviates thestraints of absorption spectroscopy. The location

d the time of the measurement can be chosenely, independent of external radiation sourcesch as the sun, the moon, or stars. Viewing the earths limb allows a height reso-ion of the measurement that cannot be reached by- or nadir-looking instruments. Because of theg path through the atmosphere provided by this

ometry, even species with very low concentrationsn be detected.

e crucial requirements for the design of a high-olution mid-infrared limb sounder are derivedm the scientific objectives and the information con-t of limb spectra. The most important specifica-

ns concern the spectral coverage and resolution,e sensitivity and radiometric accuracy, and the al-ude resolution and pointing accuracy. Basic re-

1 two flights during EASOEMIPAS-B2

enitrified vortex, SESAME campaigntable spring vortex, CHORUS campaign, ILAS validationth vortex remnants, CHELOSBA projectde spring validation of L2 code of MIPAS-Envisat

SEOde spring with filaments, THESEO

PSC, POSTA

alidationPSC, POSTAlidationitions, Envisat validationlutuplongeca

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Table 2. Target Molecules and Spectral Regions of High Scientific Interest

Channel Target MoleculesLower Boundary

cm1Upper Boundary

cm1

, CHFHNOirements for MIPAS-B and MIPAS on Envisat areilar, although in the case of MIPAS-B2 somede-offs led to different technical solutions that aresible on a balloon-borne platform but not on a

acecraft. Detailed requirement considerations forIPAS on Envisat are given in the MIPAS Scienceport.19 The following subsections outline the re-irements for the balloon application.

Spectral Coverage

e main scientific objective of MIPAS-B2 is to de-er vertical concentration profiles of trace gas con-tuents involved in the ozone depletion mechanismsd in the greenhouse effect. Inspection of spectraows that this requires at least a spectral coveragem 780 cm1 ClONO2 up to 1925 cm

1 NO.e strong dynamic range within this broad spectralerage implicitly requires a separation in different

annels. The boundaries between the channels arest conveniently placed in areas of low scientificerest. A trade-off among the number of channels,

e channel width, and the complexity led us to theecification summarized in Table 2. To keep thevantage of complete simultaneity of the measure-nts, all of these spectral channels must be re-ded at the same time.

Spectral Resolution

IPAS takes advantage of the limb geometry; henceis not necessary to derive the vertical trace gastribution directly from the line shapes. In conse-ence, the requirement for the spectral resolution istated by the typical FWHM of weak pressure-

oadened atmospheric lines in the middle tropo-here 0.05 cm1, which is typically the lowerundary of limb sounding. At higher altitude levels,e blending due to pressure broadening decreasespidly, so that spectral resolution requirements tendease. Analysis of synthetic spectra39,40 shows thate information content of the spectra for our appli-tion is only slightly degraded by using an unapo-ed spectral resolution of 0.035 cm1. Thisresponds to an apodized spectral resolution of bet-than 0.07 cm1.

Sensitivity and Radiometric Accuracy

e sensitivity i.e., the required noise-equivalentectral radiance NESR is ultimately determinedthe desired detectability and precision of retrievedce constituent concentrations. These quantities

pend on the SNR of the target spectral lines of agle spectrum with a certain radiance, which pri-

1 HNO3, ClONO2, ClO, O3, CO2, CCl4, CFCl3, CF2Cl22 N2O5, CH4, N2O, H2O, HDO, HOCl, CF4, COF2 O3,3 NO2, H2O4 NO COrily is a function of line strength, emitter amountd temperature, the number of lines or spectral gridints that can be used for the retrieval, and the timeat can be allocated for this individual measure-nt. Less abundant species with weak or only asuitable signatures, and generally upper tangent

itudes and cold temperatures e.g., in the arcticnter stratosphere, are most critical.Thus, the objectivewhich species shall be detect-le under which geophysical conditions and in whichitude regionsdrives the requirements for theSR. Inspection of synthetic spectra39,40 shows

at, as a rule of thumb, the weakest lines to betected in a single spectrum represent the black-dy radiance for an emissivity of 2% at a typicalatospheric temperature of 210 K. Hence, as-ming that for a definite detection a SNR of 4 isuested, the required NESR in a single spectrum

n be determined as the radiance of such a black-dy with an emissivity of 0.5%. Actually, this re-irement would lead to extreme low NESR values ate short-wavelength side of the spectral coverage,ich are hard to achieve for technical reasons. Int, this requirement can be met to 1500 cm1 butuld demand different technical solutions e.g.,ch colder instrument temperatures at higher fre-

encies. In practice, the situation is more flexible,several lines of the same species can be used in therieval and since longer integration times can beplied. Flexibility with respect to integration timean important advantage of balloon-borne limbission sounding, because it permits the adjust-nt of the observation scheme to the scientific ob-tives. The upper limit of spectral radianceeived at the entrance of a limb sounder observing

e atmosphere in local thermodynamic equilibriumm the lower troposphere up to 40 km is defineda blackbody spanning the atmospheric tempera-

re range 190240 K. The NESR requirementwell as the typical upper limit of the dynamic

nge is plotted in Fig. 1 for illustration.The absolute radiometric accuracy, i.e., the accu-cy to which the radiance of the calibrated spectrumknown, must be 1% within the long-wavelengthnd i.e., between 750 and 1000 cm1. Theiver for this requirement is mainly the need forcurate temperature retrievals. For the remainingectral regions, the requirement can be relaxed to10001700 cm1 and 3% at the short-

velength side of the covered spectral region. Thediometric accuracy ultimately defines the best pos-

2Cl, SF6 H2O 770 9703, ClONO2 1200 1370

1585 16851845 1925 2300

1 June 2004 Vol. 43, No. 16 APPLIED OPTICS 3337acsp2%wara

sibam

E.

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333le absolute accuracy of retrieved trace gasounts.

Field of View

e instruments field of view FOV has to ensureat the observed air volume is well defined and thate different spectral channels are collocated. It hascisive influence on the effective altitude resolution.IPAS-B2 is operating from a balloon-borne plat-m. Typical observer heights are 2840 km, de-nding on the balloon size used. Even with annitesimal FOV, the limb-observing geometry im-

es that the half-width of the weighting functionll not be smaller than a certain limit value. Withnite FOV, the half-width is growing from this limit

lue with the FOV diameter. In practice, theoice of the FOV for the instrument is a trade-offtween instrument size and complexity originatingm the need for a large telescope on one side andective altitude resolution on the other side. Theuirement was to limit the FOV diameter mapped

the lowest tangent height to 3 km. Depending one observer height, this translates into an angularmeter of the FOV of 4.5 to 5.5 mrad. Accordingly,

e specification for the instrument FOV was set torad.

Pointing Requirements

e atmosphere is highly structured in the verticalection; in contrast, it is comparably homoge-ous in the horizontal direction. Therefore thee-of-sight LOS requirement is split into re-irements concerning the elevation and the azi-th of the beam. The requirements are defined

terms of a priori pointing accuracy, stability, andowledge. The specification for the a priori accu-cy in elevation describes the accuracy with whichpredefined tangent height can be acquired for artain measurement. With the achievable heightsolution prescribed by the FOV of 3 km at the

est tangent point, an a priori accuracy acquisi-n accuracy of 1 km was regarded as sufficient.

. 1. NESR requirements thin lines and expected maximumctral radiance thick lines.

8 APPLIED OPTICS Vol. 43, No. 16 1 June 2004is corresponds to approximately 1.5 mrad a pri-i pointing accuracy.The requirement for pointing stability arises frome need to minimize amplitude modulations causedshort-term LOS fluctuations within one interfero-

am and to avoid a broadening in the wings of theV caused by long-term drifts of the LOS during

e or more, coadded, interferograms. The first re-irement, with 0.2 mrad during 10 s, is ratheringent.41 The second one, with accepted varia-ns of 1.0 mrad within 15 min maximum coadditione, is less critical.

The knowledge requirement is deduced from theed to limit the errors that are caused by a falsesignment of the tangent height during the re-eval process. To keep these errors smaller thaner systematic errors, the knowledge of the tan-

nt height has to be better than 300 m 3, cor-sponding to an angular knowledge of better than

mrad.In azimuth, the requirements are significantlyaker because of the higher horizontal homogeneitythe atmosphere. The requirements for acquisi-n accuracy, stability, and knowledge are only 1.

Instrument Description

Overview

e instrument can be divided into two independentical units: the cryogenic spectrometer and theS system see Figs. 2 and 3. The cryogenic spec-meter consists of a telescope, a Michelson inter-ometer, and a detector system. The LOSbilization system of MIPAS-B2 is made up of anitude and heading reference system AHRS pro-ing the Earth reference, an azimuth stabilization

Fig. 2. Functional scheme of the MIPAS-B2 instrument.

meansya c

B.

Aletrlatitydoasmefor

Fig -mirrof t tecto

FigSwchanism, an elevation stabilization mechanism,d a star reference system. Spectrometer and LOSstems need control electronics, which are housed inommon insulated rack.

. 3. Schematic drawing of the MIPAS-B2 optical setup: 1 scanhe double-pendulum interferometer, 6 beam-splitter unit, 7 de

. 4. MIPAS-B2 gondola nearly ready for launch from Esrange,eden, on 12 February 2002.Gondola and Structure

l these systems, together with batteries and telem-y equipment, are accommodated in an aluminumtice structure, combining high stiffness and stabil-with modularity see Fig. 4. This kind of gon-

la structure, which has been widely used intronomic applications for many years, is a develop-nt of the Geneva Observatory. We have chosen itMIPAS-B2 because of its proven versatility and

se of construction and maintenance. The struc-re has dimensions of 2.06 m 1.85 m 1.87 m. Iters an instrument mounting bay of approximately

1.2 m 1.7 m plus additional small mounting

ys for auxiliary equipment within the frameworkucture. The weight of the structure alone isghly 100 kg. The instrument itself is attached

thin the mounting bay with steel shock absorbersd thermally insulating glassfiber elements.The instrument consists of two thermally sepa-ted levels of an aluminum rail structure. The tele-pe, interferometer, and detector system constitute

e lower part of the instrument structure and areerated at cryogenic temperatures. The scan mir-, star camera, and AHRS are mounted on the up-

r level of the structure and work at ambientperature. The lower part is contained in an air-

ht insulating polyurethane box; the upper part isused in an airtight aluminum box but with a doorfront of the scan mirror that can be opened re-tely by telecommand. The two parts are con-cted with high mechanical precision by glassfiber

or, 2 star camera, 3 blackbody, 4 telescope, 5 moving partr dewar, 8 mounting frame.

1 June 2004 Vol. 43, No. 16 APPLIED OPTICS 3339eatuoff2 mbastrrouwian

rascothoprorpetemtighoinmone

elements and an elastic airtight silicone tunnel forthe incoming radiation.

The gondola is covered by an insulating blanket toprrasidgewecrame

C.

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th1,inde

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scope should provide a vertical transmission ratio of2.4; the horizontal ratio is less clearly determined,since there is no stringent specification for the hori-zobedirarme

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Thsicpesefixalltorfol

alifercapenereflthenbequwisigfiebealicu

334otect the instrument and the electronics from solardiation during daylight flight time. The uppere is open to allow radiative cooling of the heat-

nerating components of the instrument. The totalight of the instrument ready for flight, includingshpads, cover, batteries, and telemetry equip-nt, is 560 kg.

Cooling System

e cryogenic part of the instrument is operated at aperature of 215 K. On the ground, this tem-

rature is achieved by gas cooling. Liquid nitrogenevaporated in the sealed housing of the optics. Inis way the instrument is cooled by cold dry nitrogens and is kept slightly above ambient pressure, pre-nting moisture from entering the system. The

perature is regulated by controlling the amountevaporated liquid nitrogen with a low-temperaturelve.Shortly before launch of the payload, the connec-n to the liquid nitrogen supply is cut, and the in-ument is thermally floating without any activeling. The instrument temperature is slightly ris-during ascent. During night, the ambient tem-

rature at float is in the same range as thetrument temperature; in consequence, the instru-ntal temperature drift is small, and the instru-nt nearly reaches isothermy. With sunrise, theter insulation temperature is going up substan-lly because of the radiative heat load, and the in-ument temperature starts to rise again with a rate0.51 Kh.The performance benefit in SNR compared withe uncooled instrument is a factor of two in channela factor of three in channel 2, a factor of four to fivechannel 3, and a factor of five to eight in channel 4,pending on the signal radiation load.

Telescope

e maximum allowed off-axis angle of the beam ine interferometer is determined by the desired spec-l resolution and the high-frequency boundary of

e spectral coverage: OPDmax max12.

ith a maximum optical path difference OPDmax of.5 cm and a typical upper spectral boundary max of25 cm1, a maximum off-axis angle of 6 mrad ate entrance pupil of the interferometer is acceptable.e input off-axis angle of the whole spectrometer,wever, is determined by the desired height resolu-n at the tangent point. The specification for thisolution, which originates from the vertical exten-n of the weighting function, is 3 km see Section 2responding to an input off-axis angle of 2.5 mrad.is requirement, imposed by the desired height res-tion, is more stringent than the interferometricstraint of 6 mrad. To obtain the optimal SNR it

necessary to use the maximum throughput allowedthe interferometer. Therefore a telescope had toincluded in the instrument design. This tele-

0 APPLIED OPTICS Vol. 43, No. 16 1 June 2004ntal extension of the FOV, as the atmosphere canconsidered sufficiently quasi-homogeneous in thisection. Output angle pattern as well as exit pupil

e of radial symmetry to achieve optimum interfero-tric performance.

The chosen telescope is a simple three-mirrortup. All mirrors are made from Zerodur and areted with gold but without protective coating.ce the mirror surfaces are spherical and arranged

an off-axis setup in one horizontal plane, the tele-pe generates spherical aberrations. FOV defini-n, as well as stray-light suppression, is necessaryinly in the vertical direction, however. The tar-

t, the horizon, is focused on two intermediate fieldps. Both stops are placed in the vertical, or me-ional, foci of the mirrors, and are of rectangularape. The second field stop has an effective width% smaller than the first one. The entrance aper-re stop is located approximately 68 cm in front ofe first telescope mirror and is imaged on an inter-diate Lyot stop, which again is10% smaller than

e image of the entrance aperture. This setup ofps is intended to effectively prevent light that has

en diffracted at the first set of stops from enteringe system. The recollimation mirror is bent out ofe telescope plane. Thus the aberrations are dis-buted in both directions. The quality of the out-t beam is good enough to avoid degradation of theectral resolution of the interferometer; the ex-cted deterioration in the critical short-wavelengthion is below 2%.

Interferometer

e interferometer is a modified version of the clas-al two-port Michelson setup. The double-ndulum interferometer42 is characterized by aparation of the interferometer into two units: aed beam-splitterretromirror unit and a rotation-y oscillating structure with two cube corner reflec-s see Fig. 3. This arrangement has thelowing advantages:

1 Interferometric stability is improved. A stablegnment of the optical components of the inter-ometer is essential to obtain good modulation andlibration stability. In case of the double-ndulum interferometer the only moving compo-nts are the cube-corner reflectors. Since theyect incoming light back in the incidence direction,

e alignment accuracy is defined solely by the inher-t accuracy of the cube-corner reflectors and theam-splitterretromirror unit. There is no re-irement for a relative alignment of the two unitsth interferometric precision. The mechanical de-n of the whole interferometer structure is simpli-d; the difficult tasks are reduced to building aam-splitterretromirror unit that remains wellgned under cryogenic conditions and to findingbe corner reflectors with the same property.

2 Rotational bearings are rugged. High-precision bearings are commercially available andcan be easily modified to withstand cryogenic tem-pecom

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merotreq

coupeheupcomcomsabestrwiFaTh0.2

diffraenpeThthanThlas

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netoe

The change in beam deviation caused by thechange of the tilt of one cube corner mirror from 0 to5 during the rotary motion is specified to be smallerthnewocumewhatth

twtimmeguZOdizfersparma58ali

lasEvposatioghdefrakewh

metraintgr90

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be2042ratures. This again simplifies the design task,pared with a linear slide approach.

3 The rotation axis of the rotor structure corre-onds to a center of gravity axis. In consequence,ither the force of gravity nor translational acceler-ons influence the OPD velocity. This minimizese disturbances of the control loop of the velocity anduces the mechanical work that has to be done by

e actuator of the interferometer.4A high ratio of 8 between the change of OPD andchanical displacement is obtained. Only smallational movements are necessary to generate theuired path difference.

The price for these advantages is that, owing to thenterbalance of the rotor structure, the double-

ndulum interferometer is slightly larger andavier than other comparable interferometer set-s. The KCl beam splitter is combined with a KCl

pensation plate of matched thickness to achieve amon zero OPD ZOPD for all wavelengths at the

me orientation of the rotating structure. Theam-splitter substrate, compensation plate sub-ate, and the air gap between them are wedgedth an angle of approximately 5 mrad to avoidbryPerot interference patterns in the spectra.e deviation from a plane surface is smaller thanm at any point on the beam-splitting surface.

The beam-splitter substrate has two zones withferent coatings: a multilayer coating for the in-red beam in the main zone, which is not transpar-t in the visible, and a single-layer coating in aripheral zone for the laser beam in the visible.e efficiency of the infrared beam splitter is betteran 80% in the wavelength region from 4 to 15 md better than 90% in the region from 4.5 to 11 m.e efficiency of the visible zone is roughly 90% at theer frequency of 0.632 m.

The plane retromirrors of the beam-splitter unite placed in a 90 arrangement. One of the mirrorsn be aligned during flight with two stepper motorsat act on differential screws. The interferometerhibits excellent thermal stability: After thegned interferometer is cooled from room tempera-re to its operating temperature of roughly 215 K, itst be realigned by less than 10 steps in each step-

r motor direction, corresponding to a wave-frontear of approximately 80 rad and a modulations of 20% at a 10-m wavelength. During float,e temperature of the interferometer changes byout 35 K; this generally causes no need to realigne interferometer. The resistance to mechanicalock and vibration is very high: The interferome-

shows reasonable modulation after touchdown,overy operations, and transportation back to thench base.

The rotating structure contains the two cube cor-r reflectors, a counterbalance, a torque motor, pho-lectric barriers for the motion control, and a lock.an 12 rad. At the maximum OPD both cube cor-rs are used at 5 in opposite directions. In thisrst case, the angular errors from each passing of abe corner can add linearly, and the total misalign-nt between the two wave fronts can be 48 rad,ich would correspond to a modulation loss of 25%5 m. This is smaller than, but comparable with,e modulation loss caused by the finite field of view.As each part of the beam passes its cube cornerice and both cube corners are moving at the samee, the OPD is eight times as high as the displace-nt of the apices of the cube corners. With 5 an-lar motion an OPD of 145 mm on both sides ofPD is measurable, which corresponds to an unapo-ed spectral resolution of 0.035 cm1. The inter-ometer is situated in the optical path of theectrometer such that at ZOPD both retromirrorse at an image plane of the entrance aperture. Theximum possible diameter of the infrared beam ismm at the beam splitter; for safety regarding mis-gnment, only a diameter of 52 mm is used.Parallel with but outside the infrared beam a HeNeer beam provides the measurement of the OPD.ery zero crossing of the laser interferogram withsitive slope is used as a clock pulse for both thempling of the infrared interferogram and the mo-n control of the rotating structure. To avoidosts in the spectra, the time delay between thetection of the laser signal and of the different in-red channels is matched, and the OPD velocity ispt constant within 0.5% of its nominal value,ich can be chosen in the range from 1 to 6 cms.

The OPD scan region can be chosen freely, sym-trical or asymmetrical, around ZOPD with arbi-ry length, since the length-measuringerferometer system provides two laser interfero-

ams within one beam with a phase difference of, allowing absolute fringe counting.The HeNe laser source is a nonstabilized multi-de model with 5 mW of output power. The fre-ency instability generated by the mode shift of theer is estimated to 5 109 within the samplinge of one interferogram and to 5 107 over aole flight. The coherence length is sufficient to

ovide a good modulation at maximum OPD. Theer and its low-noise power supply are housed atproximately 1000 hPa in a hermetically sealed box,ich is mounted on the gondola structure. The la-

r light is coupled into the interferometer with a-m multimode fiber-optic cable. The laser is op-ally decoupled from the interferometer by a smallviation of the incidence direction from the autocol-ation direction.

The weight of the interferometer, including theam-splitter unit and the rotating structure, is.9 kg; its diamond-shaped envelope has a width of0 mm, a length of 640 mm, and a height of 250 mm.

1 June 2004 Vol. 43, No. 16 APPLIED OPTICS 3341

F. Detector System

The optics of the detector system and the detectorsare housed in a liquid-helium cryostat with a ZnSeen5finheimaperaterwaspcheffcheitniu

Si:contorcosaseffchzachwaelethFaitundivash

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sigmueqingcadethtwgapetaiis

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Abla

into deep space is used to assign radiance units to thespectra. The main critical areas in this calibrationapproach are the quality of the blackbody, which con-troze

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arvenonaglmeacYada0.4

optiomemoschtrameevpivatitiome

334trance window and are cooled to a temperature of7 K. The entrance stop of the cryostat serves asal, defining field stop of the instrument. With thelp of a spherical mirror the incoming light is thenaged onto a second stop, which acts as the finalerture stop of the instrument. The spectral cov-ge is split into four bands with dichroic beam split-s to improve the sensitivity, especially in the high-ve-number region. The separation into the

ectral bands is done after the beam definition, so allannels observe the same FOV and have the sameective entrance aperture. Depending on theannel, the final concentration on the detector isher performed by a Winston cone or by a germa-m lens.

The detectors are photoconductive backilluminatedAs blocked-impurity-band detectors.43 Unlikeventional extrinsic photoconductors, these detec-s are quite thin and are practically insensitive tomic rays. Unfortunately, the substrate layer actsa FabryPerot etalon and produces channeling

ects in the measured spectra. The strength of theanneling can, however, be diminished by maximi-tion of the incidence angle on the detector. Inannel 1, with the strongest channeling effect, its necessary to use a cone condenser as couplingment, because the very large output divergence of

e condenser leads to a very good suppression of thebryPerot interferences. For the other channelswas sufficient to use a lens and orient the detectorder a constant angle of 30 versus the incomingergent beam. The advantage of the lens optics is

more homogeneous response over the FOV and aarper FOV definition.The detector photocurrent is converted to an out-t voltage with a transimpedance amplifier. Theplification factor is determined by a feedback re-tor that is mounted together with the first stage ofe preamplifier, a double-junction field effect tran-tor, in the proximity of the detector on the coldte.

To keep the thermally induced detector currentnificantly below the photocurrent, the detectorsst be cooled below 14 K. In our system the

uilibrium temperature, which arises from the cool-of the cold plate and the heating from the electri-

l components, is between 5 and 7 K, depending ontector position. Measurements have shown thate SNR is rather independent of temperature be-een 5 and 12 K. However, the photoconductivein, and therewith the responsivity, is strongly tem-rature dependent, so it is very important to main-n a constant temperature, as a stable responsivityessential for the calibration procedure.

Calibration System

two-point calibration by looking into an internalckbody at instrument temperature 215 K and

2 APPLIED OPTICS Vol. 43, No. 16 1 June 2004ls the gain determination, and the quality of thero level, which governs the offset determination.The blackbody is a cylindrical cavity with a length-radius ratio of 4, which is mounted within the cold

partment of the instrument see Fig. 3. Therfaces are coated with Herberts 1002E black. Theissivity of this coating varies from 0.965 to 0.985 in

e complete range from 4 to 14 m, yielding anerage total emissivity of the blackbody of 0.9985 007. Additionally, the blackbody is at instru-nt temperature; residual reflected radiance origi-tes from surfaces of approximately the same

perature and produces only a negligible error.e temperature measurement of the blackbody isrformed with calibrated platinum resistors. Sig-cant effort is made to achieve a total estimatedperature measurement uncertainty of 0.1 K.

fset calibration is performed with spectra produceden the instrument is looking 20 upward into deep

ace. Since the gondola is, unlike a spacecraft, stillthin the atmosphere, these offset spectra are to atain extent contaminated with atmospheric lines,ich are removed with a special algorithm see Sub-

ction 4.D.

Line-of-Sight System

e design goal for MIPAS-B2 has been the minimi-tion of restrictions in the measuring scheme. Thetrument measures the thermal emission of theospheric constituents without any background

urce. To keep this flexibility, the LOS informationould not be derived from a defined target e.g., then or selected stars. So another method has to beed to acquire and stabilize the LOS and to deter-ne its knowledge.The attitude and heading of the balloon gondolae measured with an AHRS that was specially de-loped for this application. It is a miniaturized,rth-seeking Schuler adapted strapdown inertialvigation system44 aided by an embedded GPSobal positioning system. The inertial measure-nt unit contains two high-quality gyros and three

celerometers and uses a ten-state Kalman filter.w, roll, and pitch angle are delivered with an up-te rate of 128 Hz and an attitude accuracy of4 mrad 3.

Based on this AHRS, a pointing system was devel-ed that accomplishes the requirements of acquisi-n and stabilization of the LOS during theasurements and compensates for the oscillatingvements of the gondola45 see also the functionaleme in Fig. 5. The roll and pitch angles arensformed to the coordinate system of the instru-nt and extrapolated to nearly real time. The el-

ational component of the LOS is controlled with aotable scan mirror to keep the LOS constant rel-ve to the horizon of the Earth. The angular posi-n of the scan mirror relative to the instrument isasured with a 19-bit optical angle encoder and

adomth6IncacesbilObconango

acedIts7.3ofallofticinfthduatditwi

a supplementary heating or pressurizing system, andit is not necessary to actively cool the CCD sensorduring long-time integration. The actual operatingtemap580.1agGPofstasitacIned

I.

Thteceleanthcaan

nefortelbaan

ofdigcirticthfersibconBrsaththThthcisdoticpuallpeOP

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FigB2thefrojusted by a limited-angle torque motor. The ge-etry of the instrument allows elevation angles in

e range from 20 deep space measurements to, where the Earths surface is already in the FOV.addition, the mirror can be turned to the internal

libration blackbody. The yaw information is pro-sed and then sent with 8 Hz to the azimuth sta-ization system developed by the Genevaservatory.46 The system exerts a very preciselytrolled torque onto the flight train of the balloon

d is therewith able to direct the azimuth of thendola with a precision of several milliradians.A star reference system was developed to verify thetive pointing system and to determine the knowl-ge of the LOS relative to the ellipsoid WGS84.47main component is a CCD camera with a FOV of 5.5. For every interferogram a star picture

the night sky around the LOS is taken. Addition-y, the light of marks representing the optical axisthe spectrometer is superimposed by a special op-s onto the light of the stars without masking therared beam to determine the thermal warping ofe optics arising from the changes in temperaturering flight. The CCD star camera can be operatedtemperatures down to 50 C under vacuum con-ions. This has two advantages: The camera canthstand the conditions in the stratosphere without

. 5. Functional scheme of the LOS control system of MIPAS-. The outer box represents the gondola; the gray arrows denoteincoming infrared radiation from the atmosphere and the light

m the stars.perature of the CCD camera during the flights isproximately 40 C. The CCD sensor has 752

2 image elements with angular pixel dimensions of7 mrad. After onboard data reduction, star im-

es and the precise time and position provided by aS are sent to ground by telemetry. With the helpalgorithms for subpixel resolution, calculation ofr positions, and star pattern recognition, each po-ion on the CCD sensor can be calculated with ancuracy of 0.15 mrad 3 in the horizon system.cluding systematic errors, this leads to a knowl-ge of the LOS in elevation and azimuth of 0.3 mrad.

Onboard Electronics

e whole onboard electronics is based on transputerhnology. It consists of dedicated interferometerctronics, which are responsible for data processingd control of the interferometer, LOS electronics,at control the LOS system described above, andmera electronics, which process the star imagesd allow camera control.All of these dedicated electronic units are con-cted to a central transputer unit that is responsible

acquisition of auxiliary housekeeping data, foremetry packaging and generation of an onboardckup of all data, and for reception, distribution,d execution of telecommands.The interferometer electronics essentially consistsfour signal channels, each with a 16-bit analog-to-ital converter and digital signal processor, of acuit which generates the trigger pulses for the op-al path management and the sampling pulses fore analog-to-digital converters from the laser inter-ometer signal, and of a processor which is respon-le for management of the OPD and the drivetrol loop. The spectrometer is using a modified

ault48 sampling scheme: the analogue data ismpled with a fixed rate of 50 kHz and the data isen interpolated to the time of the zero-crossings ofe laser interferometer signal with positive slope.e sampling units for the four infrared channels ande laser interferometer refer to the same high pre-ion clock. The data is then digitally filtered andwnsampled according to the bandwidth of the op-al filter; the reduced data is sent to the main trans-ter system. The interferometer electronics alsoows alignment of the interferometer with two step-r motors and variation of detector gain, maximumD and scan speed during flight.

The main transputer system interfaces with theemetry system, which is supplied by the CNESentre National dEtudes Spatiales balloon divi-n. The downlink rate is 500 kbits; the datansfer is carried out in a synchronous telemetry

otocol. The uplink rate is 9.6 kbits and is per-med in a standard serial asynchronous protocol.The interfaces on the ground are completely iden-al to those onboard to avoid any differences be-

1 June 2004 Vol. 43, No. 16 APPLIED OPTICS 3343

tween laboratory and flight situation other than thetransparent telemetry connection.

The on-ground system consists of a server PC thatdesedatoallanthserelonmathbedivthexchera

4.

A.Co

ThwiinErternocudeterse

intferme

B.

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334codes the telemetry protocol with the help of arial communication analysis card. The decodedta is extracted from the telemetry frames and sentvisualization clients on PCs by TCP-IP. In par-el, the incoming data is written into a database,d consolidated interferograms can be read fromis database and processed during flight. Therver PC also works as telecommand server, whichays telecommands originating from different usersPCs to the telemetry system. Recently a telecom-nd and telemetry supervision capability through

e INMARSAT and IRIDIUM satellite network hasen realized in cooperation with the CNES balloonision. This allows long-distance flights beyond

e limit of 500 km of the normal telemetry andtends the measuring duration, increasing theance for observing, e.g., diurnal variation or sev-l satellite overpasses.

Data Processing

Quality Assurance, Nonlinearity Correction, andadding of Interferograms

e interferograms are checked for consistencyth respect to sampling errors, spikes, noise, andstabilities of elevation angle during the scan.roneous interferograms are discarded. Each in-ferogram is scaled to compensate for the detectornlinearity. The appropriate scaling factor is cal-lated from the corresponding dc-value, which islivered with the housekeeping data. For the de-mination of the detector nonlinearity, see Sub-

ction 5.E.Interferograms of the same sweep direction of theerferometer are coadded if the LOS and the inter-ometer are sufficiently stable during the measure-nt of the respective tangent altitude.

Phase Correction of Interferograms

r an emission spectrometer the different radiationurces and their phase relations have to be consid-d. These radiation sources are 1 the atmo-

heric port, 2 the detector port, and 3 the beam-litter emission see also Fig. 6. The atmosphericrt comprises all radiation sources that are situatedthe opposite side of the interferometer with re-

ect to the detector. The main contributions to thisrt are the scene and the self-emission of the scanrror and the telescope. The detector port containsradiation sources that are at the same side of theerferometer as the detectors. Here the main

urce is the reflection of the instrumental back-ound radiation into the light path by the wedgedostat window. The phase difference between theospheric and the detector port is close to , and

e phase relation between the atmospheric port ande beam-splitter emission is close to 2.49,50When the atmospheric port is defined as real andsitive, the detector port is real and negative, and

4 APPLIED OPTICS Vol. 43, No. 16 1 June 2004e beam-splitter spectrum is purely imaginary.erefore, even without any phase error, the result-spectrum is complex, showing a natural phase

ig. 6a. In the measured spectrum, a phase errordue to the frequency-dependent propagation timethe signal in dispersive optical components as wellin the electronics and due to sampling shifts rel-ve to the peak of the interferogram is superposedthe natural phase Fig. 6b. Because of the non-gligible beam-splitter emission, a standard phase-rection method51,52 cannot be applied. Severalase-correction methods that consider the beam-litter emission have been proposed. The methodRevercomb et al.53 requires a constant phase errortime between two calibration sequences. Espe-lly for early MIPAS-B2 flights, this requirements not fulfilled. The double-differencing method

oposed by Blom et al.54 fails for high tangent alti-des and deep space measurements because of theatively low SNR. The method of Johnson et al.55uses on one-sided interferograms and relies on theymmetry of individual lines to detect isolatedase points. Since MIPAS-B2 is collecting double-ed interferograms, the lines are always symmet-al but are moving, in case of phase errors, towarde imaginary plane. This behavior is used for therieval of the phase function described here.56

In a first step, contributions, that are nonlinearth respect to wave number are determined fromckbody measurements. For these measure-nts, radiation from the atmospheric port is muchger than the beam-splitter emission, so that the

. 6. a Illustration of the different radiation ports and theirase relations in the complex plane for one wave number. Theal spectrum is given by the vectorial sum of the three ports.Same as a, but with an additional phase error .

standard phase determination described by Formanet al.52 can be applied. The nonlinear contributionsare attributed to the optical and electrical compo-nestaphwi

phbetimfridiseatercoa

a s

whnupaph

inaThreppleby

miin

Thticwipapaprvidimth

trusplarthblaInfropaphstatrapathprcaatm

mental phase is used for the phase correction of allatmospheric and deep space spectra. For the radio-metric calibration, only the real parts of the phase-corcom

C.

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Thfunicabroffby

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1enabhamomoofInisthnonts of the interferometer. They are rather con-nt in time and are referred to as instrumentalase instr. The instrumental phase is calculatedth a spectral resolution of 12 cm1.Sampling shifts in the interferogram lead to aase error that is linear with respect to wave num-r. This linear phase error lin may change on ae scale of a few interferograms, e.g., because of

nge count errors in the electronics or a thermaltortion of the interferometer. Therefore the lin-

r phase error is determined for each coadded in-ferogram individually. Interferograms are onlydded if the phase is sufficiently stable.

The linear phase error is defined by an offset a andlope b. The total phase error is then of the form

instr lin instr a b 0, (1)

ere is the wave number and 0 a fixed wavember in the center of the spectral interval. Therameters a and b are adjusted such that for thease-corrected spectrum

1 The correlation between the real and the imag-ry part of the corrected spectrum is minimized.is reflects the assumption that the imaginary part,resenting the beam-splitter emission, is com-tely independent from the real part, which is giventhe atmospheric and the detector ports.2 The variance of the imaginary part is mini-zed. This reflects the absence of any line featuresthe beam-splitter spectrum.

e real and the imaginary parts have common op-al and electrical filter functions, which vary slowlyth wave number. Therefore the spectra are high-ss filtered to eliminate this correlation before therameters a and b are determined in an iterativeocess. Double-sided interferograms, which pro-e a high spectral resolution of both the real and theaginary parts of the spectrum, are mandatory foris phase-correction method.The imaginary part of the phase-corrected spec-m is a measure for the strength of the beam-

litter emission. If the beam-splitter spectrum isger than a few percent of the blackbody spectrum,

e instrumental phase, which was derived from ackbody measurement, can be noticeably improved.this case a new instrumental phase is determinedm a blackbody spectrum, taking the imaginaryrt into account. The improved instrumentalase then serves as input for a repetition of thetistical phase determination in atmospheric spec-. Depending on the strength of the imaginaryrt, one or two iterations are necessary to achievee improved instrumental phase. This iterativeocedure has to be performed only once for eachlibration sequence, with a pair of blackbody and

ospheric spectra. Then the improved instru-rected spectra are used, since they include theplete radiation of the atmospheric port.

Radiometric Calibration

mentioned above, a two-point calibration is per-med with spectra looking into deep space at a0 elevation angle and into an internal calibra-n blackbody. The deep space measurementsrve to determine the instrumental self-emission.th the blackbody and the deep space measure-nts are taken several times per flight typically

ery 2040 min to account for thermal drifts ofe instrument and changes in the interferometergnment. The calibration measurements are per-med at full resolution, and typically 16 interfero-

ams are coadded to reduce the noise level. Theh resolution is necessary to resolve residual atmo-

heric lines, which have to be removed before cali-ation. This removal of atmospheric lines isscribed in Subsection 4.D. After the eliminationatmospheric signatures, the spectra are smootheda resolution of 0.52 cm1 to reduce the noise levelthout degrading the filter function. The gainction G, which is a measure for the instrumentponsivity, is calculated from the smoothed black-

dy and deep space spectra SBB and SDS, by using theanck function of the blackbody temperature LBBT:

G SBB SDS

LBBT. (2)

e atmospheric spectra SA are divided by the gainction and thus converted into spectra with phys-l units of spectral radiance. To obtain the cali-

ated atmospheric spectra LA, the instrumentalset L0, which is the deep space spectra SDS dividedthe gain function, is subtracted:

LASAG L0, L0

SDSG

. (3)

th gain function and instrumental offset are deter-ned before and after each limb scan sequence ande linearly time interpolated to the measurement

e of the individual atmospheric spectra.

Removal of Atmospheric Lines from Deep Space andckbody Spectra

Deep space measurements are used as cold refer-ce source for the calibration procedure. Fromove the gondola, usually no broadband emissions to be taken into account, but there are still at-spheric lines in the spectra that have to be re-ved to yield the instrument offset. The removalthese atmospheric lines is performed in two steps.the first step, a special algorithm, called shaving,

applied to detect and remove the lines. Thereforee deep space spectra are coadded to reduce theise level, as only lines exceeding the noise level can

1 June 2004 Vol. 43, No. 16 APPLIED OPTICS 3345

be detected. The shaving algorithm detects atmo-spheric lines by fitting a Gaussian function to anisolated line to retrieve the line width. Then thecortrutrilinGathAfpameAlcathGaisflinlinis

caWulaThgameprtaling

spinsnatraadspgolinfun

E.

AknpliesgaThtiotuofillu

5.

Anvescifiespoplinert

line shape ILS. Within the scope of this paper,only few examples can be presented.

A.

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dimofththforsaof

toSinhotiodirofchcenThensasetim

meThacregwaanrabequsioprstrgleas20anreaofrepmothforondeth

334relation of the Gaussian function with the spec-m is determined: High values indicate a re-

eved line at the given position. The indicatedes can be removed in different ways: by fitting aussian curve and subtracting it, or by cutting off

e retrieved line and replacing it by a straight line.ter the shaving the spectrum is smoothed by low-ss filtering to reduce the noise level. With thisthod a preliminary offset spectrum is obtained.

though this algorithm shows good results in these of isolated thin lines e.g., in the CO2 laser band,e removal of line features that differ largely from aussian function like many Q branches is not sat-ying. Also, in the case of many strong overlappinge wings e.g., in the case of CO2 and O3, the basee is not clearly detected. Therefore the final offsetdetermined in a second step.In this step the measured deep space spectrum islibrated by use of the preliminary offset spectrum.ith the help of the forward model KOPRA,57 a sim-ted spectrum is fitted to this calibrated spectrum.is simulated spectrum is then multiplied with thein function, and the result is subtracted from theasured, uncalibrated deep space spectrum. This

ocedure delivers the final version of the instrumen-offset, which again is smoothed by low-pass filter-.2 Blackbody spectra contain a small number ofectral lines originating from CO2 and H2O in thetrument, where these gases show very strong sig-tures. The lines concerned are not used for thece gas retrievals. To avoid a contamination ofjacent spectral regions, the lines in the blackbodyectra are removed with the described shaving al-rithm before the spectrum is smoothed. As thesees are isolated and well described with a Gaussianction, further steps are not necessary.

Spectral Calibration

first-order spectral calibration is obtained by theowledge of the distance of the interferogram sam-ng. A more exact spectral calibration is not nec-sary, because a spectral shift is allowed in the traces retrieval, compensating for small inaccuracies.e main causes for these residual spectral calibra-n errors are interferogram sampling errors by fluc-ations of the reference laser frequency, stretchingthe scale by the finite FOV, and inhomogeneousmination and misalignment of the interferometer.

Instrument Characterization

essential part of the experimentalists task is torify the compliance of their instrument with theentific specifications. MIPAS-B2 has been quali-d in terms of optical properties such as FOV re-onse and stray-light sensitivity, in terms oftoelectronic properties such as detectivity and non-earity, and in terms of purely spectrometric prop-ies such as radiometric accuracy and instrumental

6 APPLIED OPTICS Vol. 43, No. 16 1 June 2004Field of View and Stray-Light Characterization

e field of view FOV is measured by moving adulated collimated infrared source filling 1% of

e FOV and the full aperture vertically and horizon-ly through the FOV. For the vertical movemente internal scan mirror of the instrument is used,ich guarantees a relative precision of the verticalasurement of better than 10 rad. For the hori-

ntal measurement an external simple turntablerror, which provides a relative precision in theer of 0.3 mrad, is used.

The experimental setup allows a full two-ensional measurement of the FOV with a spacing

0.35 mrad in the vertical direction and 0.7 mrad ine horizontal direction. As an example, a plot ofe two-dimensional distribution taken shortly be-e flight 8 is shown in Fig. 7. All data refer to theme geometrical axis, which is defined by the centerthe channel 3 FWHM.The shape of the FOV varies slightly from channelchannel, especially in the horizontal direction.ce the atmosphere is assumed to be quasi-

mogeneous in the horizontal direction, this varia-n is irrelevant. The effective FOV in the verticalection, which is obtained by horizontal integrationthe measured FOV pattern, varies very little fromannel to channel. The maximum variation of theter is less than 2% of the vertical FOV diameter.e variation of the width is less than 10%. Thissures that all channels view approximately theme air mass with the same weighting and that datats from different channels measured at the same

e can be combined in the retrieval process.To assess the stray-light properties of the instru-nt, the experimental setup was slightly modified.e modulated collimated source was chosen to ex-tly fill the full FOV. The corresponding signal wasarded as the reference signal. Then the sources slowly moved out of the FOV up to an incidencegle of 70 mrad by turning the scan mirror. Thetio of the output signal to the reference signal couldmeasured down to values of 106. In this way aantity closely related to the point source transmis-n PST could be derived. The measurement ap-oach implies that the PST does not vary withong gradients with wave number or incidence an-. The result of the measurement justifies this

sumption see Fig. 8. For angles larger thanmrad the measured PST shows very good compli-ce with a model calculation for the PST that usessonable assumptions for the reflection properties

the optical surfaces and a simple optical model toresent the instrument. For smaller angles nodel values are available. Basing on the results of

e model calculation, an Earth integration was per-med and an analysis of the effect of stray radiationmeasured spectra was carried out. The analysis

monstrated that stray light should not degeneratee quality of the spectra.

B.

Onmequthqumemeupveterwiintciethno

C.

ThtioreaNoise-Equivalent Spectral Radiance

e method to describe the noise of the whole instru-nt is to calculate the standard deviation of subse-ent spectra of blackbody measurements. In Fig. 9e NESR derived from variance spectra for one se-ence is shown. As the photon load for atmosphericasurements is usually lower than for blackbodyasurements, these variance spectra serve as theper margin for the NESR. The measured NESR isry satisfying and in all channels is significantly bet-than the specification. The result is in compliance

th a performance model of the instrument that takeso account the detector quantum efficiency, the effi-ncy of the interferometer, and the self-emission ofe instrument and assumes the background photonise as the dominant noise source.

Quality of the Phase Determination

e phase correction is performed so that the radia-n originating from the atmospheric port becomesl and positive. The radiation contribution from

Fig. 7. FOV distribution of the four channels of MIPAS-B2 measured before flight 8.

Fig. 8. Stray-light properties of MIPAS-B2: measured valuescircles in comparison with a model calculation triangles. Thenominal radius of the FOV is 2.5 mrad.

1 June 2004 Vol. 43, No. 16 APPLIED OPTICS 3347

thimrecclephfol

InatpasperrprpremsiolontosigArraacchshusreq

insphpherranconlatdaderortot

D. Contamination Problems Shaving

Deep space measurements are used as the cold ref-erence source for the calibration procedure. ThespabinsofwinoinoffthtwseoftraInnoof77tualthigfecsoanshtogbaThstrlinthof

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334e beam-splitter emission is then completely in theaginary part. If the phase is not determined cor-tly, the three different radiation ports are notarly separated any more. An error of in thease leads to an error S in the spectrum S of thelowing extent:

S ImSsin ReS1 cos

ImS. (4)

this context, ReS and ImS denote the real partmospheric and detector port and the imaginaryrt beam-splitter emission of the phase-correctedectrum. As long as the phase error is small, theor in the spectrum is dominated by the sine ex-

ession. The error in the spectrum is then directlyoportional to the magnitude of the beam-splitterission. Thus, the larger the beam-splitter emis-n, the more critical is the phase correction. In theg-wave channel 1, the spectra are most sensitive

phase errors, because the beam-splitter emission isnificantly larger than in the other channels.ound 800 cm1 the beam-splitter emissivity hasnged from 0.05 to 0.22 in the past, depending on thetual sample of the beam splitter. In the otherannels the emissivity is usually below 0.02. In theortwave channel 4, the beam-splitter emission isually so low that no statistical phase correction isuired.

The total phase error is composed of the error in thetrumental phase instr and the error in the linearase lin, which is determined with the statisticalase-correction method. For the estimation of theor in the instrumental phase, noise, drift effects,d the influence of the imaginary part have beensidered. The error in the linear phase is calcu-

ed from the noise in the spectrum by use of stan-rd error propagation formulas on the statisticaltermination of the parameters a and b. Both er-s are of the order of or below 10 mrad, so that theal phase error is typically below 25 mrad.

. 9. NESR for single spectra apodized for all channels, de-ced from variance spectra of blackbody measurements duringht 13.

8 APPLIED OPTICS Vol. 43, No. 16 1 June 2004ectral signatures originating from the atmosphereove the gondola have to be removed to yield thetrument offset see Subsection 4.D. The qualitythe removal of atmospheric signatures is assessedth simulated spectra. Such spectra with typicalise levels have been calculated for flight situationsmid-latitudes and in the Arctic. The instrumentset has then been retrieved from these spectra withe method described above. The difference be-een the original and the retrieved instrument off-t is a measure of the error due to imperfect removalatmospheric signatures from the deep space spec-. In general this error is within the noise level.some cases, however, the error may exceed the

ise level by approximately a factor of three. Onethe spectral regions concerned is the range below0 cm1 because of the rather strong CO2 signa-res. During Arctic winter flights, where the flightitude is relatively low and the abundance of HNO3h, the region around 880 cm1 may also be af-ted. Furthermore, in some cases the broad unre-

lved N2O5 band around 1250 cm1 may also lead to

error in the baseline determination. Figure 10ows a simulated uncalibrated deep space spectrumether with the original and the reconstructed

selines for channel 1 in an Arctic flight situation.e original baseline is mostly hidden by the recon-ucted one. The difference between the two base-es is shown in the residual plot. Uncertainties ofat kind are taken into account in the error budgetretrieved trace gas amounts.

Detector Nonlinearity

e Si:As blocked-impurity-band detectors show aticeable nonlinear response to the incident photon

. 10. Reconstruction of the original baseline from a simulatedcalibrated deep space spectrum for an Arctic winter flight situ-on in channel 1. The residual is the difference between theonstructed and the original baseline.

flux. This leads to a distortion of the interferogramsand to artifacts in the spectra. Any nonlinear re-sponse can be approximated by a polynomial func-tioIFint

Inriv

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Figfligspen. So the measured nonlinear interferogramGmeas is approximated as a function of a linearerferogram IFGlin:

IFGmeas IFGlin a2IFGlin2 a3IFGlin3. (5)

consequence, the measured spectrum Smeas is de-ed from the linear spectrum Slin by

Smeas Slin a2 Slin Slin a3 Slin Slin Slin.(6)

us the undisturbed spectrum is convoluted withelf, leading to quadratic and cubic artifacts. Ase quadratic artifacts are completely outside of theminal spectral range in all spectral channels ande cubic artifacts are small, the distortion of theseline in the spectra is negligible. However, theferent mean responsivities for blackbody, deepace, and atmospheric spectra have to be consideredkeep the two-point calibration valid. In channel 1e responsivity for the deep space spectra is approx-ately 10% larger than the responsivity for theckbody spectra. In channel 2 the difference is%; in channel 3 it is 1%; in channel 4 the nonlin-

rity is not measurable. The responsivity as action of the dc level is deduced from the artifactsthe blackbody spectra. Raw-data-mode not dig-lly filtered interferograms are needed for theseasurements. In channel 1, where the bandpasser ranges approximately from 750 to 1000 cm1,

e nonlinearity parameters a2 and a3 are deter-ned by minimizing the quadratic artifact in theectral region from 30 to 250 cm1. The very lowve-number region below 30 cm1 is not used be-

use of artifacts arising from the ac coupling in thenal chain. The region from 1500 to 2000 cm1 ist appropriate for determining a2 because the signalthis region originates not only from the quadratictifact but also from radiation that passes the inter-ometer twice through reflection at the cryostatndow, filters, and the detector itself . The cubictifact is calculated in the spectral region from 22503000 cm1. With these parameters, a function isated that describes the voltage of the detector ver-

s the voltage of a linear detector:

DCmeas DClin a2DClin DCref2

a3DClin DCref3, (7)

ere DCref is the dc value of the blackbody measure-nt, from which the nonlinearity parameters are

duced. The responsivity change compared with aear detector is given by the derivative of the for-la above. Thus, because the parameters a2 andand DCref are known, a responsivity R can be

lculated for each interferogram from its dc valuemeas. The whole interferogram is then scaled

th a factor R0R, where R0 stands for a referenceponsivity, which can be chosen arbitrarily but con-tently.Figure 11 shows the artifacts of a measured and arected spectrum in channel 1, taken during flight. The nonlinearity artifacts have disappeared ine corrected spectrum; only the signal between 1500d 2000 cm1, which is due to double-passing of theam in the interferometer, is left.In channels 2 and 3 the nonlinearity is determinedewise, yet only the quadratic artifacts have to been into account, because the cubic artifacts are far

low the noise level. The main error sources, thatve to be considered regarding the nonlinearity cor-tion, are the truncation of the power series and

anges in the detector temperature. The latter hasstrong influence on the detector responsivity. Inannel 1, also the part of the cubic artifact, that isperposed to the nominal spectrum, contributes toe error. In channel 3, the noise in the spectra hasbe taken into account. In total, the radiometricor due to the nonlinearity is well below 1% for allent flights.

Radiometric Accuracy

r the estimation of the total radiometric error, thelowing error sources and their impact on the ra-metric accuracy are considered:

1 Errors due to inaccuracies in the phase deter-nation blackbody, deep space, and atmosphericectra; see Subsection 5.C,2 Errors due to the imperfect removal of atmo-heric signatures in the deep space spectra thatrve as offset spectra see Subsection 5.D,3 Uncertainties in the correction of the detectornlinearity see Subsection 5.E,4 Emissivity and temperature errors of the inter-l blackbody see Subsection 3.G,5 Error due to the linear time interpolation of thelibration measurements,6 Noise in the calibration measurements.

calibration sequence with blackbody and deepace measurements is performed before and afterch limb sequence. For the calibration of the limbectra, gain and offset are linearly time interpolated

. 11. Nonlinearity artifacts measured in channel 1 duringht 10. The initial spectrum is plotted in black, the correctedctrum is plotted in gray.

1 June 2004 Vol. 43, No. 16 APPLIED OPTICS 3349

to account for drifts between the calibration measure-ments. At the time of a calibration sequence, theinterpolation error is zero. The maximum error oc-cuanvaqucreint0

stamusoqublainintsethwi

thstaerrMdenusemuatise

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As measurements at high and low tangent altitudesare performed comparatively close to calibrationmeasurements, the interpolation error for these alti-tu

timmefrowanwathanimFoartwth13sabeth

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335rs in the middle between the calibration sequencesd is estimated to be half the difference between thelues of the first and the second calibration se-ence. Between the extreme values a linear in-ase and decrease of the error is supposed. Theerpolation error of the gain function is typically.5% with this estimation.

The noise in the calibration measurements is of atistical origin, but its implication for the spectrast be considered as a systematic error for two rea-

ns: 1 All atmospheric spectra of one limb se-ence are calibrated with the same two pairs ofckbody and deep space spectra; therefore an errorthe calibration spectra propagates systematicallyo all calibrated atmospheric spectra of one limb

quence. 2 Due to the smoothing of the spectra,e noise errors are no longer spectrally independentthin the width of the smoothing kernel.The effect of each independent source of error one calibrated spectrum is calculated separately withndard error propagation formulas. For the totalor, all independent errors are added geometrically.

ost of the described errors are wave-number depen-nt; thus the total error is calculated for each wavember and is given as an error spectrum. As thensitivity of the trace gas retrieval is different forltiplicative errors and for additive errors, the rel-ve gain error and the offset error are determined

parately.Figure 12 shows the different contributions to theative gain error for a tangent altitude of 21 km,

timated for flight 10. Only the main error contri-tions are shown. The total gain error is1%, ande main contributions are the error due to the linearin interpolation, the uncertainty of the detectornlinearity correction, and the temperature error ofe blackbody. In the higher-wave-number regions,ise in the calibration measurements also becomesportant. The only error that significantly variesth tangent altitude is the time interpolation error.

. 12. Analysis of systematic error sources for flight 10. Onlymain contributions to the total gain error are displayed.

0 APPLIED OPTICS Vol. 43, No. 16 1 June 2004des is smaller than shown here.The radiometric offset error is dominated by the

e interpolation error of the deep space measure-nts. Other significant contributions originatem the error of the phase correction in channel 1here the beam-splitter emission is relatively larged from noise in the deep space spectra for higherve numbers. The offset error is compared with

e NESR of the coadded spectra: below 900 cm1

d for high tangent altitudes the NESR is approx-ately a factor of two smaller than the offset error.r these altitudes, as many as 20 single spectrae coadded. For the lower altitudes, where onlyo spectra are coadded, the NESR is larger thane systematic error. In the region from 900 to00 cm1, the NESR and the offset error are of theme order of magnitude, and for higher wave num-rs the NESR is approximately 24 times largeran the offset error.This error estimation, which was performed in de-l for flight 10, is representative for the followinghts as well. The analysis reveals that frequent

libration measurements are crucial for high accu-cy of the calibrated spectra. In total a radiometriccuracy of 0.6%1.2% is obtained.

Spectral Resolution and Instrumental Line Shape

precise knowledge of the ILS is an important pre-uisite for trace gas retrieval. In an ideal Fourier-nsform spectrometer the ILS is equal to a sincction, whose width is defined by the maximumD OPDmax. The finite FOV of a real instrument

nerates a shift of the center of the ILS to smallerve numbers. Asymmetries in the FOV pattern

use asymmetries in the ILS function. Both effectscome stronger with increasing wave number.boratory measurements are very accurate but dot necessarily represent the ILS during flight con-ions and at the relevant wave-number points.r this reason a calculation of the ILS directly fromht spectra was developed that allows the determi-

tion of the ILS over almost the entire wave-numbernge.Since the ILS can be regarded as approximatelystant in a sufficiently small wave-number range,atmospheric lines within this range are degradednearly the same ILS. On the basis of this as-

mption a method was developed that deconvolutesltiple lines within the mentioned small ranges of

e spectrum simultaneously. This obviates theed to search for isolated lines, which are normallyed for ILS determination. Flight spectra of uppergent heights are best suited for this new kind of

S retrieval, since they contain optically thin lines.r these lines a change of the trace gas concentra-n causes a scaling of the line profile only; henceere is almost no influence on the line shape. Typ-l uncertainties of trace gas concentrations of 10%% in the volume mixing ratio cause no more than a

1%IL

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Table 3. Performance of the LOS System

QuantityElevationmrad, 3

Azimuthmrad, 3error in the determination of the FWHM of theS.Within a small window the measured spectrumut is a convolution of the incoming spectrum Sinth the ILS:

Sout ILS Sin. (8)

ith the help of the Fourier transform convolutioneorem, this can be written as a product:

FTSout FT ILSFTSin. (9)

. 13. Comparison of the deconvoluted ILS of flight 6 at threeve numbers with the ideal sinc function of an instrument with4.5-cm maximum OPD shift is omitted. The decrease of theximum and the growing FWHM with increasing wave numberboth an effect of the finite divergence pattern inside the inter-

ometer. The theoretical ILS curves of an instrument with anerferometer divergence of 6.188 mrad are omitted because therealmost no visible differences from the deconvoluted curves.

. 14. FWHM of the ILS obtained by deconvolution from spec-measured during flight 6 dotted compared with the optimalHM for an instrument with the same physical parameters

lid curve; for details, see text.nally we obtain a simple estimation of the modu-ion function MOD and the ILS:

ODx FTSoutFTSin

, ILS FT MODx.

(10)

cause of the slow variation of the ILS with theve number, this method works with wave-numberndows of 1030 cm1 in width. The restriction tomall wave-number range generates an error con-bution from the lines next to the borders of thenge. This error can be decreased iteratively byrecting the measured spectrum with the men-ned lines and a first guess of the ILS:

Sout,new RSout RR Sin ILS

R RSin ILS, (11)

ere R is zero outside and unity inside the decon-lution window and R is the opposite. The physicsan interferometer suggests that the modulationction is smooth; so it can be fitted with a polyno-

al function of low order. In consequence, noises only a small effect on the results as long as thean SNR within the deconvolution window is larger

an two.As a typical example, the results obtained for flightre compared in Figs. 13 and 14 with a theoretical

S, assuming uniform illumination of the entireld of view FOV. This comparison shows the per-mance of the real instrument versus an ideal in-ument with the same physical parameters. Theative deviation of the FWHM increases from 0.5%700 cm1 to 0.7% at 950 cm1 in channel 1. Noymmetry of the ILS is noticeable. In channel 2 theative deviation of the FWHM increases from 0.6%1100 cm1 to 1.4% at 1400 cm1. In channel 4 thelculation was possible only near 2100 cm1 andes FWHM deviations of 1.5%. The correspond-NortonBeer strong-apodized ILS deviates in theHM up to 0.8% in channel 1, up to 0.9% in channel

and 1.0% in channel 4. Unfortunately, water-por contamination of the instrument leads to aall error in the line shape in channel 3 during the

libration. For this reason the deconvolution is notpletely trustworthy in this channel. The simi-

ity of the results in the other channels gives con-ence, however, that the FWHM values can beerpolated between channels 2 and 4.

Acquisition 1.5 13Stability within 15 min 0.22 4.4Knowledge 0.3 1.7

1 June 2004 Vol. 43, No. 16 APPLIED OPTICS 3351

H.

AstaiInforThfligthinfselgeerrshEvwefligbesw

sefligThdemodifsetemancortucorthplathrem

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335Line of Sight

described in Subsection 3.H, the LOS system con-ns an inherent quality control with a star camera.this way the performance can be precisely evaluatedeach measurement sequence performed at night.

erefore it could be shown that during the first eighthts small oscillations in the LOS were induced by

e use of GPS velocities to aid the AHRS. The GPSormation at that time was degraded by the so-calledective availability for civil users. The degradationnerated errors in the velocities, which map into theor of the attitude. These errors dominated the

ort- and mid-term stability of the LOS system.en with this artificial degradation, the stability isll within the specifications see Table 3. Afterht 9 there is a substantially improved performance

cause the selective availability of the GPS has beenitched off by the U.S. government.As a typical example, the results of the last limbquence of flight 10 are shown in Fig. 15. Thisht demonstrates the capabilities of the system.e stepped line with its scale on the right-hand sidenotes the commanded elevation angle; the dia-nds with the scale on the left-hand side denote theference between commanded angles and the ob-rving angles determined with the star camera sys-

. The solid curve between the diamonds showserror estimation of the AHRS itself, which can berelated to the difference in LOS. Even after a

rn in azimuth from 340 to 45 at 15:31 UTC, therelation holds and confirms the platform error of

e inertial navigation system. This error in thetform of the inertial navigation system arises

rough a small scaling error, which we could notove. But this error is known and considered in

. 15. Assessment of the pointing performance of flight 10 dur-the night limb-scanning sequence before the cut at 15:55. The

pped line indicates the commanded elevation angles in degreesht-hand axis. The diamonds indicate the differences betweencommanded LOS and the LOS derived from the star camera

tem left-hand axis. The solid curve between the diamondsws an error estimation of the AHRS itself, which is consistenth the LOS errors as characterized by the star camera. Theor in the LOS after the azimuth turn arises through a smallling error in the AHRS. The offset of 0.5 mrad is due to thermal warping of the instrument.

2 APPLIED OPTICS Vol. 43, No. 16 1 June 2004e flight planning. The systematic offset of approx-ately 0.5 mrad is due to the thermal warping ofe instrument during ascent. Calibration mea-rements of the optical axis of the cryostat withpect to the optical axis of the telescope are per-med before flight and are taken into account inese results. This value varies slightly from flightflight, but stays nearly constant during one flight.us it has an effect only on the acquisition uncer-nty.The relevant quantity for the trace gas and tem-rature retrievals is the tangent height that is cal-lated from both the balloon height and the LOS.cause of the varying balloon height and the coad-g of interferograms to improve the SNR, the active

inting system stabilizes a constant elevation onlyring the measuring time of one interferogram.ring the turn time of the pendulum of the inter-ometer a new elevation is calculated to achieve astant tangent height, resulting in a slightly vary-elevation angle during one commanded tangent

ight. The peak-to-peak variations during theown night limb-scanning sequence of flight 10 werethin 100 m. The overall accuracy of the LOS sys-

and the knowledge is clearly sufficient to avoidjor contributions to the retrieval error by the LOScertainty. We do not have very stringent require-nts concerning azimuth acquisition and azimuthbility, and the well-proved mechanisms of the Ge-

va Observatory easily fulfill these specifications.The final pointing information is used to calculatee height and position of the tangent points to pro-e localization of the spectra for the retrieval, tak-into account that there can be distances between

e observer and tangent point of as great as 600 km.

Conclusion

IPAS-B2, a cryogenically cooled Fourier transformectrometer for limb-emission sounding has beenerated successfully for a series of balloon flights.

. 16. Complete limb sequence measured on 24 September 2002mid-latitudes of a narrow spectral region inside channel 3 ded-ted for the retrieval of NO2: tangent altitudes between 6 andkm are displayed. Most lines are due to NO2.

This paper has provided a detailed description of theinstrument requirements and their justification.The requirements have been discussed in terms ofspdiocuanfocritsecovmeacreqfeaillush

Messeofcom

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futtatadorth

zeschdeduWdeinSwfrothfligLaFricadenisthUn

Re1.

2. M. T. Coffey, W. G. Mankin, and A. Goldman, Simultaneousspectroscopic determination of the latitudinal, seasonal, anddiurnal variability of stratospheric N2O, NO2, and HNO3, J.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.ectral coverage and resolution, sensitivity and ra-metric accuracy, as well as FOV and pointing ac-

racy. The actual realization of the instrumentd the design trade-offs have been described with aus on the spectrometer and the processing algo-hms necessary to produce calibrated spectra. Alection of crucial characterization measurementsering optical properties, sensitivity, and spectro-tric properties has been presented. These char-

terization measurements show that the instrumentuirements are attained and that the instrumenttures are to a large extent understood. For anstration of the quality of the spectra, see Fig. 16,

owing a narrow spectral region at 6 m.Intercomparisons of data retrieved fromIPAS-B2 measurements with other well-tablished and closely collocated in situ and remote-nsing measurements have proved the high qualitydata provided by MIPAS-B2. Examples of inter-

parisons include, e.g., CH4, N2O, and H2O.23,58With more than a dozen successful flights untilw, the instrument has contributed to stratosphericemistry in particular and to atmospheric and cli-te research in general.59 Meanwhile, the focus isthe validation of spaceborne atmospheric chemis-instruments. Later in the life of the satellite

nsors the focus will shift to a synergistic exploita-n of the mutual benefits of spaceborne and air-rne sensors.The main technical activities for the foreseeableure, except for careful maintenance, are the adap-ion to new launch areas such as the tropics and theaptation to new technologies, such as laser diodesimproved electronics, to simplify the operation of

e instrument.

We thank the colleagues at the Institut fur Pro-ssinstrumentierung und Elektronik of the For-ungszentrum Karlsruhe and at the ObservatoireGene`ve, Switzerland for their dedicated support

ring development and operation of the instrument.e express gratitude to the experts from the Centrelancement des ballons stratospheriques of CNESAire sur lAdour and Toulouse, France, from theedish Space Corporation in Esrange, Sweden, andm the Central Aerological Observatory in Russiaat performed extremely successfully the launch,ht, and recovery operations in the past years.st but not least, we thank the colleagues of theeie Universitat Berlin for the excellent meteorolog-l support during many campaigns. The workscribed herein was supported by the Bundesmi-terium fur Bildung und Forschung, Germany, by

e Directorate General for Research of the Europeanion, and by the European Space Agency.

ferencesC. B. Farmer, O. F. Raper, and R. Norton, Spectroscopic de-tection and vertical distribution of HCl in the troposphere andlower stratosphere, Geophys. Res. Lett. 3, 1316 1976.Geophys. Res. 86, 73317341 1981.G. C. Toon, The JPL MkIV Interferometer, Opt. PhotonicsNews 210, 1921 1991.C. Camy-Peyret, J. M. Flaud, A. Perrin, C. P. Rinsland, A.Goldmann, and F. Murcray, Stratospheric N2O5, CH4, andN2O profiles from IR solar occultation spectra, J. Atmos.Chem. 16, 3140 1993.M. R. Gunson, M. M. Abbas, M. C. Abrams, M. Allen, L. R.Brown, T. L. Brown, A. Y. Chang, A. Goldman, F. W. Irion,L. L. Lowes, E. Mahieu, G. L. Manney, H. A. Michelsen, M. J.Newchurch, C. P. Rinsland, R. J. Salawitch, G. P. Stiller, G. C.Toon, Y. L. Yung, and R. Zander, The Atmospheric TraceMolecule Spectroscopy ATMOS experiment: deployment onthe ATLAS Space Shuttle missions, Geophys. Res. Lett. 23,23332336 1996.D. G. Murcray, A. Goldmann, F. H. Murcray, and W. J. Wil-liams, Stratospheric distribution of ClONO2, Geophys. Res.Lett. 6, 857859 1979.F. H. Murcray, F. J. Murcray, D. G. Murcray, J. Pritchard, G.Vanasse, and H. Sakai, Liquid nitrogen-cooled Fourier trans-form spectrometer system for measuring atmospheric emissionat high altitudes, J. Atmos. Ocean. Technol. 1, 351357, 1984.B. Carli, F. Mencaraglia, and A. Bonetti, Submillimeter high-resolution FT spectrometer for atmospheric studies, Appl.Opt. 23, 25942603 1984.J. C. Brasunas, V. G. Kunde, and L. W. Herath, CryogenicFourier spectrometer for measuring trace species in the lowerstratosphere, Appl. Opt. 27, 49644976 1988.D. G. Johnson, K. W. Jucks, W. A. Traub, and K. V. Chance,Smithsonian stratospheric far-infrared spectrometer anddata reduction system, J. Geophys. Res. 100, 309131061995.H. Fischer, F. Fergg, H. Oelhaf, D. Rabus, and W. Volker, Si-multaneous detection of trace constituents in the middle atmo-sphere with a small He-cooled high resolution Michelsoninterferometer MIPAS, Beitr. Phys. Atmos. 562, 2602751983.H.