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Envisat GO MOS An Instrument for Global Atmospheric Ozone Monitoring
European Space Agency Agence spatiale europeenne
SP-1244 May 2001
Envisat
GO MOS
An Instrument for Global Atmospheric Ozone Monitoring
European Space Agency Agence spatiale europeenne Short Title: SP-1244 'Envisat - GOMOS'
Published by: ESA Publications Division ESTEC, P.O. Box 299 2200 AG Noordwijk The Netherlands Tel: +31 71 565 3400 Fax: +31 71 565 5433
Technical Coordinators: C. Readings & T. Wehr Editor. R.A. Harris Layout: Isabel Kenny Copyright: © European Space Agency 2001 Price: 30 Euros ISBN No: 92-9092-5 72-8 Printed in: The Netherlands
ii Foreword
This report is one of a series being
iii
Contents
Foreword ...... m
List of Acronyms ...... ix
Chapter 1: Introduction
1.1 The European Space Agency's Present Atmospheric Chemistry 1 Sounding Missions 1.1.1 Upcoming Mission: Envisat . .1 1.1.2 Ongoing and Future Mission: The GOME Instrument .2 on ERS-2 and Metop 1.2 GOMOS Short History . .3 1.3 Scientific Rationale . .3 1.4 The Observation Principle .. .5 1.4.1 Vertical distribution, resolution and accuracy .. 5 1.4.2 Geographical coverage . . .7 1.4.3 A short history of the stellar occultation technique ... 7 1.4.4 Difficulties associated with stellar occultation .. 8 1.4.5 This Report . . .. 8
Chapter 2: Scientific Objectives
2.1 Motivation . . 11 2.2 Current Science Issues . . 12 2.2.1 Monitoring Profiles of OJ and Trends .. 13 2.2.2 Increasing Arctic OJ Depletion . .13 2.2.3 Mid-latitude Ozone Trends . .13 2.2.4 Trends in Temperature, H20 and stratospheric aerosol .14 2.2.5 Stratospheric Dynamics ...... 16 2.2.6 Mesospheric Ozone and Clouds . .17 2.2.7 Recovery of Stratospheric Ozone .17 2.3 Ozone Depletion . ... 18 2.3.1 Stratospheric Gas-Phase Chemistry ...... 19 2.3.2 Heterogeneous Chemistry ...... 20 2.3.3 Polar Ozone Depletion . .21 2.3.4 Middle-Latitude Ozone Trends . .23 2.3.5 Consequences of Ozone Depletion . .23 2.4 GOMOS Scientific Objectives . .24 2.4.1 Primary Scientific Objective: Ozone Trend Observations .25 2.4.2 Secondary Scientific Objectives . . .. 26 2.4.3 Long-term, campaign and permanent objectives ... 27
Chapter 3: Instrument Requirements
3.1 The Required Measurement Principle . .29 3.2 Instrument Requirements: Spectral Coverage and Resolution .30 3.3 The Contribution of GOMOSMeasurements . .32 3.3.1 Monitoring of 03 Profile and Trends .33 3.3.2 Other Studies ...... 35
v Chapter 4: Instrument Concept
4.1 Introduction ...... 39 4.1.1 The GOMOS instrument design overview .. . .. 39 4.1.2 Steering-Front Mechanism Optics (SFM) .. . .42 4.1.3 Telescope . .42 4.1.4 UV-visible spectrometer optical design .. .43 4.1.5 IR spectrometer chain optical design . .44 4.1.6 Fast photometers optical design . .45 4.1. 7 Star tracker optical design .. .46 4.1.8 UV-visible/IR detector design . .46 4.1.9 Photometer detection design . .47 4.1.10 Star-acquisition and tracking-unit detection design ... .48 4.2 Instrument Electronic Design . .48 4.3 Mechanical Design . .48 4.3.1 GOMOS interface structure (GIFS) .48 4.3.2 Optical bench . .49 4.3.3 Opto-mechanical cover (OMC) .. .50 4.3.4 Telescope Baffle Assembly (TBA) .50 4.4 Thermal Design .... 50 4.5 Software . . ... 51 4.6 Instrument Operation 51 4.7 How to Use GOMOS 53 4.7.1 Modes, Transitions and Commanding Overview 53 4.8 Instrument Performance Summary 55
Chapter 5: Characterisation and Calibration
5.1 Introduction . .61 5.2 Definition of Calibration versus Characterisation .61 5.3 Characterisation and Calibration Database . .62 5.4 Ground support equipment ...... 62 5.5 Wavelength Ranges, Assignment and Dispersion ...... 63 5.6 Spectrometer Line Spread Function ...... 65 5.6.1 Contributors to systematic errors ...... 65 5.6.2 Measurement Method ...... 66 5.7 Spectrometer Radiometric Uniformity ...... 67 5.7.1 Definition of radiometric uniformity ...... 68 5.7.2 Measurement of the uniformity map ...... 68 5.8 Spectrometer Linearity 68 5.9 Instrument Radiometric Sensitivity 69 5.10 Instrument Signal-to-Noise Ratio ...... 70 5.11 In-flight Calibration ...... 70 5.11.1 Introduction ...... 70 5.11.2 The GOMOS Calibration database ...... 73
Chapter 6: Mission Planning
6.1 Introduction . . 75 6.2 Motivation . . 75 6.3 Elements of the GOMOSmission planning .. 76 6.4 Criteria characterising mission objectives .. 77 6.5 Mission planning tool .78 6.6 Results . .81 6.7 Summary 81
vi Chapter 7: Scientific Data Processing and Data Products
7.1 Introduction ...... 83 7.2 Level 0-1b Processing ... 83 7.3 Level 1b Data Product ... 87 7.4 Level 1b-2 Processing . ... 88 7.4.1 Preparation of the inversion process ...... 90 7.4.2 The spectral inversion module ...... 90 7.4.3 The vertical inversion module .92 7.4 .4 Level 1b-2 processing loops .92 7.5 Level 2 Data Products . .93 7.6 Characteristics of GOMOS products .94
Chapter 8: GOMOS Validation
8.1 Internal Consistency of GOMOS Measurements .97 8.1.1 Measurements at the Same Place ...... 97 8.1.2 Comparison of Temperature Derivations .97 8.1.3 Comparison of Ozone Derivations .... .98 8.1.4 Self-consistency check of N02, NOP 03 .98 8.2 Geophysical Validation . . 99 8.2.1 Introduction . . 99 8.2.2 Campaign activities . . .99 8.2.3 Independent Sources of correlative data ...... 99 8.2.4 Analyses using models ...... 102 8.2.5 Activities of the Envisat Validation Team ...... 103 8.3 Discussion and Conclusions ...... 104
Chapter 9: Concluding Remarks . . 105
References ...... 107
vii
List of Acronyms
ACVT Envisat Atmospheric Chemistry Validation Team ADEOS Advanced Earth Observing Satellite CCD Charge Couple Device CFRP Carbon-fibre reinforced plastic CNES Centre National d'Etudes Spatiales CO ALA Calibration for Ozone through Atmospheric Limb Acquisitions CTM Chemistry-transport model ECMWF European Centre for Medium-Range Weather Forecasts Envisat Environmental Satellite ERS Earth Remote Sensing Satellite ESA European Space Agency EUMETSAT European Organisation for the Exploitation of Meteorological Satellites FM Flight model FOV Field of View FWHM Full width half maximum GCM General circulation model GIFS GOMOS interface structure GOME Global Ozone Monitoring Experiment GO MOS Global Ozone Monitoring by Occulation of Stars HIRD LS High Resolution Dynamics Limb Sounder ICU Instrument Control Unit IPCC Inter-Governmental Panel on Climate Change IR Infrared LSF Line spread function Metop Meteorological Operational Satellite MIPAS Michelson Interferometer for Passive Atmospheric Sounding MLS Microwave Limb Sounder NASA National Aeronautics and Space Administration NASDA National Space Development Agency of Japan NAT Nitric acid trihydrate NDSC Network for the Detection of Stratospheric Changes NILU Norsk Institutt for Luftforskning NRL Naval Research Laboratory NRT Near real time NWP Numerical weather prediction OMA Opto-mechanical assembly OMC Opto-mechanical cover OSIRIS ODIN Spectrometer and IR Imager System oss Optical support struts PEB Payload equipment bay PMC Polar mesospheric clouds POAM Polar Ozone and Aerosol Measurement PSC Polar stratospheric cloud QTH Quartz-Tungsten-Halogen RKA Russian Space Agency SAGE Stratospheric Aerosol and Gas Experiment SAOZ Systeme d'Analyses par Observations Zenithales SATU Star acquisition and tracking unit SCIAMACHY Scanning Imaging Absorption Spectrometer for Atmospheric Chartography SFM Steering-front mechanism optics SMR Sub-Millimeter Radiometer SNR Signal-to-Noise ratio UARS Upper Atmospheric Research Satellite UK United Kingdom UV Ultraviolet VMR Volume mixing ratio WMO World Meteorological Organisation
ix • 1. Introduction
1.1 The European Space
Agency's Present _____ AATSR Atmospheric Chemistry Sounding Missions _,,,_ SCIAMACHY ,_____ MWR The European Space Agency (ESA)is "'~Ka-Band a key-player in global atmospheric Antenna chemistry research using spaceborne instruments. Satellite instruments are t------DORIS the ideal tools for process studies and trend observations on a global scale. ,__ X-Band ESA's activities related to chemistry Antenna sounding include ongoing and future GOME activities and the upcoming launch of Envisat, which will carry --+---di- ASAR the GOMOSinstrument amongst Antenna others. In the future, atmospheric chemistry missions could be realised within the frame of the Earth Observation Envelope Programme. Within this programme, the Earth Explorer Missions will provide opportunities for research and technology demonstration missions.
1.1.1 Upcoming mission: Envisat atmospheric composition sounding, Fig. 1-1 The Envisat MIPAS,SCIAMACHYand GOMOS. Scientific Payload In mid 2001, ESA will launch the environmental satellite, Envisat, an The Envisat satellite has an ambitious advanced polar-orbiting Earth and innovative payload, as depicted in observation satellite, which will Figure 1.1, which will ensure the provide measurements of the continuity of most of the data atmosphere, ocean, land and ice over measurements of the ESA ERS a five-year period. Envisat will be satellites. The Envisat data will ESA's largest Earth observation support Earth sciences research and mission so far. In the summer of allow monitoring of the evolution of 2001, an Ariane-5 launcher will place environmental and climatic changes. Envisat in a near-polar orbit at 98.5° Furthermore, they will facilitate the inclination. Envisat will carry ten development of operational and scientific instruments including the commercial applications. Besides three instruments dedicated to seven other scientific instruments,
1 Envisat will carry the following three longitudes. MIPAShas been described instruments for measuring the in ESA 2000 composition of the atmosphere. SCIAMACHY GOMOS The Scanning Imaging Absorption GOMOS is the acronym for Global Spectrometer for Atmospheric Ozone Monitoring by Occultation Chartography (SCIAMACHY)has the of Stars. It is a completely newly following spectral ranges: 240-1750 developed instrument, exploiting nm contiguously, 1940-2040 nm, stellar occulation, measuring 2265-2380 nm; resolution of 0.2-1. 3 in the ultraviolet, visible and infrared. nm. It features various operational The major objective is monitoring of modes exploiting different the global vertical ozone distribution measurement principles: (1) Nadir from the upper troposphere to the mode: solar radiation (backscattered upper mesosphere. This report is from the surface), cloud, aerosol or concerned with GOMOS. air. Observed species include 03, N02, H20, CO, N20, CH4, BrO, C02, uncer MIPAS special conditions also S02, OClO and H2CO. (Total columns or in some The Michelson Interferometer for cases stratospheric profiles with Passive Atmospheric Sounding coarse resolution.); (2) Limb mode, (MIPAS)is, like GOMOS, a completely backscattered solar radiation: new development. It consists of a Stratospheric profiles of temperature, Fourier transform infrared 03, N02, H20, CO, N20, CH4, C02, spectrometer with a contiguous BrO (if enhanced) and aerosol spectral coverage of 685 to 2410 cm-I extinction. The vertical resolution is and resolution of 0.035 cm-I. MIPAS 3 km; (3) Once per orbit a solar scans the limb of the atmosphere in occultation will be performed to the aft direction or perpendicular to measure atmospheric constituents the flight direction, covering the and produce 1 km resolution profiles upper troposphere, stratosphere and of temperature, 03, N02, CO, N20, mesosphere. The measured species CH4, BrO, C02; (4) Lunar are ozone, the source gases H20, CH4, occultations will be performed when N20, CO, C2H2, C2H6, CFC-11, CFC- possible. 12, CFC-22, CC14, CF4, SF6, HOO, the radicals NO, N02, ClO (if enhanced), reservoir and sink species HN03, 1.1.2 Ongoing and future mission: N205, ClON02, HN04, HOCl, and the GOME instrument on ERS-2 particles (cloud, aerosol, polar and Metop stratospheric clouds). The Global Ozone Monitoring Since thermal emission in the IR is Experiment (GOME)is currently very sensitive to temperature, MIPAS operating onboard the ERS-2 satellite, will be able to precisely measure which was launched in 1995. GOME temperature. MIPASdata products activities are being continued in will include geolocated calibrated limb cooperation with EUMETSATon the emission spectra, vertical profiles of future Metop missions (first launch pressure, temperature, 03, H20, CH4, planned in 2005), which will include N20 and HN03. The horizontal the improved GOME-2 instrument. The resolution will be approximately 300 instrument consists of a grating to 500 km along track. The spectrometer operating from 240 nm stratosphere and mesosphere to 790 nm (UV-visible-near infrared) will be covered at all latitudes and with 0.2 to 0.4 nm spectral resolution.
2 Species absorbing in this region ESA-developed and ESA-led include o., 02, 04, H20, HCHO, S02, atmospheric chemistry instrument on NO, N02, ClO, OClO and BrO. The Envisat, while SCIAMACHYremains a instrument's main operational mode is nationally-funded instrument. nadir view, measuring back-scattered solar radiation. The maximum swath is As an ESA facility instrument, 960 km and the spatial resolution GOMOS does not have a principal varies between 40 km x 40 km and investigator. Instead, the scientific 40 km x 320 km. lead of the instrument lies in the hands of ESA, with support from the The GOME-2 instrument on Metop will GOMOS Science Advisory Group have a larger swath width (1920 km, (SAG).The members of the GOMOS but the old swath width will also be SAG are presently, in alphabetic possible), a better polarisation order, J.-L. Bertaux, Service measurement and improvements in d'Aeronomie du CNRS, France, the optics and the calibration. M. Chipperfield, University of Leeds, United Kingdom, F. Dalaudier, Service The present operational data product d'Aeronomie du CNRS, France, contains the calibrated radiances and D. Fussen, IASB, Belgium, irradiances and the geophysical A. Hauchecorne, Service d'Aeronornie, products ozone and N02 total column France, E. Kyrola, Finnish measurements. Meteorological Institute, Finland, G. Leppelmeier, Finnish 1.2 GOMOS short history Meteorological Institute, Finland, G. Megie, Service d'Aeronomie du In January 1988, ESA issued a call for CNRS, France, M. Pirre, LPCE/CNRS, proposals for Announcement of France, H. Roscoe, British Antarctic Opportunity instruments to be Survey, United Kingdom and included in the Earth Observation P. Simon, Belgian Institute for Space Polar Platform mission, the earlier Aeronomy, Belgium. name of Envisat. Among the responses received was the proposal for Global 1.3 Scientific rationale Ozone Monitoring by Occultation of Stars (GOMOS).The GOMOSproposal Stratospheric ozone is the main came from the European scientists absorber of solar ultraviolet (UV) J.L. Bertaux, R. Pellinen, P. Simon, radiation. Without ozone, life as we E. Chassefiere, F. Dalaudier, S. Godin, know it on the Earth's surface would F. Goutail, A. Hauchecorne, not exist. Any significant decline in H. Le Texier, G. Megie,J.P.Pommereau, stratospheric ozone is therefore of G. Brasseur, E. Kyrola, T. Tuomi, serious environmental concern. The S. Korpela, G. Leppelmeier, G. Visconti, ozone concentration and distribution P. Fabian, S.A. Isaksen, S.H. Larsen, of ozone also affects the temperature F. Stordahl, D. Cariolle, J. Lenoble, structure in the stratosphere. Any J.P. Naudet and N. Scott. change in stratospheric ozone can therefore have an effect on the heat An industrial Phase-A study was budget of the atmosphere and could conducted for the French Space impact the climate. Agency, CNES, in 1989-1990. Afterwards it was decided at the ESA Stratospheric ozone production and Member States Ministerial Conference destruction, plus stratospheric in Madrid in 1992, that GOMOSwould transport and troposphere• fly as an ESA-funded instrument on stratosphere exchange, control ozone the Envisat platform. After MIPAS, levels. The only source of ozone is the GOMOS has thus become the second combination of molecular and atomic
3 oxygen. Atomic oxygen is produced and the careful monitoring of ozone through dissociation of molecular and substances related to ozone oxygen by solar UV radiation. Ozone destruction is therefore necessary. is destroyed by the absorption of solar UV radiation, which constitutes the Besides the necessary ozone UV filter effect of the ozone layer. monitoring, several scientific Fragments of the destroyed ozone questions regarding the ozone molecule, molecular and atomic depletion processes are still open. oxygen, can re-combine to form ozone Although measurements suggest that molecules again. Besides this natural most of the ozone decline occurs in process of ozone destruction, ozone is the lower stratosphere, in line with depleted by a variety of catalytic current hypotheses, observations of cycles involving the oxides of trends in ozone profiles lack sufficient hydrogen, nitrogen, bromine and accuracy for definitive statements to chlorine. Man-made emissions of be made. Worse, current models of ozone depleting substances play a stratospheric chemistry do not predict key-role in the drastic reduction in the magnitude of the observed dedine ozone amounts observed in the Arctic in levels of ozone. The observation of and Antarctic regions. ozone trends is not only important in the lower stratosphere, but in the The understanding of those processes higher stratosphere and mesosphere involving gas-phase and hetero• as well. geneous chemistry has significantly improved in the past years. However, GOMOS, MIPASand SCIAMACHYon despite the significant improvements Envisat will explore different in our understanding of stratospheric observation techniques to address chemistry and dynamics, the science issues related to the ozone problem. is still far from being completely The instruments are highly cornp.e• resolved. For example, reductions in mentary in coverage, resolution and ozone levels are also observed at mid• atmospheric parameters observed. latitudes, but the mechanisms GOMOS has the great advantage of a responsible are not clear. Thus, very simple observation technique stratospheric ozone research still and comparatively easy algorithms needs further observation of ozone for the retrieval of the atmospheric and other parameters relevant to the parameters. It has the potential to problem. In order to understand long• become a relatively inexpensive term changes of the stratospheric standard ozone-monitoring composition, accurate measurements instrument. As GOMOS measures are necessary over decades. GOMOS the attenuation of stellar light responds to both this requirement, by through the atmosphere, the ozone monitoring with high accuracy atmospheric transmittance can be and high vertical resolution. directly retrieved from the ratio of the stellar light outside the Stratospheric ozone content has been atmosphere to that passing through declining worldwide since the 1970s. the atmosphere. The stellar The decline is of the order of 4% to occultation technique enables 8% per decade. After international GOMOS to measure with high agreements regarding the reduction of vertical resolution and with a global ozone destroying substances, there is coverage. Since the instrument is in hope that the destruction of ozone principle self-calibrating, linear will slow down and that eventually ozone trends should be observable recovery will occur. This is, however, with an accuracy in the order of a process that will take many decades 0.1 % per year.
4 1.4 The observation spherically symmetric. This vertical principle retrieval is straightforward, exploiting the so-called 'onion-peeling' technique. GOMOS is being implemented on Envisat with its overall field of view Besides the extreme simplicity of oriented opposite to the velocity the retrieval algorithm compared vector, allowing the instrument to to other methods, the occultation observe the successive setting of technique has one enormous various stars while the platform advantage readily expressed from the moves along its orbit. The principle of mathematical form of Equation (1.2): stellar occultation is quite simple. an absolute estimate of N(z) When the star is high above the is obtained from the ratio of two horizon, the light spectrum of the star measurements taken with the
F0(A.) is recorded by GOMOS, without same instrument within a few seconds any atmospheric absorption. A few interval so very high accuracies are seconds later, the light spectrum of possible. The method is inherently self• the same star, observed while setting calibrated, and even if the spectral through the atmosphere, is recorded. sensitivity of the instrument is The spectrum F(A.,z) is modified by the changing with time, the ratio and the absorption of all atmospheric column density N(z) will be measured constituents, integrated over the line• correctly during the relatively short of-sight from the satellite to the star, time of a single occultation. This according to the Beer-Lambert law. protection against long-term drift is of When only ozone absorption is course ideal for the study of trends of considered (for simplicity) the ozone and other constituents. spectrum can be written as: This self-calibration characteristic is shared with the solar occultation (1. 1) technique, used very successfully in the past by the US instruments SAGE and SAGE II, whose results are often where A is the wavelength, z is the taken as a reference for vertical ozone altitude of the line-of-sight above the distributions. However, one should horizon, N(z)(molecules/ cm2) is the note that, by definition, solar occulta• integrated quantity of ozone along the tions occur during a transient photo• line-of-sight and C\ the absorption chemical situation, which is more cross section of ozone. complex than the plain night-time situation. Furthermore, stellar From Equation (1.1) one derives: occultation measurements can be performed both during night and day I F (A.:.) over a wide geographic region, N (r ) --log( ) (1.2) providing a much denser global 5 In principle, it is sufficient to know distribution, sampled by the the total ozone to predict how much occultation geometry, are strongly harmful ultraviolet radiation reaches peaked, with no contribution from the the ground. Howeve, the ozone• regions lying below the altitude z of destruction processes depend heavily the line-of-sight, and a rapidly on the altitude and simulations and decreasing contribution from upper available trend data confirm that the layers. This ensures a well-behaved ozone decrease also depends strongly solution for vertical profile retrieval. Fig. 1-2 A set of synthetic on the altitude. weighting functions The altitude of the measurement is representative of a Therefore, in order to validate the entirely defined by the direction of the vertical occulation. The limits of the acquisition understanding of the ozone star and by the position of the are 20 to 21.5 km, 50 to evolution, one must obtain ozone spacecraft. The position of Envisat 51.5 km and 80 to 81.5 measurements from the whole will be known to an accuracy of 30 m km, respectively, from bottom to top. The stratosphere, with a good altitude or better, ensuring accurate second plot is a zoom of resolution and to high accuracy. knowledge of z. Therefore, the the lowermost weighting location of the observed air mass, and function emphasising its shape around the As shown in Figure 1.2, the the tangent altitude of the line of maximum. weighting functions of the vertical sight can be determined without considering the attitude of the spacecraft. Altitude accuracy is important, because an error of 100 m 120 in the tangent altitude would result in a 1% error in the ozone density 100 estimate in the altitude region abcve ••...... the ozone peak. E 80 ::! ...... • During one occultation GOMOS will Q) 60 "O measure stellar light in 0.5 sec .•..•::J :;:; 40 integration time intervals. This < corresponds in the worst case 20 (occultation in the orbital plane) to an interval of ~z = 1.7 km of altitude 0 projected at the limb, which is equal 0.0 0.5 1.0 1.5 to a quarter of a scale height. The Weighting function weighting functions are accordingly modified as in Figure 1.2. The 30 vertical resolution is therefore 1.7 km in the case of an occultation in the orbital plane and better than 1.7 km for oblique occultations. The accuracy of the retrieved geophysical parameters depends on the temperature and brightness of the respective star. Up to about 30 km, where the Chappuis band is used to determine ozone, the accuracy does 15~~.._....._....__._ _.__.__.__.__. not depend on the star temperature. 0.0 0.5 1.0 1.5 The accuracy at 25 km altitude is Weighting function about 2% (for a star with magnitude 0) to 10% (magnitude 2) and degrades 6 rapidly above and below that height the same star are aligned along a for a cold star (temperature up to constant latitude circle, regularly about 6000 K). Above 30 km, where spaced in longitude. (An example of UV absorption is used, the ozone the distribution of occultations will be profile retrieval accuracy strongly shown in Chapter 6: Mission depends on the star temperature. A Planning, Figure 6.8.) In addition, star around 10000 K provides about because there are 14+11/35 orbits the same accuracy for the altitude per 24 hours, nearly the same range 25 to 60 km. A star hotter than longitude sampling applies three days 10000 K provides an improved later, and sampling on day two and accuracy for the upper stratosphere day three are interlaced with those of and lower mesosphere. The best day one. Since the correlation time of accuracy can be achieved around ozone is more than three days 60 km. For example, at 60 km [Frederick 1984], it will be of great altitude a 30000 K star provides a interest to regroup three days of orbit retrieval accuracy around 0.5'Yo to provide a more complete global (magnitude 0) or 3°/ci (magnitude 2). coverage. GOMOS mission planning will be discussed in a separate 1.4.2 Geographical coverage chapter when an example of the global coverage will be given. Inherent self-calibration and algorithm simplicity are two 1.4.3 A short history of the stellar characteristics shared by both solar occultation technique and stellar occultation. However, the stellar occultation method has The stellar occultation technique has superior geographical and local time been reviewed by Smith & Hunten, coverage for two reasons: 1989. It has been used several times to probe the vertical structure of from a Sun-synchronous polar planets (and Titan), exploiting orbit like the Envisat orbit, solar refraction. GOMOS will exploit the occultations occur only in a absorptive properties of the occulted limited range of high latitudes atmosphere, as described on detail by (North and South), preventing e.g. Hays and Roble, 1968. The ozone monitoring in the mid technique has only been used to latitudes. observe the Earth's atmosphere a few times in the past from orbit: for the there is only one Sun, but there detection of H2 with the Copernicus are many stars. satellite [Atreya et al., 1976] and for the vertical profile of ozone with Simulations have shown that the OA0-3 [Riegler et al., 1976,1977]. occultation of 25 to 45 stars can be This observation yielded the first observed in a single orbit, distributed direct observation of the secondary over all latitudes. It can be noted that maximum of ozone at 90 km. when a star is observed on one orbit, it can also be observed during the However, the quantity of ozone, which following orbit at almost the same was derived from the measurements latitude. As the star is fixed in an around 50 km was larger (by a factor inertial system, the orbital plane has of three) than predicted by models for moved by only a small fraction reasons which were never elucidated. ("'1/15) of a degree and the Earth has It might have been a combination of rotated by 25°. Therefore, the imperfect orbit knowledge (10 km) succession of points of occultation of and the presence of some other 7 absorber besides ozone. The problem GOMOS.These fast variations of light is that with only one wavelength intensity do not occur simultaneously monitored during the occultation, one at all wavelengths so the spectrum of can never be sure of the identification a star seen through the atmosphere is of the absorber, and indeed in most not only affected by absorption, but is cases there is more than one absorber also deformed by scintillations. This at any wavelength, modifying problem is resolved by the use of two Equations (1.1) and (1.2). fast photometers, which allow the measurement of scintillations and The advent of multi-element detectors reconstruct the actual deformation of has allowed enormous progress to be individual spectra. Furthermore, the made in exploitation of the technique analysis of the scintillation signal can of stellar occultation by using be used to retrieve the vertical fine instruments capable of making 'multi• structure of the atmosphere that spectral' occultations: the absorption causes the scintillation. of the atmosphere is measured simultaneously at many wavelengths, On the day-side, the bright limb is an ensuring the proper identification of extended source competing with the the various absorbers and stellar signal, calling for a small discriminating their individual instrument field-of-view,which in contribution for each line-of-sight. turns requires an accurate pointing. This is because the cross sections, a.,I. Clearly, day-side occultations will of gaseous absorbers vary as a achieve less accurate results than function of wavelength, their spectral night-side occultations, but they are signature. considered necessary, to avoid a possible bias linked to photo• 1.4.4 Difficulties associated with chemistry. stellar occultation 1.4.5 This report The most obvious difficulty is the weakness of the stellar fluxes This report discusses the GOMOS compared to the levels of solar instrument on Envisat. The aim is radiation. This calls for a relatively to present the scientific rationale, large optical collecting area compared instrument concept, data to the optics requirements of a solar processing and products in sufficient occultation instrument. Another detail for the scientific reader. important difference to solar occultation is the significant variation Chapter 2 presents the scientific in temperature and brightness rationale and derived mission between the stars. This results in a objectives, followedby the instruments corresponding variability of requirements in Chapter 3. The measurement precision and accuracy. instrument concept is presented ir• Chapter 4, with an emphasis on the Atmospheric refraction causes loss of technical details necessary to light, by dilution, when lower parts of understand of the technical concept the atmosphere are probed (below and modes of operation. Instrument 30 km). Here, the light path is not a characterisation and calibration are straight line. In addition to that, the described in Chapter 5. short-term variability of the density and temperature of the air along the The fact that GOMOSuses stars as light path results in scintillations in light sources means that, most of the the light intensity detected by time, choices have to be made as to 8 which star to use amongst several The overall conclusion is that alternatives lying in the instrument's GOMOS, an instrument for field-of-view. stratospheric and mesospheric ozone monitoring, as its primary objective, A selection procedure has been particularly well suited for trend developed to optimise the star observation because it exploits the selection in order to fulfil the mission self-calibrating principle of stellar objectives, as described in Chapter 2, occultation. The occultation principle in the best way. This mission make the data analysis relatively planning strategy is described in simple and the use of stars instead of Chapter 6. the Moon or Sun as the light source results in a very good global coverage, The GOMOS data products are including tropical regions, mid• discussed in Chapter 7. This includes latitudes and polar regions. the scientific data processing and a discussion of the retrieval approach. Chapter 8 outlines the planned data validation activities. 9 2. Scientific Objectives 2.1 Motivation Atmospheric Ozone The stratospheric ozone layer protects the Earth's surface from ultraviolet 35 • Contolns 90% of Atmospheric Ozone • Beneficial Role' Acts as Primary UV (UV)radiation. It extends from the Radiation Shield tropopause, at a height of around 30 • Ctrrent Issues: Ii> Stratospheric Ozone - Long- Term Global Downwad Trends .._25 - Springtime Antarctic Ozone Hole 8 km (in polar regions) or 15 km (in (The Ozone Layer} Each Year the tropics), to the stratopause at ~ - Springtime Arctic Ozone Losses _Q 20 inSeveral Recent Years around 50 km. The stratosphere is ~ characterised by an increase in ~ 15 • Contains 10% of Atmospheric Ozone .z • Harmful Impact' Toxic Effects on temperature with height, caused by Humans and Vegetation the absorption of solar UV radiation • Current lssues: - Episodes of High Surface Ozone by ozone. The highest concentration 51-) "Smog" Ozone in Urban and Rural Areas of ozone is typically at an altitude of /, around 25 km, as shown in Fig. 2-1. 0 5 10 15 20 25 Ozone Amount The stratosphere contains about 90'Yr, (pressure, milli- Pascals} of the total atmospheric ozone. Normally the stratospheric ozone layer absorbs UV-B radiation almost completely, preventing it from dynamics, with a possible impact on Fig. 2-1 Typical reaching the Earth's surface. the climate system. Furthermore, a distribution of ozone. (WMO, 1998) Ultraviolet radiation (UV-Bat 290- cooling of the stratosphere favours 320 nm and UV-Cat 200-290 nm) is ozone depletion in the polar regions of strongly absorbed by the DNA the globe via heterogeneous chemical molecules of living cells and can alter processes acting on solid or liquid or stop the reproductive processes of cloud particles. these cells. Therefore, a substantial reduction of stratospheric ozone can Stratospheric ozone has decreased increase the risk of skin cancer and globally since the 1970s. The most other genetic disorders caused by spectacular ozone loss occurs in the UV-B radiation in flora and fauna. Antarctic region, where the ozone loss every year is now large. Each October, The absorption of UV by ozone is the the total ozone now falls below half its most important heat source in the former value and at some altitudes stratosphere. A decrease in the ozone concentration already falls stratospheric ozone concentrations below detection limits. As well as in would therefore have an impact on the Antarctic, the ozone concentration stratospheric temperatures, which is declining elsewhere. Since 1991, would lead to a disturbance in the significant ozone reduction has also 11 ozone layer (over the next few Combined Trend(1 & 2 sigmo) 1980-96 decades). Today global ozone destruction is still continuing. ' However, in 1998, the trend of '', increasing global chlorine loading '' 'I appeared to be stagnating, possibly as '.•.,....•. a result of the Montreal Protocol. In a ..•...... • ~ best-case analysis [Engel and Schmidt, 1999], chlorine loading in the stratosphere was found to decrease after 2000, but not reach pre-ozone-hole values for at least six decades. Current stratospheric chemistry models do not quantitatively reproduce the observed decline in ozone. Hence it is essential to further -15.0 -12.5 -10.0 -7.!5 -s.o -2.5 0.0 2.5 !5.0 Trend·("/decode) improve atmospheric composition observations and to continue to monitor trends in ozone, and in its profile, but with improved accuracy. Fig. 2-2 The mean, annually averaged trend been observed at northern mid- and Only in this way can it be confirmed in the vertical distribution high-latitudes. Fig. 2-2 shows ozone that the hypotheses about the details of ozone that has trends for latitudes between 30°N and of ozone depletion are accurate and occurred over northern mid-latitudes from 1980- 50°N during the 1980s, as a function whether the provisions of the 1996 (heavy solid line) is of altitude, derived from different Montreal Protocol are having the calculated using the observation systems. In the mid• desired effect. trends derived from SAGE I/II, ozonesondes, Umkehr latitudes, the largest ozone loss and SBUVmeasurements. occurs in the chemically most active The combined altitude region around 40 km. Also in 2.2 Current Science Issues uncertainties are shown for 67% confidence limits the 25 km region with the highest (light solid lines) and 95% ozone concentration the decrease in Despite the significant advances in confidence limits (dashed ozone is significant. In polar regions the understanding of polar ozone lines). The results below 15 km are a mixture of the ozone loss is at its highest in the depletion over the last few years, stratospheric and lower stratosphere, where there are still significant uncertainties tropospheric trends and heterogeneous ozone depletion in the understanding of stratospheric the exact numbers should be viewed with caution. occurs. chemistry which must be addressed. (SPARC/IOC/GAW 1998) In particular, further work is needed The main cause for the observed to help attribute the observed mid• ozone destruction since 1980 is latitude 03 trend to different proposed anthropogenic pollution, namely the causes, and to investigate the release of chlorofluorocarbons (CFCs), interaction of other trends (e.g. in which produce free chlorine in the temperature and H20) with that of lower stratosphere, and halons, which stratospheric 03. In addition to tee release bromine radicals. Regulations, efforts towards advancing such as those in the Montreal understanding of the chemical and Protocol and its ammendments, have dynamical processes, continued reduced the release of CFCs and monitoring of ozone and its profile is other source gases so that a decline essential to confirm the observed in ozone destruction can be expected trends, and their changes, especially and eventually a slow recovery of the as the stratospheric halogen loading 12 becomes stabilised, and starts to southern counterpart, the Arctic reduce, following implementation of vortex is highly variable and mobile, the Montreal Protocol and its often moving above middle latitudes amendments. under the influence of forcing by planetary waves and by tropospheric 2.2.1 Monitoring profiles of 03 weather systems. In recent years and trends (1994/95 to 1996/97 and 1999/ 2000) significant OJ loss has Statistical studies show that been observed in the Arctic, due to significant reductions in ozone have favourable meteorological conditions occurred at northern mid-latitudes (i.e. lower temperatures). Further and, although details of the study of the Arctic is necessary to mechanism are not clearly investigate whether this variation is a understood, a consensus has emerged consequence of natural variability or that these reductions are at least a consequence of lower stratospheric partly due to the presence of reactive temperatures in the Arctic spring chlorine derived from man-made related to stratospheric radiative halocarbons. Measurements strongly cooling caused by the increase of suggest that most of this decline in greenhouse gases. In this case the ozone amounts occurs in the lower Arctic ozone depletion will become stratosphere, in agreement with more severe. current hypotheses about the actual mechanism. 2.2.3 Mid-latitude ozone trends However, measurements of trends in Despite the observations of the ozone profiles lack the accuracy of downward OJ trend at mid-latitudes, measurements of trends in the ozone the causes have not yet been firmly column and current models of established. Mid-latitude ozone stratospheric chemistry do not depletion in the lower stratosphere is reproduce the observed decline in probably dominated by heterogeneous ozone amounts. To resolve this chemistry on the surface of sulphate conundrum it is essential to continue aerosols. Whereas ozone depletion to monitor trends in total ozone to cycles involving the HO" radicals, are current accuracies and trends in its most important at altitudes below profile with improved accuracy. about 20 km, cycles involving the halogen radicals (BrO and ClO) also The total ozone (60°N to 60°S) play a significant role. Important declined by 2.9% per decade in the insights into the relative roles of period from 1979 to 1994. different catalytic cycles, in controlling Consequently, the ozone changes in the ozone abundance in the mid• the stratosphere (where about 90°/r,of latitude lower stratosphere, have come the total ozone resides) must be from in-situ measurements made on monitored globally with an accuracy the NASAER-2 aircraft, suggesting of 0.3% per year. that the understanding of the fast gas phase chemistry is satisfactory. 2.2.2 Increasing Arctic 03 depletion In May, for an assumed ClO concentration of about 25 pptv, the It is now clear that conditions ozone loss rate would be more than favourable to ozone loss are found 1% per month. Analysis of data from most years within the Arctic vortex, the MLS instrument on the UARS leading to ozone depletion. Unlike its satellite reveals ClO concentrations 13 Emissions of NOxand other gases 6 I I from aircraft exhausts are expected to Modal with observed T', aerosols affect stratospheric ozone. However, Model, no aerosol changes, no T' 4+-. '-,_ ..._ the incomplete knowledge of the '--- Model with aerosol changes. no T' 0 SBUV/SBUV2 Data details of the chemical and I I .. TOMS V7 Data meteorological processes in the very ~ 2 '-'ii. >o low stratosphere and upper a; E 0 troposphere, where long-distance c 0 c( aircraft fly, limits the ability to pred.ct c the impact of aircraft emissions in 0"' N 0 -2 I this region. ~ 2.2.4 Trends in temperature, H20 -4-i 45° N and stratospheric aerosol -6 v 78 BO 82 84 86 88 90 92 94 96 98 Observations indicate that stratospheric water vapour is Time (years) increasing. Measurements obtained at a single station at northern mid• latitudes indicate that water vapour concentrations in the lower Fig. 2-3 Observed and exceeding 100 pptv at middle stratosphere increased between 198: calculated total ozone latitudes in the lower stratosphere in and 1994 [Oltman and Hofmann, anomalies from 1979 to 1997 at 45"N.All cases winter, implying a significant year• 1995]. Futhermore, HALOEsatellite shown are smoothed by a round contribution of reactive data from 1991 onwards indicate that 25-month running mean to chlorine to mid-latitude ozone loss in these postive trends extend throughout average over both the annual cycle and the QBO. this region. It is now clear that BrO is the stratosphere [Evans et al., 1998; Red triangles show the present in significant amounts Nedoluha et al., 1998]. These increases zonally averaged TOMSV7 outside the polar vortex. Observations are larger than can be explained by the data and the blue circles show the combined of BrO in the lower stratosphere increase in the chemical production of SBUV/SBUV2 data. All demonstrate that reactions involving water resulting from the observed model results shown bromine account for more than 50% changes in atmospheric methane, and include chlorine and bromine increases. In of the catalytic destruction of ozone they may be indicative of other longer addition, the red line by halogens at 20 km altitude. term changes in the stratosphere or at shows the model results the tropical tropopause. A general including observed aerosol variations and zonal mean A recent model study showed that, in increase in water vapour temperature and planetary the presence of the contemporary concentrations would increase HOx wave temperature chlorine and bromine stratospheric radicals and would also enhance ozone amplitudes (T') estimated from National Centers for loadings, the changes in aerosol loss through an increased incidence of Environmental Prediction loading during the 1980s and 1990s, PSCs. However,it is important to note (NCEP)observations. The following major volcanic eruptions, that reliable long-term measurements black line is similar to the red line except the correlate with variations in modelled of water (i.e. greater than 15 years) planetary wave and observed mid-latitude levels of have only been made at this one site, temperature amplitudes ozone. Although, it is clear that mid• with shorter records from satellites and are not used in the model. The green line shows latitude aerosol is also playing a role other stations. The cause of these model results with in determining ozone amounts, the trends is not clear and more constant 1979 aerosol magnitude of the effect is larger than measurements are needed. observation used throughout the calculation. can be reproduced in the model (WMO,1998) (Fig. 2-3). Thus it appears that the If the increase in stratospheric water knowledge of the chemistry and/ or vapour is related to a warming of the dynamics is incomplete; further tropopause this will be important as quantification is required. the processes involved in 14 Fig. 2-4 Observations from --- (aj a high-altitude aircraft of mixing ratios of (a) ozone and (b) water vapour (ppmv), in April 1984 at Northern mid-latitudes during a tropopause fold. •• Note the fine resolution of the features, with tongues of air of thickness >00 20 mbar at 100 mbar - about 1 km. (WMO, 1985) 38 Latitude (deg NJ " stratospheric/ tropospheric exchange will increase the upper tropospheric are poorly understood. This transport humidity as long as the water vapour determines the amount of water concentration is below saturation. If vapour present in the stratosphere the air is near saturation, the and it is crucial for predictions of: cloudiness increases. The effect of increased cloudiness is not a clear a) stratospheric concentrations of forcing signal, as the effect of clouds the shorter-lived substitutes for depends strongly on altitude (e.g. fog CFCs (transport from the acts to cool the Earth's surface). Hence troposphere to the stratosphere, a high-resolution global climatology of mostly in the tropics); humidity in the upper troposphere is an important climatic variable. b) the amounts of tropospheric ozone observed away from The amount of sulphate aerosol in the polluted areas (transport from the stratosphere is also important stratosphere to the troposphere, because the heterogeneous reactions mostly in tropopause folds at involved in ozone loss occur on mid-latitudes). aerosols both at mid-latitudes and in polar regions. The amounts of Because its concentration is much sulphate aerosol in the stratosphere greater in the stratosphere than in increased significantly in 1991 and the troposphere, ozone is a 1992 as a result of the eruption of particularly useful tracer for Mount Pinatubo in June 1991. observing stratosphere-troposphere Because of the lack of significant exchange, as illustrated in Fig. 2-4. volcanic eruptions since 1992, the The exchange of water vapour is still amount of aerosol in the stratosphere poorly understood, despite being is now less than at any time since studied for many years. satellite measurements started in 1979. Measuring the stratospheric Water vapour is a potent greenhouse aerosol requires a high vertical gas. The amount present in the upper resolution because of the strong troposphere is therefore important for vertical and horizontal gradients of the radiation budget. Global warming aerosol concentration. 15 2.2.5 Stratospheric dynamics concentration of ozone in the lower stratosphere at middle latitudes is tne Stratospheric circulation is meridional transport of air, eventually characterised by general ascent in the depleted in ozone, from high latitudes equatorial region, poleward transport in polar filaments formed at the edge and a descent at high latitudes. This of the polar vortex. The contribution global circulation is controlled by of this mechanism to the trend of extra-tropical pumping [Holton et al. ozone observed at middle latitudes 1995]. Extratropical planetary waves has yet to be quantified. The rate of propagate from the troposphere to the mixing of air out of the polar vortex is upper stratosphere where they expected to vary with altitude; it is dissipate and decelerate the mean probably most efficient in the very low zonal wind, induce the mean stratosphere. poleward transport and so redistribute ozone. When looking at The transport of air which is poor in the trends in stratospheric ozone at ozone, from the tropical upper middle and high latitudes, one has to troposphere to the mid-latitude lower consider a possible evolution of the stratosphere in subtropical poleward transport, due to changes in intrusions, should also be considered this planetary wave activity, as a as a potential contributor to the m.d• contributory factor. latitude ozone budget. This mechanism is responsible for the Extratropical pumping also imposes dynamically induced mini-holes upward equatorial motion hence the visible on global satellite maps of total global injection of tropospheric air into ozone. the stratosphere. However,the detailed mechanisms involved in this process The irreversible mixing of air corning still need to be clarified. The main from different latitudes and altitudes mechanism is deep convection in occurs in turbulent layers generated cumulo-nimbus clouds reaching the by the breaking of gravity waves. tropopause and inducing very cold They are generated in the troposphere temperatures. This leads to the above mountains, above convective condensation of water vapour and clouds, and in the jet stream. They strong dehydration of air injected into propagate upwards in the middle the stratosphere (i.e. the so-called 'cold stratosphere, with an amplitude trap). However,the lowest water increasing as the inverse of the vapour mixing ratio (hygropause) is not square root of the density, until they encountered at the tropopause but a reach a condition of instability and few kilometres above. Some additional break, inducing turbulent layers and dehydration may occur in the very low the acceleration of the mean wind in stratosphere to explain this feature. It the direction of the phase is possible that local cooling occurs, propagation. They are the main leading to the condensation of a contributor to the mesospheric fraction of the water vapour into circulation and may also play a role equatorial stratospheric clouds. One of in the stratospheric circulation. the hypotheses put forward to explain Small-scale motions induced by the this is the transient cooling associated breaking of gravity waves, coupled with the propagation of Kelvinwaves with molecular diffusion, transport and gravity waves. the various chemical species through the atmosphere. Many local studies of Another important mechanism to be gravity waves and turbulent layers considered in interpreting the have been performed using in-situ 16 balloon measurements and ground• conditions i.e. when the observer is in based radars and lidars, but nothing the dark and the clouds are still is known concerning the global illuminated by 'Earth grazing' sunlight. climatology of these phenomena in the stratosphere. PMCs appear only at high latitudes during summer. This feature is 2.2.6 Mesospheric ozone and explained easily by the fact that clouds mesospheric air descends during summer conditions at high latitudes Ozone in the mesosphere is produced and the temperature reaches its by the photo-dissociation of 02 and minimum value (below 150 K; the the subsequent reaction of 0 with 02. coldest region of the entire It is destroyed by reactive hydrogen atmosphere). Then the amounts of radicals, i.e. H, OH and H02. The water vapour exceed the local photochemical lifetimes of the oxygen saturation vapour pressure and ice and hydrogen radicals are only particles form around condensation hours in the mesosphere so only nuclei, which could be either the distributions of the long-lived meteoritic dust particles or large molecules that produce hydrogen clusters of ionised water. radicals (i.e. H20 and H2), are likely to be affected by atmospheric In his review of the subject, Thomas, transport. However, despite the 1991, advocated that the appearance apparent simplicity of this situation, of PMCs is a strong indicator of global there exist significant disagreements, change. He argued that the first which need to be resolved, between reported observation was made in the predictions of current models and 1885, though there were a large observations. number of talented observers before. The explanation would be that These centre on the variations in methane has been increasing steadily mesospheric ozone: diurnal and semi• since around 1850 (growingpopulation diurnal variations and those and associated development of associated with variations in solar agriculture and livestock), with a activity. The amplitude of the diurnal present growth rate of 1'Yo per year. variation increases from a few percent Indeed, oxidation of CH4 is an at an altitude of 50 km to 80°/c, at important source of H20 above about 70 km. This mainly reflects variations 50 km, and therefore an increase of in the equilibrium between 03 and 0. CH4 at ground level will be reflected The equilibrium value of the ratio some years later by an increase of H20 [0]/[03] increases rapidly with altitude at mesopause level. The saturation and exceeds unity above vapour pressure decreases by a factor 65 km while below this altitude almost of 5 for a decrease of 4 K for summer all the 0 is converted to 03 after dusk. mesopause conditions, so that even a moderate climatic temperature change Polar mesospheric clouds (PMC)are (induced by increased C02) could cirrus-like clouds of small particles, cause an increase in global PMC confined to a small altitude range cover. (82-85 km) where the atmospheric temperature is at a minimum (i.e. the 2.2. 7 Recovery of stratospheric mesopause). They were first observed ozone from the ground and called 'noctilucent clouds' as they could only The peak in bromine loading is be observed under specific twilight expected to occur between the years 17 amount of ozone for the first two Assimilated COMEtotal ozone KNMI/ESA weeks of March was 16% lower than 22- 9-00 12h observed in the 1980s. In September 2000, the Antarctic ozone hole developed with extremely low ozone values, which occurred earlier in the season than ever observed before. Fig. 2-5 shows assimilated GOME observations These recent observations have no data • reinforced the concern that even if • • • • D ~ • D D D • • • D D < 150 175 200 225 250 275 SOO 325 300 375 400 4Z5 «10 47(1>500DU halogen loading decreases eventually, changing dynamical conditions causing a colder stratosphere would support ozone-depleting conditions Fig. 2-5 Antarctic ozone 2000 and 2010 and, when coupled during the polar winter. This would hole September 2000 with the expected decline in chlorine lead to a delay in the recovery of the observed by GOMEon ERS-2, assimilated by loading, would result in only a small ozone layer. KNMI. (Courtesy KNMI, decrease in total effective halogen The Netherlands) loading between now and then, with a Therefore, it is presently far too much sharper fall thereafter. early to assume that the ozone layer Stratospheric ozone is expected to would automatically recover if recover as the loading of stratospheric international agreements on reduction chlorine and bromine decreases. Large of halogen emissions (Montreal ozone loss in the Arctic, similar to that Protocol) were effective. A continued seen in the winters of 1995/96, monitoring of ozone, species involved 1996/97 and 1999/2000, can be in ozone depletion, and of dynamical expected to continue for at least the conditions, is therefore required with next decade in cold winters. This high accuracy, vertical resolution and picture may change if stratospheric global coverage. temperatures continue to decrease, in which case recovery would take longer, Further study of the 03 depletion or if there are further changes in at 40 km is also important. In this atmospheric composition. photo-chemically controlled regime, the influence of changes in chlorine During the winter of 1999-2000 large and temperature should be ozone losses were observed inside the discernible in the local 03 Arctic stratospheric polar vortex. abundance. The ability of GOMOS These ozone losses were observed to produce accurate ozone density from ground- based and ozone-sonde profiles in this critical region will measurements. At altitudes around make it possible to study closely 18 km cumulative losses of over 60% the coupling of ozone to chlorine occurred between January and density and temperature. March. These are among the largest chemical losses at this altitude observed during the 1990s. Satellite 2.3 Ozone Depletion observations (e.g. by the ESA Global Ozone Monitoring Experiment GOME) While stratospheric ozone is formed showed a clear ozone minimum over by one principal set of reactions, the the polar region during February and photodissociation of 02 followed by March. The average polar column combination of molecular and atomic 18 oxygen, there are numerous Fig. 2-6 Cycles of the Hydrogen Cycle reactions, which destroy ozone in gas reactive gases that control loss of ozone by gas-phase phase. There are also heterogeneous chemistry reactions, on or at the surface of particles, which affect ozone-depleting species. 2.3.1 Stratospheric gas-phase chemistry In the stratosphere, ozone is in balance between production and destruction, with a time constant of a -~ few days to many years. In the mid• upper stratosphere ozone is formed by the dissociation, by ultraviolet sunlight, of molecular oxygen (02) to atomic oxygen (0), followed by a reaction between the atomic oxygen and molecular oxygen to give ozone (03). It is removed by a variety of catalytic cycles, which can be of the form: X+O---+XO+O 3 2 XO + 0 ---+ X + 0 (2.1) 2 Globally, one of the important ozone depleting cycles is that containing N02, which will be measured by GOMOS. See also Fig. 2-6, the odd nitrogen cycle. As well as these (and other) short• lived radicals there are a number of important reservoir gases, so-called because they store Cl, Br, OH and NO in less reactive forms, such as HCl, ClON02 (from the reaction of N02 and ClO) and HN03. Fig. 2-6 summarises some of the known interactions that occur between these families of radicals and reservoirs. The cycles involving the oxides of hydrogen and nitrogen arise naturally though there is concern that the anthropogenic production of N02 (from current commercial aircraft and future high-altitude aircraft) might increase stratospheric N02, thereby changing the amount of stratospheric 19 TOTAL OZONE (DOBSON) an agricultural fumigant and soil 90 steriliser. Reactive bromine is many times more effective at removing ozone than reactive chlorine. 60 N Because of the varied lifetime of 30 ozone in the stratosphere, its distribution is determined by a UJ 0 :::> combination of transport and I- j:: 0 chemistry. As mentioned in -c ...I Section 2.2.5, transport is particularly important in the lower 30 stratosphere, where the photo• s chemical lifetime of ozone becomes 60 long and it is transported poleward and downward from the photo• chemical source region. Fig. 2-7 90 J F M A M J J A s 0 N D shows clear evidence of this transport MONTH - the ozone column maximum does not occur in the tropics, the region of maximum production, but at high latitudes. Fig. 2-7 Variation of total ozone, although this effect is now ozone with latitude and believed to be small in the 2.3.2 Heterogeneous chemistry season (WMO,1990) stratosphere. This may be contrasted with the situation for stratospheric In addition to the gas-phase chlorine where only 15-20% arises chemistry outlined above it is now from natural sources - most known that heterogeneous chemistry originates from man-made on aerosols and particles plays an halocarbons, e.g. chlorofluorocarbon important role in stratospheric ozone (CFCs). Anthropogenic sources also depletion. contribute significantly to the stratospheric bromine loading. Liquid sulphate aerosols are present throughout the lower stratosphere. Until recently CFCs were widely used Their abundance can be enhanced in domestic spray-cans and foam significantly following major volcar.ic products, and are still used in eruptions, as happened after the refrigerators. CFCs are very inert, eruption of Mt. Pinatubo in June therefore non-toxic, but have very long 1991. Under cold polar conditions life-times (70 to 150 years) in the (temperatures less than 200 K), these lower atmosphere. Therefore, they liquid aerosols take up water and survive long enough to reach the grow. This yields polar stratospheric stratosphere through convection and clouds (PSCs) which can be either dynamical troposphere-stratosphere liquid, supercooled solutions, or solid exchange processes. Only in the particles. stratosphere can UV radiation eventually break up the molecules and When PSCs form, heterogeneous release the chlorine atoms, which can reactions convert the chlorine then enter into ozone-depleting cycles. reservoirs HCl and ClON02 into active Bromine-containing halocarbons were chlorine via reactions such as: used in many fire extinguishers, and methyl bromide is still widely used as ClONO + HCl--->;Cl + HNO (2.2) 2 2 3 20 This heterogeneous reaction occurs between each August and October, rapidly on ice and solid nitric acid over half of the total ozone is trihydrate (NAT)as well as on liquid destroyed. This did not happen before aerosols composed of super-cooled the 1970s when the stratospheric ternary solutions of H2S04, HN03 and halocarbon loading was much water. smaller. The depletion occurs in the lower stratosphere, where the It appears that liquid droplets must catalytic cycles described in Section be supercooled to at least a few 2.3.1 cannot operate as the degrees below 195 K before solid abundance of atomic oxygen is too particles can form. However, once small. formed, the solid particles may survive at temperatures above 195 K The catalytic cycles which operate in although their surface area will the polar lower stratosphere involve decrease at the higher temperatures. ClO and BrO while competing non• The rate of chlorine activation on depleting pathways involve OClO, these particles is a function of their whose presence is therefore indicative physical condition (solid or liquid, of significant amounts of ClO and so water content). Because of the of the potential for ozone depletion. current incomplete understanding of Large amounts of ClO are possible via the physical properties of PSCs, the the heterogeneous reactions occurring ability to predict the future on or in PSCs. The heterogeneous evolution of Arctic ozone depletion reactions also suppress the amounts is limited. This is particularly of N02, which would otherwise react important in winters in which the with the ClO to form ClON02 and conditions for the formation of PSCs prevent ozone depletion. are marginal. This is also true of ozone depletion in locations where The mechanism for N02 removal is via PSCs are marginal, such as the edge the reaction of N02 with N03 (at of Antarctica. night) to produce N205, followed by heterogeneous reaction of N205 to 2.3.3 Polar ozone depletion form HN03, which in turn can be incorporated into the aerosols. If it is It has been demonstrated cold enough for ice to condense unequivocally that the presence of around the nitric acid trihydrate reactive chlorine and bromine from (NAT)core of a PSC, natural anthropogenic halocarbons leads to deposition occurs and the particles the destruction of stratospheric ozone eventually fall out of the stratosphere, and that their presence accounts a process known as denitrification. quantitatively for the formation of the Antarctic ozone hole. Reactive chlorine In the Arctic in winter the conditions compounds have also been observed when significant ozone depletion could in the Arctic in the lower stratosphere arise are quite commonly observed. in winter, indicating chlorine-catalysed There is now clear evidence of ozone destruction, by the same significant and widespread ozone loss mechanisms as occur over Antarctica. in the Arctic in winter, with features Indeed, in cold winters, large ozone similar to the early years of the depletion has now also been observed Antarctic ozone hole being seen in the over the Arctic. winters 1995/96, 1996/97 and 1999/2000. Maximum loss rates of up In the Antarctic, ozone depletion in to 2% per day have been observed in the spring is now so severe that the Arctic at altitudes around 18 km. 21 March 1996, Arctic air in the stratosphere, which had earlier been depleted in ozone, was displaced as + +January 340 far as the southern UK, where total ++ O February 0 ozone was the lowest in the data + + record. Temperatures in 1996/7 were 320 + + + + + ++ ++ + lower still and ozone losses exceeding + 0 s + + + 20% were identified by March 1997. + ++ 0 e 0 0o .,300-• o ++ 0 ++ + - -~ c + 0 0 0 0 0 + N 0 0 However, the largest ozone losses 0 0 0 +o + 0 continue to be observed over oo o 0 :§ 280c- 0 Antarctica each spring. In recent 00 0 years, there have been increased ozone losses at higher and lower 260 altitudes leading to larger overall 0 declines in total ozone amounts in 240---C-~-~~- this region. The inter-annual 1950 1960 1970 1980 1990 2000 variability in the ozone depletion year means that the depth and the areal extent of this depletion have not increased monotonically each year. Fig. 2-8 Trends in total At these altitudes the local losses, Total ozone in January and February ozone in summer in integrated through the winter and (Antarctic summer) has decreased by Antarctica, measured by the Dobson spectro• spring, were as large as 50-60%. over 20% since the 1960s, as shown photometer at Halley at These ozone losses occur during in Fig. 2-8. There has also been a 76"S (Courtesy of periods of chlorine activation. marked decline in the maximum J.D. Shanklin, British Antarctic Suroey, UK). Stratospheric temperatures during the ozone values that occur during the 1995/96 winter were the lowest in the final warming, suggesting that previous 30-year record, leading to dynamical effects are involved. Fig. 2-9 Seasonal and conditions that were particularly latitudinal variation of trends in column ozone conducive to ozone loss. In early When temperatures rise at the end of from TOMS. (WMO, 1998) the polar spring, the recovery of reactive chlorine compounds into the chlorine reservoirs takes place. This TOMS column trends (DU/decade) recovery usually occurs later in SON~~~~~~~~~~~~~~ Antarctica than elsewhere because of the lower temperatures and the widespread denitrification. Analysis of recent northern hemisphere winters 30N has shown the substantial chlorine ...... activation that occurs there each Ql ...... IJ winter. For any particular year the :::i = 0 duration of this activation is 0 determined by the evolution of -_J lil~iiili!!lifflllliliii[ifliii1111Qii•lll•!•iiiiilil•!lli temperatures during the winter. Measurements, made since 1991/92 30S from NASA'sUpper Atmosphere Research Satellite (UARS),have shown that, when activation occurs, it 60S<-+--+~1--+--+--+___,1--+--+--+~1--+---+-' usually occurs throughout the polar J F M A M J J A S 0 N D J vortex and that the maximum Month concentrations of ClO in the northern 22 and southern hemispheres are comparable. OZONE TRENDS 2.3.4 Middle-latitude ozone trends 1979-1996 Annual Trends, 40-50 N [%/Yr+/- 2cr] 55 At northern mid-latitudes, total ozone n is declining by between 4% and 8°/r, 50 per decade depending on the season. This rate excludes variations in ozone 45 due to solar cycles and other known natural atmospheric phenomena. 40 Fig. 2-9 illustrates this trend from satellite data. 35 'E 30 The observed decreases in ozone arise ~ primarily as a result of ozone l:en ·a; 25 destruction in the lower stratosphere, :i:: between altitudes of 15 and 25 km. 20 Since 1979 there is good quantitative agreement between the trends derived 15 from the measurements made by ozone sondes and those by the Stratospheric 10f- .tt. Umkehr Aerosol and Gas Experiment (SAGE) •SAGE I/II o SBUV(/2) satellite instruments. However,below 5' 20 km the uncertainties in the trends derived from SAGEdata are too large 0 to be useful. Other ozone losses occur -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 Trend (%/year] near 40 km where gas-phase destruction was first predicted, but there is too little ozone present at this altitude for this to have a significant effect on trends in the ozone column. probably reduce crop yields, as well Fig. 2-10 Ozone trends Figs. 2-2 and 2-10 show the magnitude as having an uncertain effect on other (% per year) from 40-50°N over the period of these trends. biota because of the ability of UVBto 1979-1996. (WMO, 1998) destroy DNA. The cause of the trend at mid• latitudes has not been unequivocally Because the absorption of sunlight by determined. It is likely to be at least ozone warms the stratosphere, partly related to increases in without compensating mechanisms stratospheric halogens, but changes ozone depletion must lead to a cooling in transport may also have had a of the stratosphere. This would have significant effect. an uncertain effect on climate, as well as changing the rates of many 2.3.5 Consequences of ozone chemical reactions in the stratosphere depletion because they are temperature• dependent. Because ozone absorbs ultraviolet light, without compensating Ultraviolet radiation at the Earth's mechanisms ozone depletion must surface has been observed to lead to an increase in ultraviolet at increase under the Antarctic ozone the surface. This would increase the hole. Ozone depletion in Antarctica incidence of human skin cancer and has also been at least partly 23 responsible for the observed seasonal to enhance the tendency to cool the decreases in stratospheric surface associated with stratospheric ternperatures. ozone depletion. Observations under clear skies have confirmed that periods of reduced 2.4 GOMOS Scientific amounts of ozone outside Antarctica Objectives are associated with increases in the amount of ultraviolet radiation at The foregoing part of this chapter the Earth's surface. However, records gives an overview of the ozone of ground-level ultraviolet radiation problem and the present state of are not yet of good enough quality, knowledge. There is still a or length, to allow the separation of considerable need for further and the smaller increases associated improved measurements regarding with cloudy conditions from the the chemical processes involved in large natural variability. Not only is the ozone chemistry and of long-term this increase in ultraviolet radiation changes of the ozone layer. The of concern in its own right, due to scientific issues described in this the potential biological effects of the chapter are numerous and will change, but ultraviolet radiation also require the data from a large plays an important role in the number of spaceborne, airborne chemistry of the troposphere. and ground-based instruments in order to measure the large number Recent calculations have supported of relevant atmospheric the suggestion that stratospheric parameters with the required ozone loss outside Antarctica leads temporal and spatial coverage and to a cooling of the Earth's surface. resolution. Models indicate that since 1979 the observed ozone loss has offset at least Atmospheric chemistry is therefore 30% of the warming effect of one of the key mission targets for increases due to increases in other the Envisat satellite. Three greenhouse gases over the same instruments, namely GOMOS, MIPAS period. The offset may be even (see ESA 2000) and SCIAMCHY,are larger than this as the decline in dedicated to atmospheric composition stratospheric ozone could have sounding. They will provide unique affected climate indirectly through and highly synergistic data sets with chemical reactions driven by respect to measured atmospheric enhanced ultraviolet radiation parameters and spatial coverage and reaching the troposphere. resolution. The increased penetration of GOMOSis an excellent space ultraviolet associated with ozone instrument for ozone monitoring. Since depletion increases the amounts of there are very many stars to use as hydroxyl radicals in the troposphere light sources, the stellar occultation (if all other controlling variables are technique has the potential to provide unchanged). Any increase in hydroxyl much more complete spatial coverage radical concentrations will reduce the than is available from solar lifetime of methane (an important occultations. Therefore, measurements greenhouse gas) and may, via the at nearly all latitudes are possible even oxidation of sulphur compounds, lead though Envisat will be in a polar Sun• to an increase in the reflectivity of synchronous orbit. Within the Envisat clouds. Both of these effects will act mission it is even more valuable as the 24 higher-level data analysis can benefit • the degree of Arctic (polar) ozone from the atmospheric parameters loss under different measured (more or less) simultan• meteorological conditions eously with MIPAS and SCIAMACHY. (Section 2.2.2). The key objective of GOMOS is the increased data for determining long-term monitoring of ozone, with height-resolved trend analysis global coverage, high vertical resolution (Section 2.2.3). and long-term stable accuracy. This objective can be achieved with stellar monitoring, which will indicate occultation, making GOMOS an ideal the beginning of expected instrument for the requirement. 'recovery' in ozone Operational stellar occulation (Section 2.2.7). instruments should follow the Envisat mission to continue GOMOS-type The primary scientific mission measurements in order to get the objective points to the use of an required coverage for long-term trends. instrument applying the occultation A derivative of GOMOS, COALA, is technique, as this is self-calibrating, presently being studied as a possible protecting the measurements from operational stellar occultation long-term instrument drifts. Stars instrument for ozone profile trend have been chosen as light sources in observation. COALA is somewhat order to provide sufficient global smaller than GOMOS and focuses only coverage. on GOMOS' primary mission objective: ozone monitoring with high vertical The occultation technique has one resolution, high accuracy and global significant advantage, which is that coverage. an absolute estimate of the molecule density is obtained from the ratio of Because of the significantly better two measurements taken with the global coverage compared to the solar same instrument within a few occultation technique, GOMOS and seconds. This makes the method COALA could prove to be the inherently self-calibrating, since even preferred solution for ozone profile if the spectral sensitivity of the monitoring. instrument is changing with time, the ratio will be measured correctly. This 2.4.1 Primary scientific protection against long-term drift is objective: ozone trend ideal for the study of atmospheric observations trends. The primary scientific objective of GOMOS measures in the UV and GOMOS is the global monitoring of the visible wavelength regions where stratospheric and mesospheric vertical ozone shows strong absorption. The ozone distribution with high accuracy, measured wavelength range is 250 nm high vertical resolution and full global to 675 nm, covering the ozone coverage. The expected high accuracy Huggins and Chappuis band. The UV of the ozone profile measurements wavelength range is particularly well should allow ozone trends to be suited for mesospheric and upper studied over the lifetime of Envisat in stratospheric measurements. The the stratosphere and mesosphere. visible band is well suited for probing the stratosphere, including the lower GOMOS ozone trend observations will stratosphere, where the UV signal is address: no longer strong enough because of 25 the UV absorption by ozone. Roughly, which can strongly affect the the UV yields the best results for the partitioning of chlorine and nitrogen mesosphere and stratosphere above compounds. 35 km and the visible band for the stratosphere below 35 km. Stratospheric water vapour is fundamental to the budget of many A highly desirable follow-on trace gases in the stratosphere. It is programme with GOMOS or COALA• therefore important to determine its type instruments could provide long• three-dimensional distribution and term trend observation. long-term trends. Water vapour is measured in a near-infrared channe. 2.4.2 Secondary scientific at 926-952 nm. objectives A further secondary objective is to As pointed out in earlier sections, N02 study stratospheric dynamics. This and N03 play important roles in the requires measuring atmospheric nitrogen chemistry relevant to ozone. temperature and density profiles. Temperature and air density will be Observations of N02 and N03 will derived from a near-infrared channe. help to address: (756-773 nm). Complementary to this, a high-resolution temperature profile the degree of can be retrieved from observations of denoxification/ denitrification in atmospheric scintillation, which will the Arctic winter and the be measured using two fast availability of N02 to deactivate photometers, one sensitive to blue ClO. and the other to red. the abundance of lower The measurement of temperature is stratosphere N02, which varies also needed to support the primary with the aerosol loading. mission objective, ozone monitoring. The ozone cross-sections in the future NOxobservations relevant Huggins band (310-350 nm) are to the ozone recovery. As NOx strongly dependent on the atmospheric dominates ozone temperature. Therefore, a long-term loss on a global scale, NOx variation of UVabsorption might be observations are important to due to a temperature variation and interpret ozone changes could be wrongly attributed to an ozone as halogens decrease. variation. Temperature measurements are possible with the A-band of 02 at GOMOS will therefore observe N02 760 nm. 02 is a perfectly mixed gas and N03 (if enhanced) in the UV• and the air can be assumed to be in visible band. hydrostatic equilibrium. Therefore, its scale height is directly connected to the OClO and BrO are key species in the atmospheric temperature. The 02 polar ozone depletion processes. They measurements make it possible to can possibly be detected with GOMOS relate measurements of ozone if their concentration is enhanced. density (and other species) to the air Measuring the aerosol extinction is an density to yield the mixing ratio additional secondary objective. [03]/[air], which is a quantity most Aerosols are important for a number readily used in models. Furthermore, of reasons, including the provision of temperature and air density are surfaces for heterogeneous reactions, essential parameters for studying 26 atmospheric dynamics, including mixing of gases. The mission objectives have also been described by Bertaux, 1999. 2.4.3 Long-term, campaign and permanent objectives The scientific objectives can be divided into three groups. The first consists of long-term objectives. These objectives will be satisfied according to the overall GOMOS mission plan during the whole mission lifetime, in a more or less ozone monitoring regular manner. The most important (l.'i-511km> of these objectives is the monitoring of stratospheric ozone. ()_,c:olumn o, profile aaosol column ;u•nr<:olprofile air density profile \t>: profile The second group consists of t >: density profile 11,t r protile \t I; during night campaign type objectives. These have and terminator di• or·,·ullaliont urnal condition more limited scope than those of the around pdcs <>Cl<> & Br<> during nigh! long-term objectives. The activation of and terminator di- 11m;1l condition air "'"""Y profile a campaign objective is based on an 01 prom. o, prom. agreement among the partners in the lrl'Rpf'ral•ft GOMOSmission planning. proflle He< r profilc ~iilll rPS<>kltiow aerosols prof Ir fitmptnttwrr profilt The third group consists of a few \'/1011 ocrtdtations ( )~profile permanent objectives. For example, pn.tsihlysptYijkd temperature lalihlde bands profile whenever possible, certain stars (e.g. <)?profile Sirius) should always be occulted and predefined locations (e.g. selected validation sites) should be prioritised. Therefore, these objectives have an overriding priority. stratospheric chemistry: lower Fig. 2-11 GOMOSlong• In more detail, the three groups and middle stratosphere term mission objectives according to the altitude include: (15-35 km), regions involved Long-term objectives stratospheric dynamics: large scale dynamics, polar vortex, The following different objectives have planetary waves, been identified: stratospheric dynamics: small stratospheric ozone monitoring scale processes, (15-50 km), • mesospheric noctilucent clouds mesospheric ozone monitoring, variation (80-87 km). stratospheric chemistry: upper The objectives are summarised in stratosphere (35-50 km), Fig. 2-11. 27 Campaign type objectives collocated simultaneous measurements with uniform geophysical validation of GOMOS global coverage. products: • campaigns: Campaign-specified validation of gas concentration latitude regions and priorities for retrieval for particular species, gases, collocated measurements e.g. special events, for example, overpass flights over validating volcanic eruption: ozone and ground based instrument or aerosol profiles (with coincident regions with highest accuracy) around the validating satellite instrument, eruption, validation of diurnal condition. stellar spectra, with increased (Depending on the validating occultation starting altitude. instrument). Permanent objectives comparisons with MIPASand SCIAMACHY: • fixed region measurements, at specified times, • fixed star measurements, comparison of ozone and The permanent objectives have aerosols profiles, overriding priority over the other mission objectives. But it must be noted comparison of N02, air, 02 and that the permanent and other mission temperature profiles, objectives can be fulfilled- or nearly !OO - at the same time, because of the large comparison of H20, OClO and number of occultations per orbit. More BrO profiles. generally, almost all mission objectives are to some extent fulfilled in any interesting diurnal condition: mission plan. The degree of fulfilment, depending on the validating of course, varies. The best results are instrument (MIPASday and obtained for mission objectives for night, SCIAMACHYnadir and which observations are optimised. limb modes day, SCIAMACHY occultation mode twilight), 28 3. Instrument Requirements The primary objective of GOMOS is atmosphere. The orientation of the the high accuracy monitoring of the field of view is opposite to the orbital three-dimensional global distribution velocity vector. During the star's of ozone. Additional objectives are occultation, the ultraviolet, visible measurements of aerosols, air and near-infrared spectra of the star density, temperature, N02, N03 (if are continuously observed. enhanced), Rp and possibly BrO. The coverage is global for the altitude As the star sets through the range of the tropopause up to 100 km atmosphere, its spectrum becomes using day and night observations. The more and more attenuated by the vertical resolution is 1.7 km or better, absorption of various trace-gases in depending on the obliqueness of the the atmosphere. Each trace gas is occultation. characterised by a known, well• defined, spectral signature. On the To fulfil the scientific objectives, ground, these attenuated spectra will GOMOSwill observe stars as they set be divided by the unattenuated stellar behind the Earth's atmosphere, and spectrum measured a few tens of measure spectral changes in the seconds earlier above the atmosphere, observed starlight intensity through so allowing the absorption spectra to atmospheric absorption. From the be derived very accurately. This absorption at different wavelengths radiometric self-calibrating method is and altitudes the concentration of the insensitive to longer-term sensitivity Fig. 3-1 Principle and Geometry of the GOMOS various gases and aerosols will be drifts of the instrument's hardware stellar occulation retrieved. and is therefore capable of fulfilling measurements 3.1 The Required Measurement Principle .--1 GOMOS . . J.·•·~· •·.;; k .. GOMOS uses the stellar occultation Pointing _____directlon "'., ·.'-.•••r,-b·,..J'!O~·· ...:---.~·· . technique. A sketch of the GOMOS ~-·-~~~~-~·~~:--c-- measurement principle is shown in Polar Orbit 1 Fig. 3-1. It is located on the nadir• facing side of Envisat with its overall field-of-viewopposite to the orbital velocity vector. The instrument's line• of-sight can be successively oriented towards stars and maintained while the star sets behind the Earth's 29 spectrum of the sky just above and fo:J-t=iartieyl below the star. These spectra are 1o·"f\ used for removal of the signal due tc the scattered sunlight. 1 o" N" ~ .£ 1 a·19 z 3.2 Instrument 0 ~ \ ~•r [t-Jo21 Requirements: Spectral UJt3 (fJ 1 o-20 ·- (fJ (fJ ~ ,, 103Hu\JQ;ns! Coverage and Resolution a:0 o UJ 10·211 The cross sections of ozone, N02 and z ·~1 -: 1§3:Cilappui~ 2 0 N03 in the 250- 700 nm band are 10·22 shown in Fig. 3-2. Ozone absorbs strongly in the ultraviolet, which allows it to be measured at high 1 o·23 altitudes. This can be seen by performing simulations of the 300 400 500 600 WAVELENGTH (nm) atmospheric transmission T(l,z)as a function of tangent altitude z and wavelength 1,by multiplying the transmissions of various gases and Fig. 3-2 Cross sections of the challenging requirement of aerosols (Fig. 3-3). At lower altitudes 03, N02, and N03 reliably detecting very small changes where the ultraviolet light is highly and long term trends in the three• attenuated, ozone can be measured dimensional distribution of ozone by observing the Chappuis band in concentrations. the visible. Measurements in both bands may consequently be used to During daytime observations, the retrieve tangential column densities of sunlight scattered by the atmosphere ozone over a wide range of altitudes. is superimposed on the star's signal. Model calculations for transmissions In order to retrieve the star's signal in the infrared of 02 and H20, Fig. 3-3 Simulation of atmospheric transmission without this component, the imaging respectively, are shown in Figs. 3-4 at 250-675 nm spectrometers also record the and 3-5. Suitable multi-element detectors are charge coupled devices (CCDs), semiconductor devices composed of intrinsic Si and a matrix of charge 0.99 confirming electrodes, which define 0.98 pixels. By coincidence, there is an 0.97 ~ almost ideal match between the 3 quantum efficiency curve q(l)of silicon 0.6-f;,·1'.'.r) __./ 0.96 ~- aerosols • g as a function of wavelength and the 0.95 ~.. domain over which ozone absorbs 0.4 ·.o.:"i-":o:"_:-:::: ..-f·.o:.oj_ 0.94 .o radiation. At short wavelengths there s 0.93 ~ are technical limitations which still 0.2 0.92 allow the GOMOS spectral range to extend down to 250 nm, where s(03) is 0.91 at its maximum value. At long 0.0 '········ 250 300 0.90 wavelengths ozone absorbs 350 400 450 500 550 600 650 wavelength (nm) significantly up to 800 nm (the CCD sensitivity limit is at 1000 nm). 30 In the Huggins bands (310-350 nm), Atmospheric Transmission of 02 at 22 km (convolved with PSF) 03 cross-sections are strongly 1.0 dependent on the atmospheric temperature. Therefore, a long-term variation of absorption of the 0.8 ultraviolet as measured by GOMOS might be due to a temperature, as opposed to an ozone, variation. To avoid this, GOMOSmeasures 0.6 temperature via the A band of 02 at 760 nm. Since 02 is a perfectly mixed gas and the air is in hydrostatic equilibrium, 02 density is directly connected to atmospheric /~5 760 765 770 ternpera tu re: wavelength (nm) I dT I d [ O_,] --+- --= (3.1) T d: [O_,J dz observe in the spectral bands given in Fig. 3-4 Atmospheric Table 3-1. transmission of 02 at 755-775 nm This measurement of 02 also makes it Several aspects were taken into possible to relate all observed densities consideration in deciding on the of ozone and other trace gases to the spectral resolution. One important air density, to compute the mixing aspect is that, if there is an unknown ratio [03]/[air] directly, without atmospheric absorber, which is not reference to other data which might taken into account in the ozone bring additional error sources. This is retrieval process, it can bias the 03 important because mixing ratio rather retrieval. If, in addition, this absorber than density is the quantity more often has a trend during the lifetime of used in models and comparison of GOMOS,one might confuse this with GOMOSmeasurements with models is an ozone trend. Therefore, the therefore easier. observed spectra must be compared with model spectra in order to identify Stratospheric water vapour H20 is systematic discrepancies. This fundamental to the budget of many consideration called for a complete trace gases in the stratosphere. It is monitoring of the spectrum with a Fig. 3-5 Transmission of therefore important to determine its resolution of about 1 nm. H20 around 950 nm three-dimensional distribution and long-term trends. The strongest H20 absorption band in the CCD silicon domain is at 936 nm, therefore Atmospheric Transmission of HZO (US standard atmosphere 1976) at 26 km (convolved with PSF) GOMOSalso covers this wavelength range. The chemistry of ozone is also very much connected to reactive nitrogen gases. N02 absorbs in the whole visible domain and N03 (existing only at night) can be 0.90 observed at 662 nm. 920 930 940 950 960 In view of these considerations from wavelength (nrn) the scientific objectives, GOMOSwill 31 Band Wavelengths Resolution Main Usage definition of the tangent heights UV-visible 250 nm - 675 nm, 1.2 nm 03, N02, N03, encompassed by each measurement, Aerosol, (OClO, better than 30 m, independent of the BrO) spacecraft attitude. Near IR (1) 756 nm - 773 nm, 0.2 nm 02 (density and temperature) Near IR (2) 926 nm - 952 nm, 0.2 nm H20 Simulations have shown that, in 'Blue' photometer 470 nm - 520 nm, 50 nm scintilla tions, order to retrieve ozone density with turbulence an accuracy of better than 1% in a 'Red' photometer 620 nm - 700 nm, 50 nm scintillations, turbulence significant range of altitude, a signal to noise ratio of 100 was required for each spectral pixel (ofwidth 0.3 nm). Given the brightness of the brightest Table 3-1 GOMOS Several atmospheric trace gases have stars, this determined the size of the wavelength ranges and spectral features with structure on a optical collecting area for GOMOS. their usage wavelength scale in the range of 1 nm: in particular there are the N02 Small-scale structures in the features around 410 nm, the ozone temperature/ density profile (and Huggins bands, S02 (ofvolcanic therefore refraction index) induce fast origin) around 300 nm and OClO (S02 variations of the stellar intensity and OClO might be detectable under during occultation, as observed from certain conditions, but are not an the Salyout space station [Gretchko et objective or design driver for GOMOS). al., 1981]. Because the index of In such cases, differential absorption refraction of air depends on the in limited portions of the spectrum wavelength, scintillations are not may be used to determine the collocated at all wavelengths, and the quantity of gas, by measuring the star's image will be distorted by structure in the spectrum of the chromatic refraction. However, transmission (DOASmethod). measuring the scintillations at high frequency (1 kHz) with two fast For 02 and H20 in the near IR, the photometers at different wavelengths spectral signature has variations on a allows the reconstruction of the scale smaller than 1 nm, calling for a scintillations at all wavelengths, so resolution of about 0.2 nm. that each spectrum can be corrected for the scintillation effect. The signal-to-noise (S/N) ratio has to be specified for a given integration In dayside, in order to account for time, which is connected to vertical contributions from scattered light, the resolution. With the Envisat orbit at instrument has to simultaneously 800 km, the tangent height of the observe the spectrum of the target line-of-sight to a star changes by star, of the sky above the star and of 3.4 km/s for the fastest occultation, the sky below the star. The plane of one that occurs in the orbital plane. dispersion must always be parallel to Therefore, an integration time of 0.5 s the limb of the atmosphere. will give a vertical sampling better than 1.7 km or one quarter of an atmospheric scale height (as small as 3.3 The Contribution of 1 km for many occultations). Vertical GOMOS Measurements sampling provides a limit to vertical resolution when the signal-to-noise The mission objectivesof GOMOSare ratio is large. An important advantage discussed in Chapter 2. This section of the stellar occultation technique is describes how GOMOSdata can be that it provides a very accurate used to achieve the scientific objectives 32 3.3.1 Monitoring of 03 profile and Limiting magnitude Numbers of stars trends my" 0 4 0 < my s 1 11 Because GOMOS measurements of 1 < my s 2 34 ozone only rely on two measurements 2 < my s 3 120 of the same star closely spaced in 3 < my " 4 340 time, there are no calibrating devices in GOMOS that can drift with time. All drifts in GOMOS spectral parameters cancel when the ratio is star is important; hot stars being Table 3-2 Observable stars taken. This immunity to drift applies favoured for the ultraviolet (ozone at not just to total ozone but to the high altitudes), while cool stars are measurements of the profile of ozone. preferred for the infrared (H20 at Furthermore, in the lower 936 nm). stratosphere, GOMOSwill measure ozone using its visible absorption With the capabilities of GOMOS as bands, which are not temperature presently defined, it will be possible to sensitive. Hence not even errors in perform useful occultations with atmospheric temperature combined about 500 stars; they are distributed with drifts in atmospheric as shown in Table 3-2. temperature will give rise to drifts in GOMOS ozone in this crucial altitude Among the 500 stars, only about 200 region. are occulted for a particular orbit and time of year. Not all of them can be a) Night-side and day-side observed in each orbit due to a lack of occultations time. A natural criterion for the selection of a star is its brightness Because of the presence of scattered and temperature. A scientific strategy sunlight on the day-side, stellar has been established, taking into occultations are much more difficult account the actual star distribution to achieve than on the night-side. and desired geographical coverage However, day-side occultations are and scientific objectives (see necessary as there is diurnal Chapter 6 on mission planning). variation in N02 above 20 km and in ozone above 50 km {from c) Ozone trend photochemical models). The relatively narrow entrance slit of the An estimate of the trend capabilities spectrometer (200 µm, or the size of is made in analogy with an earlier 10 spectral pixels) will limit the day• computation made by Frederick, side limb contribution entering the 1984. He addressed the problem of spectrometer. It should be noted that the detection of an ozone trend from the spectral resolution of the stellar one single ground station, performing spectrum is not affected by the slit a total of n measurements x at width, but rather by the blur circle of regular intervals M, over a duration the telescope, the pointing stability D =rust. Annual, semi-annual and and the grating performance. quasi-biennial variations are assumed to be regressed. The calculation made b) Number of stars that can be by Frederick is valid for observed measurements fully decorrelated spacially and in time. GOMOS The brightest stars give the best measurements, however, are not fully ozone retrievals. Also the colour of the decorrelated, which leads to a slightly 33 modified calculation of the minimum If the globe is divided into K detectable trend. geographical regions (for each altitude), the minimum trend can be The minimum trend d that can be calculated for each region as: detected with a confidence interval of 95% is: (3.4) 8 = 1.96 u(x) ~ (3.2) !Y.t \}NC,ind A 4% variance of ozone at 40 km Nindis the number of independent (sstv = 4%) and a worst precision of 5Cfo measurements. If Ncoris the number on stars of magnitude 3 of correlated occultations and Ntotthe (sinst = 5%) is assumed, leading to total number of occultations during s = 4.4% (for Ncor= 8). When the K GOMOS' lifetime, the number of regions are 9 latitude bands of independent measurements is 2000each, the minimum trend that Ninct= Ntotf Ncor'with Ncorbeing can be detected by GOMOS is shown approximately 8 (3 to 4 in space and in equation (3.5). 2 to 3 in time) for the lower stratosphere. In the middle When the world is divided into 100 stratosphere Ncoris approximately 4 geographical regions where the ozone and in the upper stratosphere 1. trend must be studied, as suggested Na is the number of independent from Dobson-network analyses [WMO, measurements per year with D being 1990], the minimum detectable ozone the total lifetime of GOMOS (nominal trend is shown in equation (3.6). 4 years) and Na= 25¥14.33¥365/Ncor· The mean time between two Note that these are the extra limits to independent measurements is linear trends imposed by noise on GOMOS measurements. They do not M = l/Na. include limits imposed by non• The total variance of each periodic variations in ozone (e.g. by independent measurement is given volcanic eruptions) nor do they apply by: to nonlinear trends. 2 It can be noted that a higher 7 7 CT rr-(x) = tr" + inst (3.3) precision is obtained for tangential stu N Cl!/ column densities than for local concentrations. These primary data 0. 09% /year in the lower stratosphere 8GoMos(lat,K = 9) = 0.07%/year in the middle stratosphere (3.5) { 0.05%/year in the upper stratosphere 0.3%/year in the lower stratosphere 8 lat, K = 100 = . . (3.E) GOMoi ) { 0.2%/year m the middle and upper stratosphere 34 will also be archived for trend studies activity, S02 might be also detected in as a function of altitude. the lower stratosphere with its strong spectral signature around 310 nm. In summary, GOMOS will be able to Aerosols of volcanic origin may also respond to the accuracy requirement be characterised from their spectral for ozone trend observations as noted signature, different from aerosol in Section 2.2.1. On the other hand, between eruptions. GOMOS will obviously not be able to monitor over the required periods Improvements to models often (decades) because of the limited life• require simultaneous measurements of time of Envisat, but a successful many gases. Other instruments on operation of GOMOS would prove that Envisat are complementary to GOMOS this observation technique could in this respect. However,modern provide a highly desirable option, model assimilation techniques, if possibly a key-instrument, for future applied to chemical as well as ozone monitoring space programmes. dynamical measurements, should mean that simultaneity of 3.3.2 Other studies measurements is not essential. By means of such assimilation, Other gases, S02 and volcanic measurements on other satellites or aerosols from the ground can be integrated into a meaningful whole and can thereby GOMOS measurements of N03 and allow useful tests of model predictions. OClO are particularly important as they are potentially superior to Atmospheric turbulence and high• measurements of these gases by other resolution temperature profile instruments, which observe sunlight rather than starlight: N03 only exists Scintillations of the stellar at night; and OClO is only present in intensity are recorded with the small amounts until sunset, building photometers at two different up rapidly during the early part of the wavelengths in order to correct each night. The profile of N03 is difficult to spectrum for the effect deduce with any accuracy from of scintillation. Scintillations result ground-based measurements, and from the fine structure of the total N03 has only been measured atmospheric vertical temperature from a few sites. The rate of build-up profile. The spatial vertical frequency of OClO is only known to limited spectrum, obtained by Fourier accuracy; its geographic variability is transform of the photometer signals, not known at all. Hence GOMOS is in is representative of the vertical a position to make uniquely valuable fluctuations of the temperature, measurements of N03 and OClO. which in turn are representative of motions in the atmosphere In addition to ozone, the trends in (waves, turbulence, advection, other constituents (N02, N03, H20, T, convection). aerosols ...) will also be measured, and correlated with ozone trends for Single-wavelength scintillations possible explanations of ozone during a stellar occultation were depletion mechanisms, by comparison obtained from the Salyut space with models. station [Gretchko et al., 1981, 1997] and interpreted in terms of In conditions of polar ozone depletion, vertical structure of temperature. OClO might be measured with bright GOMOS should yield for the first time stars. In conditions of volcanic a climatology of small scale dynamics 35 parameters important for studying the The horizontal structures of both mixing of species. scintillations and line densities are of great scientific interest, and Furthermore, since there are two GOMOS offers a unique observation photometers, one can determine the opportunity. The slant column actual high-resolution vertical densities obtained from tangential profile of the temperature [Theodore, observation are fully valid, but the 1998] from a careful analysis of the vertical profiles have to be used with time correlation of the two signals at great caution because of the large two different wavelengths. This will horizontal extension - possibly over allow the determination of the several hours local time, in particular climatology of temperature perturba• in high latitudes and polar regions - tions induced by gravity waves and will of the observation. give very valuable information for processes sensitive to temperature as Polar stratospheric clouds (PSCs} for instance the formation of PSCs. In some cases, the horizontal optical Tangential occultations thickness of PSCs is so large that the star tracker will lose the stellar signal Stars in the orbital plane stay in the The fact that the star tracker loses the orbital plane during an occultation, star could possibly be used as an and the line-of-sight of GOMOStracks indicator for the presence of PSCs. This the star at the same azimuth (0°).This could at least make the detection of the is the definition of a vertical occultation. cloud possible, allowing the production of statistics of PSC occurrence and Stars outside the orbital plane are their cloud top altitudes. observed to move in azimuth at the horizon while they are setting behind Noctilucent clouds the atmosphere. In a frame coordinate system related to GOMOS, during one Noctilucent clouds are seen from the orbit, a given star describes a cone ground during twilight. They are the around the pole of the Envisat orbital 'visible' part of polar mesospheric plane, of half-angle a. When a star clouds (PMC),most of which can only lies between an angle of a = 62° and be observed from space. These are a = 66° the cone is tangent to the patchy thin layers (2 km) of small ice Earth's limb, at the point where it particles (r < 0.1 µm) at 83-85 km of reaches its minimum altitude. This is altitude, with vertical optical the definition of a tangential thicknesses of 10-4 and tangential occultation, in which the horizontal optical thicknesses of about 10-2 displacement of the tangent point to (wavelength dependent). They are the star is much larger than the connected to the condensation of vertical displacement. water vapour during the cold conditions of the summer For a given star, the angle a varies by mesosphere. 1° per day at most. Therefore, tangential occultations of one star are In emission, PMCswould be detected seen at each successive orbit at an by the GOMOSspectrometers, increasing altitude by steps of mainly in the two background bands 4 km (or less). For a given bright (the bands on the CCD above and star, tangential occultations cluster below the star image). The actual around a particular day of the year. brightness is about the same as 36 Rayleigh scattering at 50 km altitude and aerosols. Furthermore, it can and should give a significant signal, make measurements just above thick which could be interpreted in terms convective clouds without being of Mie theory and particle size and perturbed. nature. In absorption, the 1% absorption over 2 km altitude would be Limb radiance and 03 retrieval measurable with the fast photometers, which would give the exact height, On the dayside, the limb radiance is vertical extent and vertical distribution. due to Rayleigh and aerosol Because the absorption is so small, the scattering of solar light. The spectrum effect might be detectable only in of the recorded light contains absorption spectra of the brightest absorption by ozone and other stars. The emission spectrum would gases and may be analysed in always be observable. terms of their vertical profile. Though the retrieval algorithm is much With its capability to detect the more complex than that for occultation, presence of PMCs at each occultation, it will be useful to compare values of GOMOS should establish an ozone retrieved by the two techniques. unprecedented corpus of data on GOMOShas the capability to perform a these phenomena over the whole 'fictitious star mode' occultation, in globe. Of particular interest will be its which the tracking mirror is no longer ability to document the north-south piloted by the image of a star in the asymmetry and to monitor the star tracker, but rather programmed to evolution of PMC cover and the perform a motion as if there were a location of its equatorial boundary. star to be occulted, sweeping the Validation observations would be whole range of altitudes across the made with high-latitude lidar bright limb. The limb radiance as a stations. function of altitude can be measured and a SCIAMACHY-typealgorithm Water vapour in the lower applied. This type of data analysis for stratosphere GOMOSwould be experimental. GOMOS measurements of water Atmospheric transport vapour will have an uniquely good vertical resolution for remote sensors, As a result of extensive recent efforts and can cover locations where by the scientific community, the balloon-borne sondes cannot be dynamics of ozone in the lower launched. This good vertical stratosphere near the poles are now resolution is particularly important in represented fairly well in numerical the lower stratosphere and upper models (see Figure 3.6). However, troposphere. It is this altitude where poleward transport and descent are the signal-to-noise ratio of GOMOS less well understood. GOMOS measurements of water vapour will be observations of temperature (from at a maximum. density) as well as of ozone, with good vertical resolution, should help to GOMOS will be particularly well greatly enhance the understanding of suited for the study of air dehydration this transport and the processes near the tropical tropopause, as it has involved. the capability to retrieve simultaneously high-resolution temperature profiles, water vapour 37 Fig. 3-6 Ozone measured by the ground based UV• visible spectrometer SAOZ at Faraday in Artarctica 450 during winter tne spring 1994, compared to predictions by the 400 Cambridge SLIMCAT3-D model (Courtesy of A.M. Lee, Cambridge Centrefor 350 Atmospheric Science) 5 0 -; 300 I,,,"' 2 rv jh_..; ~ 250 .• CG -0 - 200 150 0 May June July Aug Sept Oct 38 4. Instrument Concept 4.1 The GOMOS instrument feeding the telescope, is piloted by the design overview SATUand moved to keep the star in the slit during the whole occultation. 4.1.1 Introduction The star acquisition probability will GOMOScontains two different be better than 91°/c, (obviously a spectrometers, A and B, to meet both function of the star magnitude) and is the spectral coverage and the spectral specified for both day and night-side resolution requirements. operation. Because of the signal Spectrometer A covers the whole attenuation through the atmosphere, UV-visible range 250-675 nm. a star cannot be acquired close to the Spectrometer B is used to cover the limb. Therefore, GOMOSalways 02 band at 760 nm and the H20 band acquires setting stars outside the at 936 nm, with the spectral sampling atmosphere. In average, GOMOSwill defined by the dispersion and pixel observe 40 stars per orbit. size (20 µm in the spectral direction, 27 µm in the vertical direction). The GOMOSis located on the nadir-facing plane of dispersion is always parallel side of Envisat with its overall field-of• to the limb. view opposite to the orbital velocity vector. The limitations on the overall Two fast photometers, with motion of the mirror motion allow bandwidths in the blue and the red, measurements only from -10° to +90° are also fed by the same telescope in azimuth (where 0° is the orbital and measure fast variations of the plane), and in the range of +62° to stellar flux (scintillations) with a +68° (with respect to nadir direction) 1 kHz sampling rate all along the in elevation. The elevation range occultation. The star acquisition and corresponds to a tangential altitude tracking unit (SATU),with a 100 Hz range of -30 km to +280 km. Early sampling rate provides the necessary simulations with the actual star field information for the acquisition of the showed that this 100° azimuth angle star, the tracking and guidance. The restriction still allowed to perform a image of the star must be kept within large number of occultations per orbit the entrance slit of the spectrometer, (30 to 50, depending on the season). in spite of the motion of the star on the horizon, scintillations and micro• In order to cover the whole spectral vibrations induced by the Envisat range, there are four CCDs. Each of subsystem and other instruments. them has a band of 5 to 10 lines The optical bench of GOMOS, devoted to the measurement of the including the telescope, is fixed to the star spectrum, which are binned body of Envisat, and a flat mirror, together electronically. In order to 39 remove the background signal (scattered sunlight) from the star UV-vis/IR signal, two bands of 7-10 lines are spectrometers devoted to the measurement of the Steering background above and below the Photometers Mirror altitude of the star. Star Tracker All blocks of lines (their separation Optomechanical Assembly and height are programmable by ground commands) are binned electronically so that all pixels of a Mechanical Instrument Science column in a band are added together Drive Control Data in the output shift register to form Electronics Unit Electronics ~I one spectral sample. Three spectra PEB Panel are transmitted to the ground for each CCD: one for the star, the two OBDH Bus Science (S/C Control) Data Bus others for the sky background. The electronic on-chip binning is preferred over numerical addition to reduce the Fig. 4-1 Instrument block read-out noise, since there is only one diagram readout per spectral sample. The readout noise is a limiting factor Fig. 4-2 GOMOS 3-D when dim stars are observed, and drawing also in the case of strong absorption, The integration time is 0.5 sec. An occultation will begin when the star is seen at an altitude of around 120 km (to avoid any atmospheric absorption). The occultation will end when the stellar sensor loses the star signal in the lowest atmospheric layers (cloud top, for instance). Then the mirror is sent to a new position to track the next star to be occulted, according to a mission scenario whicn will be computed in advance on ground. The GOMOS instrument design fulfils two basic functions. The first is an optical measurement function to acquire spectral data. The second is the pointing function, which enables the instrument to maintain the star image within the field-of-viewas the star sets through the atmosphere. The orientation of the field of view of the instrument will be in the direction of setting stars, i.e. opposite to the direction of the instrument's velocity vector. The block diagram of the 40 Fig. 4-3 Spectral coverage 250 375 405 675 756 773 952 of the GOMOSdetection units Dispersion [nm/pix] 0.312 Spectral resolulion [nm ] 1.2 Spectral sampling [nm ] 0.32 I I I I lJVIS-1 lJVIS-2 ••IR-1 IR-2 FP-1 H l·'P-2 H 470 - :120 11111 6:10 - 700 nm SAT!! 62:1 - 1J:i0 11111 instrument is shown in Fig. 4-1 and a controlled optical bench. The optical 3-D drawing is shown in Fig. 4-2. bench and the telescope are mounted together and fixed on the instrument The instrument's optical design is interface structure via carbon-fibre based on a single telescope. Via a struts. beam splitter in its focal plane this feeds simultaneously a medium The steering front mechanism is a resolution UV-visiblespectrometer, a two-stage mechanism for line-of-sight two-channel high-resolution near-IR pointing. The first stage, the spectrometer, a two-channel fast orientator, is used to move the photometer, and two star trackers. instrument's line-of-sight from one Fig. 4-4 GOMOSdetailed The spectral coverage of the detection star direction to another. The optical bench layout (OBD units is shown in Fig. 4.3. The orientator does not move during the = optical beam dispatcher) detailed optical bench layout is shown in Fig. 4-4. An important feature of GOMOSis TELESCOPE ASSEMBLY the fact that for stellar observations, it functions as a slitless spectrometer, i.e. the star image itself is fed to the spectrometers and to the photometer. The main reason for the choice of this concept is that it allows the use of the entire stellar signal, very important in view of the poor signal provided by stars of magnitudes as weak as 4. The price paid for this optically efficient design is that a high performance pointing system has to be used to keep the star image fixed at the input of the spectrometers (in Detection Module order not to degrade the spectral UVIS Spectrometer resolution and the spectral stability). Mechanism Star tracker (redundant) All the detection units are implemented on a thermally- 41 acquire the image of a star in a large angular range (± 26°). The SFM rotates and maintains the mirror in a commanded position. track this star in a narrow angular field (± 2° in both azimuth and elevation) with a 5 Hz control bandwidth. steer light from the star into the telescope via a plane mirror. The first two objectives were realised through a two stage mechanism composed of a coarse stage ensuring a mirror rotation over a large azimuth angular range, and a fine stage ensuring a mirror rotation over a reduced azimuth and elevation angular range. Both stages are used in order to achieve an accurate initial rallying positioning, while the fine stage carries out the tracking function used during the actual occultation. Pointing performance : as a constituent of the GOMOS instrument tracking loop, Fig. 4-5 Steering Front tracking of the stars. The tracking the SFM has been submitted to very Mechanism and input phase is performed by the fine stringent performance requirements mirror pointing system mounted on top of which impose strict design constraints. the orientator. The quick and The SFM design has also taken into accurate tracking function (100 Hz, account various constraints like better than 20 microradian tracking spacecraft perturbing torques and accuracy) is controlled in closed loop forces, spacecraft microvibrations, larger (i.e. feedback-loop via the star perturbations induced by mechanism en tracking unit back to the mechanism the spacecraft, mechanism lifetime, control unit) using the star-tracker mass and power. A torque control correction information as calculated concept has been chosen for the fine within the instrument control unit. stage mechanism due to the fact that it allows a low-pass rejection of An opto-mechanical cover is mounted microvibrations and the minimisation of around both the optical and the perturbations induced into the platform steering front mechanism to provide the adequate straylight baffling and Fig. 4-5 shows a photograph of the thermal insulation. steering front mechanism and the input mirror. 4.1.2 Steering-front mechanism optics (SFM) 4.1.3 Telescope The basic objectives of the SFM are The Cassegrain telescope focuses light to: from the star and the nearby sky on 42 to the entrance plane of the imaging Pupil Knife-edge distribution at 85 % spectrometer. It uses a parabolic primary and a hyperbolic secondary Spectral direction Spatial direction mirror, which minimises astigmatism UVIS sub-pupil 20 pm 25 pm IR and Photometer sub-pupil 25 pm 30 pm and coma. SATU (tracking) 40 pm 45 pm SATU (acquisition) 45 pm 50 pm The main design drivers were: low mass The primary and secondary mirrors Table 4-1 Telescope knife• are made of Zerodur and are edge distribution including the diffraction • high stiffness isostatically mounted on a rigid at end of life (4 years structure. Each mirror is optically operation) • long term stability of the inter divided in two areas with silicon-oxide mirror distance protected aluminium used for the IR-spectrometer /photometer and The main optical requirement for the SATUchannels. Yttrium/ oxide-coated telescope is expressed in terms of a silver has been chosen for the knife-edge distribution (KED) along UV-visible spectrometer channel to the spectral and the spatial axes. ensure a good UV reflectivity. Table 4-1 summarises the requirements. Focal length 1050 mm Table 4-2. Telescope Mirror separation 250mm dimensions The relative location of the pupils is Primary mirror size 323 x 197 mm2 shown in Fig. 4-6. The entrance pupil Secondary mirror size 176 x 111 mm2 is located on the primary mirror. It provides adequate collecting areas according to photometric 4.1.4 UV-visible spectrometer requirements for the back-optics. The optical design rectangular entrance pupil is split into four sub-pupils dedicated to The UV-visible spectrometer provides UV-visible spectrometer, a medium spectral resolution (1.2 nm) IR spectrometer/ fast photometers and over a wide spectral bandwidth (from to the optical chain of each star• 250 nm to 675 nm). tracker. The spectrometer design and its Table 4-2 summarises the telescope accommodation on the optical bench dimensions. The minimisation of the have been optimised to avoid straylight has been a driver of the important flux losses, mainly in the telescope design and is ensured by: UV part of the spectrum. The UV• visible beam is folded only once, in stringent mirror reflectance requirements. IR-Spectrometer baffle design, which prevents any + Photometer UVIS-Spectrometer focal plane direct illumination and minimises indirect illumination. use of additional fins on the structure and black painting of surfaces which can be illuminated by starlight or by SATU-1 Fig. 4-6 Telescope mirror scattered sunlight. pupil 43 the optical beam dispatcher. IRl : 756 - 773 nm (02 band) Dispersion and imaging are performed by the same component, a • IR2 : 926 - 952 nm (H20 band) concave reflective grating. The design is based on a Rowland configuration. The spectrometer is based on a Littrow configuration. The main The major design constraints are the design drivers are the imaging quality transmission, mainly in the UV band and the transmission, and the fact (250-350 nm), and the imaging that the IR spectrometer collimating quality. Consequently an aberration• optics are also used for the corrected ion-etched holographic photometers' input beam collimation. grating was chosen as the main element, with the blaze wavelength of The use of a Littrow configuration in 305 nm and with the UV band high orders of diffraction (order 16 for efficiency at its maximum at this IR2 and order 13 for IR1) enables wavelength. both spectra to be diffracted in the same direction, so the grating blaze Since the UV-visiblespectrum length is angle is optimised for the middle of 27.4 mm and the useful length of the both IRl and IR2 spectral bands. The CCD is 27 mm, the spectrum is imaged flux diffracted in both useful bands is on two CCDs via a folding mirror therefore at maximum. located close to the image plane. A minor drawback is that the spectrum The separation between the IR and corresponding to the gap is lost (375- the photometer channels is performed 405 nm). On the other hand, an in the focused beam which optimises advantage of this design is that the use the dichroic splitter performances in of two CCDs enables partial compensa• terms of wavefront distortion and of tion for the grating's residual spectrum transmission and reflection curvature and therefore improves the coefficients. The IR beam is reflected spectrometer image quality and (its associated spectral band is removes a single-point failure. narrower than that of the photometers) and the incidence angle In the UV-visible spectrometer the on the dichroic splitter is minimised. main straylight sources are the intrinsic grating straylight and the The diffracted IR1 and IR2 spectra are unused orders of diffraction, which folded by the dichroic splitter and are superposed on the useful then focused toward the detection spectrum or reflected toward the plane. The collimator is slightly off• detectors by the mechanical parts centred and tilted with respect to the surrounding the spectrometer. This spectrometer input beam, so that the straylight is minimised by filters output beam is spatially separated located in front of each detector and from the input beam. by a dedicated baffle surrounding the grating. The grating 0 order is A dichroic splitter located close to the eliminated by a light-trap. IR spectrometer image plane separates the IRl and IR2 spectra, which are 4.1.5 IR spectrometer chain imaged on two CCD detectors, similar optical design to the UV-visiblespectrometer ones. The main straylight sources of the IR The IR spectrometer provides a high spectrometer are the unused orders of spectral resolution (0.14 nm) over two diffraction, which are superimposed on narrow spectral bands: the IRl and IR2 spectra. They are 44 rejected by narrow band-pass filters, which are located before the detectors. Folding Fig. 4- 7 shows a sketch of the IR spectrometer optical layout. A photo• graph of the IR optical spectrometer unit is shown in Fig. 4-8. 4.1.6 Fast photometers optical Dichroi« heum design splitter The fast photometers' optical channel provides images of the observed star Photnnu-u-r beam in two spectral bands: IR spcct ro• dispatcher meter input input hcum photometer band 1: 650 - 700 nm beam photometer band 2: 470 - 520 nm The main constraint concerning the collimated and separated from the IR Fig. 4-7 IR-spectrometer photometers channel is the spot size beam in the IR optical spectrometer and photometer optical because the detectors are small (14 x unit, refocused by dedicated optics layout 14 pixels CCD). The photometer beam (see Fig. 3-8). A dichroic splitter shares the same telescope sub-pupil located after the focusing optics as the IR input beam and is separates the red and blue Fig. 4-8 IR Optical Spectrometer Unit 45 Fig. 4-9 CCD Readout focal length of 630 mm. The main structure 1353 Columns constraint affecting the star tracker (*=programmable) channel is the star spot size, which must be 30 urn (85 % encircled First line• Line width• energy) in the tracking field of view Line separation· (FOV),and 40 µmin the acquisition FOV in order to meet the acquisition and tracking accuracy requirements. This optical quality is achieved using the Cassegrain telescope and a focal reducer with a 0.6 magnification. The 111111111111111"1111111--+Vw star tracker's spectral band is 625- Output Register 1050 nm, determined by a high-pass filter located close to the detector plane, which filters it. The upper limit of the SATUspectral band photometer channel bands. Each corresponds to the limit of sensitivity spectral band is imaged on a of the detector. dedicated detector. 4.1.8 UV-visible/IR detector design The focusing optics only perform a good chromatic correction over the red For the UV-visible/IR detector, photometer band, so an additional GOMOSuses CCDs with 143 lines of optical correction element is located 1353 pixels. Two of these are used in between the dichroic splitter and the the UV-visible spectrometer and two blue photometer detector. A band-pass in the IR spectrometer. filter is located in front of each detector to select each spectral band. The CCD operation is optimised for GOMOS such that only the part of the The overall photometer channel is image containing information on the composed of the following elements: star spectrum and the adjacent background will be read out and • the telescope processed. For each CCD, three bands are read out. Each of these bands • the optical beam splitter consists of a programmable number of lines summed on-chip in the output • the IR optical spectrometer shift register (see Fig. 4-9). The devices are thinned, back illuminated • the detection module, which and treated with an antireflective includes the focusing optics and layer, which is optimised in the UV the additional optics for band. During the integration period chromatism correction, the phot the devices work in inverted mode 1/ phot 2 dichroic splitter, the (multipinned mode) and thus exhibit filters and the detectors. extremely low dark current. Even though the inverted mode is inhibited 4.1. 7 Star tracker optical design during readout (resulting in a somewhat larger dark current), the The nominal and redundant star additional dark charge generated is tracker channel provides, on each kept small due to the relative short Star Acquisition and Tracking Unit readout time (with respect to the (SATU)detector, an image of the integration time) and contributes only observed star, with an equivalent about 1.5% of the overall dark charge. 46 The UV quantum efficiency (Fig. 4- 80 --r--r--r--r--r--r--r--r--1--r--r--r--r--r--1 10) is achieved using a complex I I I I I I I I I I I I I I I ....., I I I I I ..•• backside treatment process, which ';(2_ --L--L--L--L-- 0 70 consists of an ion (Boron) implant, •.....• I laser annealing, anodisation, >- 60 - - r - - r - - r - - r/- r - - r - - r - -:- - -:- - -:- - -:- - -~ - -:- - - :-- - o I stripping and anodisation. The ion c: Q) 50 implant and laser annealing process ·o --~--~--~-,~--~--~--~--~--~--~--~--~,-~--~--: creates an electrical field that allows :E 40 I w I I I I I I I I I UV photo electrons to reach the E 30 --L--L--L--L--~--~--~--~--~- potential well. If this electrical field is ::::J .•.. I I I I I I I I I I I I I not provided, UV photo-electrons c: 20 --~--r--r--r--r--r--r--r--r--r--r--r--r--r- would be recombined quite close to ctl ::::J 10 --·--·--·--·--·--·--·--~--~--~--~--~--~--~--· the air/ silicon interface and would 0 I I I not reach the associated potential 0 well. The anodisation and stripping 0 0 0 I.{) r-- 0 process allows the minimisation of N N C'? the ion implant thickness (which Wavelength [nm] increases the UV sensitivity) and creates a Si02 layer which acts as an anti-reflective layer. The thinning of the substrate is performed by dynamic LSF (and scintillation in the Fig. 4-10 UVIS/IR CCD chemical etching after having sealed lower atmosphere) insures that the Quantum Efficiency the CCD substrate on a thick spatial information will in fact cover substrate, which allows to several lines. In view of this, 7 lines maintaining flatness and rigidity. of binning have been chosen for both star and sky band as the optimum The main characteristics are: width (selectable during in-orbit opera tion). • pixels : 20 µm x 27 urn 4.1.9 Photometer detection design image zone : 143 x 1353 pixels The photometer uses a CCD with the memory zone : 142 x 1353 pixels following main characteristics: dark signal : < 25 pA/ cm2 at begin frame Transfer mode of life at 20°C useful image zone : 14 x 14 pixels image zone charge capacity : 5.9 x 105 e pixel size : 23 urn x 23 urn (100% aperture) register zone charge capacity : 1.2 x 106 e • quantum efficiency (average) CCD linearity : < 0.6 °/c, in PHOTl band (470-520 nm) 0.2 Due to the optics line-spread• function (LSF)the star image (point in PHOT2 band (650-700 nm) source) will be spread out in the 0.4 spatial direction and will cover several CCD lines. The instrument's technology : photomos, hurried static LSF width is close to the CCD channel line width, but adding also the 47 Fig. 4-11 Photometer 4.2 Instrument Electronic readout scheme Design Summed in readout The architecture consists of three register parts: • an instrument control unit (ICU), in charge of the instrument management, sequencing, TM/TC, housekeeping, active thermal control, pointing loop management, Summed digitally after ADC auxiliary data generation. It interfaces on the instrument side with the science data electronics and mechanism drive electronics and on the platform side with the • dark current (beginning of life) on-board data handling bus 1 nA/cm2 (through the data bus unit) and the power buses. Since there are no imaging requirements for the photometer, all • a science data electronics, in 14 rows will be summed in the readout charge of all detector powering, register. Furthermore, five columns sequencing and control, centred around the star position will spectrometer and photometer date. be summed digitally after the acquisition, processing, formatting analogue-digital converter. and transfer to the platform low bit-rate interface, star tracker data Using the 'uniformity' mode acquisition, pre-processing and (instrument mode) it is possible to transfer to the ICU. access all pixels. A sketch of the photometer readout scheme is shown • a mechanism drive electronics. in Fig. 4-11. This equipment drives the steering front mechanism under 4.1.10 Star-acquisition and ICU control. tracking-unit detection design A block diagram of the instrument The main characteristics of the CCD electronic design is shown in device used for the SATUdetection Fig. 4-12. chain are: • frame transfer mode 4.3 Mechanical Design • image area : 288 x 384 pixels The GOMOS instrument's structural hardware, split into five functionally • quantum efficiency: independent structural parts as shown in Fig. 4-13. 600 nm: 0.4 800 nm: 0.4 4.3.1 GOMOS interface structure 900 nm: 0.2 (GIFS) • dark current (begin of life) The main function of the GIFS is to 48 electronic units) together and to Fig. 4-12 Electronic provide the mechanical interface with Design the Envisat platform. The GIFS interfaces on one side with the satellite (through a set of fixation points) and on the other side with the following instrument units: steering front mechanism and its connector bracket optical supporting structure struts and optical bench central "''' machining (ball joint) telescope baffle optomechanical cover (OMC) optomechanical assembly (OMA) electrical harness and the associated connectors brackets purging pipe bracket and a set of purgmg covers • optical bench main panel thermal blankets inserts for telescope and optical bench units In addition the GIFS provides handling points for ground operation harness fixation device and and optical cubes for instrument connectors bracket fixation devices alignment. The GIFS main panel is a sandwich structure (carbon-fibre reinforced plastic (CFRP) skins, aluminium honeycomb core). The thickness of the skins (1 mm) and of the core (70 mm) complies with the stiffness and stability specification. 4.3.2 Optical bench The optical bench is mounted on the GIFS by means of the optical-bench support struts and provides a highly stable mechanical interface with all the optical bench units on one side and with the telescope on the other side. The optical bench includes the Fig. 4-13 Structural following hardware: Elements 49 • grounding hardware • support thermal hardware heater mat carrier which provide The OMC is light proof in order to support for the heaters prevent any straylight arising near the focal plane. In addition the OMC • active thermal control hardware provides an interface for fixing the hardcover used during on ground • alignment devices (optical cube) purging phase. Each part of the OMC is made of several flat CFRP sandwich The optical bench has to meet facings with an aluminium core. Flat stringent dynamic and thermoelastic panels are linked together with requirements in order to satisfy bonded CFRP corner profiles. instrument pointing and stability performance requirements. Therefore, 4.3.4 Telescope baffle assembly the optical bench main panel (TBA) comprises a sandwich plate with CFRP skins (2.5 mm) and an The telescope baffle surrounds the aluminium core. Thick skins are telescope in order to protect it from necessary to ensure adequate straylight inputs, and to provide the stiffness as well as to provide a high telescope with a smooth and thermal conduction (minimisation of controlled thermal environment. The thermal gradients). The optical bench telescope baffle supports thermal is isostatically mounted on the GIFS hardware for the thermal control of in order to minimise the interface the telescope environment and is loads and induced distortions. made of CFRP skin sandwiches with aluminium cores, isostatically The optical support struts (OSS) are a mounted on the GIFS, and including set of four struts and one ball joint on straylight fins. All internal surfaces of a diaphragm supporting the optical the telescope baffle are painted black bench in a defined and fixed position The whole telescope baffle is wrapped with respect to the GIFS and the in a multilayer insulation. steering-front mechanism optics (SFM).The struts consist of CFRP with an orientation of the fibres 4.4 Thermal Design optimised to obtain a quasi-zero coefficient of thermal expansion for a Thermal regulation of the GOMOS strut after glueing of the titanium opto-mechanical assembly (OMA)is end-fittings. performed independently from that of the spacecraft, while the electronics 4.3.3 Opto-mechanical cover (OMC) panel depends partly on the internal thermal environment of the payload The OMC is a secondary structure equipment bay (PEB). mounted on the GIFS, which aims to: The OMAis thermally insulated from • protect the instrument core from the PEB panel and ejects heat via Sun inputs, albedo and Earth three thermal panels integrated in the thermal radiation opto-mechanical cover (OMC)on the front of the instrument. These • insulate the instrument from space thermal panels guarantee an overall and provide a 'smooth' thermal temperature below the maximum environment permitted operating temperature. The correct temperature is achieved using 50 a distributed network of heating 4.6 Instrument Operation elements and thermistors, controlled by the instrument control unit. The The Envisat platform is four-axis electronics unit, located in the PEB, stabilised and permanently nadir• releases its heat energy by radiation oriented. GOMOSis located on the into the regulated interior of the nadir-facing side of Envisat with its spacecraft. overall field-of-viewopposite to the orbital velocity vector. 4.5 Software The GOMOSoperation scenario is composed of three major activities, The GOMOSICU Software runs on a starting with uploading of all 1750 microprocessor located in the command, operation and star ICU. selection parameters. Each star acquisition will be time-tagged and Its main functions are the following: executed sequentially. Prior to the expected arrival of each star, the the management of the interface instrument will direct the line-of-sight with the ground, (i.e. the steering front mirror) in the direction where the star is expected the handling of the instrument to appear at the programmed modes. This involves handling the rendezvous time. Once the mode transitions requested by the rendezvous time is triggered, the ground macro commands, instrument will start searching a ±0.3° cone for the brightest stellar the on-board monitoring and object. Once this is found, the anomaly management, instrument will lock on to it and direct the line-of-sight, using the fine the pointing control. This is the steering mechanism, centring the main function of the software and image of the star on the tracking the most complex. It is a 100 Hz detector. process, with very tight deadlines within each 10 millisecond cycle. In order to perform the acquisition This function manages all the and tracking of targets, several 15 different pointing sub-modes pointing sub-modes are implemented, and the transition between them. as sketched in Fig. 4-14. The The most constraining sub-modes transitions between these pointing are centring and tracking, where sub-modes are performed the computations to be performed automatically and do not need any by the software are important user interaction. However the sub• compared to the allocated mode status is monitored in deadlines, telemetry. the management of the star All GOMOSoperations use these observation programme. In order to pointing sub-modes in different observe stars, the pointing function sequential orders. is commanded by some macro• commands that contain the The typical sequence of a star sequences of stars to be observed, occultation measurement will be: thermal control. 51 Fig. 4-14 Instrument pointing loop maintain coarse ;------+ current pointing f------11 ~ fine rallying rallying position ,g= .§ .s ie <;e <; •• 0 ••'"' star centering detection ..r::i"" i.-- star tracking 14--- 14--- ""' 0 t;;a .::•• t: •• orbital motion IQ -e e0 compensation ~ .:;. (fixed position) ·s 1. The steering-front mechanism 3. At the commanded rendezvoustime optics (SFM)will be controlled to the 'detection pointing sub-mode' maintain its current position, will be started. This acquires the i.e. it is not moving. star image within a specific detec• tion window of the SATUCCD. The 2. After receiving an observation satellite's orbital motion will be command the SFM will be rallied to compensated by a mirror counter• the commanded position by: rotation in order to allow a success• ful star detection. When the star coarse rallying of the SFM has been successfully detected, the coarse stage (azimuth only). pointing will automatically switch to During this sub-mode the fine the 'centring' sub-mode. stage will be controlled to main• tain its current position. At the 4. The 'centring pointing sub-mode' end of this phase the coarse stage will perform a centring and actuator will be switched off. stabilising of the star image on the SATUCCD in order to move it fine rallying of the SFM upper inside the tracking field-of-view. stage (azimuth and elevation) in order to point the mirror to a 5. If the star image is successfully fixed position with respect to centred in the SATUCCD, the the platform. The orientation is tracking phase will be started. The such that the instrument's line• SFM fine stage will be moved to of-sight will be close to the maintain the target star within the predicted star direction. If the tracking field-of-view located in the commanded position has been centre of the SATUCCD matrix. successfully rallied, the pointing The pointing control must be function will switch to the performed precisely; a line-of-sight 'detection pointing' sub-mode at stability of typically 20 µrad over a the rendezvous time. 0.5 sec period can be reached. 52 6. At the end of the tracking duration Occultation mode: performs the current SFM position will be acquisition and tracking of a maintained (i.e., the mechanism star or, alternatively, performs stops). the observation of a fictitious star, i.e. the orbital motion is In case of a fictitious star observation, compensated so that the the pointing switches to the 'orbital instrument points in th motion compensation' sub-mode at direction of a fictitious star. the end of the 'fine rallying pointing' sub-mode. In this sub-mode Envisat's Linearity monitoring mode: orbital motion is compensated and no performs periodic monitoring of star detection is attempted. At the the instrument's radiometric end of a defined duration, the response linearity by the pointing goes to 'maintain current observation of stars with position' sub-mode. variable integration times. In case of the 'uniformity monitoring' Uniformity monitoring mode: mode, the pointing switches to the performs periodic monitoring of 'keep rallied position' sub-mode after the instrument pixel to pixel termination of the 'fine rallying' sub• response uniformity by the mode. In this sub-mode (like in the observation of an extended 'maintain current position' sub-mode) uniform target (i.e. Earth limb). the SFM fine stage is controlled to a fixed given orientation, with respect to Spatial spread monitoring mode: the instrument reference frame. The performs periodical monitoring SFM orientation at the end of the 'fine of the star spectrum and star rallying' sub-mode is kept. The star image location by the tracker is in its 'centring' mode. At observation of the instrument the end of a defined duration, pixel to pixel response. correspond-ing to the 'uniformity monitoring' mode duration, the Pause mode: intermediate pointing function goes to the operation mode performing 'maintain current position' sub-mode. preparation activities for observation sequences. 4. 7 How to Use GOMOS Fig. 4-15 GOMOS Operational Modes 4. 7 .1 Modes, Transitions and Commanding Overview The GOMOS instrument implements two categories of operational mode (see Fig. 4-15): a) Support modes The support modes are used to achieve or maintain full instrument operational conditions. b) Operation modes There are five operation modes: 53 Type of Occultation Linearity Uniformity Spatial Fictitious measurement data spread star Spectrometer yes yes yes A/8 binned spectra Spectrometer yes yes A/8 33 unbinned spectra Photometer summed yes yes image Photometer pixel map yes yes SATU tracking yes yes yes window coordinate Table 4-3. The various types of science data produced by GOMOSduring the different operation modes c Q) c c c ~ Q) 0 ~ -0 ·- c Q) c: :; .9 .9 c "8 0"'~Q) .~ c 0 1;.1 Oil Oil E.2 E CJ c 0 .9 c c g ·- "' '-" i.;: = .~ ·~ ' Q) e! ·;:: a ~ ·5 ~ .0 ;::l ~ 8. uQ) ~-0 ·- - Q) c CJ ] 8. Objectives ·; c 0.. "' Q) ~ if "' 0.. ., CJ 0 operation ·- c 8 8. ·'= E -0 CJ fl Q) •••• E ~ ..Q"' .> Occultation Mode perform spectral measurements (fictive star) ./ ./ ./ from bright limb scattered light Linearity monitoring of radiometric Monitoring ./ ./ ./ ./ ./ response linearity by ob- servation of a stable target Uniformity monitoring of pixel-to-pixel Monitoring ./ ./ ./ response uniformity by observation of an extended, uniform tarzet e.I!..Earth limb Spatial Spread monitoring of pixel-to-pixel Morutoring'" ./ ./ ./ ./ ./ response by observation of a stable target Pause preparation of observation ./ sequence; ability to switch immediately to any of the mea- surement models OJ Two types of occultation mode are available: • synchronous occultation mode, repeating a list of star observations over several orbits (evolving star rendezvous parameters are interpolated between first and last orbits). • asynchronous occultalion mode, executing a list of star observations once. Synchronous and asynchronous observations cannot be mixed within one orbit. (l>The result of the spatial spread monitoring mode will be the information on which pixel of the CCDs the star image will be displayed. With this information specific macro-commands will be generated for the proper setting of the CCD columns and rows used for the star occultation. Table 4-4 Sub-mode sequences used by the operation modes 54 The types of science data of the ~ different operation modes are .,, -o " - .g .E ee g e 0 ~ ~ ee 0 ~ e -c v summarised in Table 4.3. ~ <;:: ·cc"""" ..r::""·.:::: " 0 " 0 ~ .E .c " >N " ~ .= ·~ ~ 5 ·.: - 0 ~ E I ec § I "8 ~ ~ .0 ~ > "I Q. bl] "" ·- ~ ~ """ ~ E -E~ E~;;; 1·.;:: ~ e ~·E E 3 aper~" E Q. "" "";..::::: h~ -c "= -o .S mod g_ ~ "" E ~ " ·~2 Table 4.4 summarises the sequences ~ Q. "' " of sub-modes used by the operation Occultation Mode (1) (1) ./ I ./ modes. Table 4.5 summarises the (real or fictive star) parameters specifying the observation Linearity ./ ./ ./ ./ ./ entry in an observation sequence. Monitoring Uniformity Monitoring 4.8 Instrument Performance Spatial Spread I ./ ./ ./ I ./ I ./ Summary Monitoring The instrument performance is 11) for the first orbit and the last orbit (if synchronous observation) summarised in Table 4.6. Table 4-5 Summary of parameters specifying the observation entry in an observation sequence Acknowledgements: MATRAMARCONI SPACE GOMOS Team and K.-D. Mau from DORNIERsupplied part of this text. 55 Requirement description Requirement SPECTROMETERS Star characteristics Visual magnitude range: max. -1.6 to min. 2.4 to 4 fo: Defined by the 95% probability of the stars with 30000 K and 3000 K temperature respectiv tracking system to detect the star The spectral bands UV: 250 - 375 nm Defined by the full-width half-maximum VIS: 405 - 675 nm (FWHM)of the spectral response function !Rl: 756 - 773 nm IR2: 926 - 952 nm Spectral resolution 1.2 nm in UVVIS Defined by the FWHMof the spectral 0.2 nm in NIR rejection function Spectral sampling 0.3 nm in UVVIS Spectral distance in nm between the centre 0.05 nm in NIR of two adjacent pixels Spectral rejection UVIS: 85%,within 4 samples monochromatic Defined by the minimum spectral width NIR: 85% within 4.5 samples containing 85% of the signal Spectral stability knowledge in dark limb. 0.07 nm in UVVIS Error in knowledge of star star-spot 0.015 nm in !Rl stability, on spectrometer CCD, when 0.018 nm in IR2 considering the SATUbarycenter position (peak to peak value) Spectral stability knowledge in bright limb. 0.22 nm in UVVIS Definition as for dark limb 0.025 nm in !Rl 0.030 nm in IR2 Relative spectral accuracy 1.5 nm in UVVIS Stability of the wavelength associated with 0.3 nm in !Rl one detector pixel between any two occulations 0.3 nm in IR2 Dispersion direction Tangential to earth limb within ±2 deg. Angle between wavelength dispersion plane and tangent plane of earth limb Dispersion Stability 0.12 nm in UVVIS Spectral drift between any two pixels over 0.02 nm in !Rl one occulation. Mainly the stability of the 0.02 nm in IR2 each CCD position w.r.t. the others Relative dispersion accuracy 1.5 nm in UVVIS Spectral drift between any two pixels over 0.3 nm in !Rl 0.02 nm in !Rl instrument nominal lifetime. Mainly the 0.3 nm in IR20.02 nm in IR2 stability of each CCD position w.r.t. the others Spatial extent of spectra Band Target (star) Background Spatial width (angle in the vertical direction) Binned lines 2 - 14 3 - 19 of the CCD binned lines UVIS arcsec 18-70 arcsec 24-96 arcsec !Rl / IR2 arcsec 13-52 arcsec 18-70 arcsec Polarisation sensitivity <1% (Goal) Relative difference in sensitivity to two perpendicular polarisation directions (P1 - P2) / P1 - P2) in percent Linearity response 1% Maximum deviation of the response w.r.t. perfect linearity of any spectral element for a variation of any specific flux within a dynamic -ratio of 1 to 10, i.e. local linearity over each decade. Gain setting dependant 56 Actual Performance Visual magnitude range: max. -1.6 to min. 4.6 to (J.8 for stars with 30000 K and 3000 K temperature respectively UV: 248 - 371 nm VIS: 387 - 693 nm IR!: 750 - 776 nm IR2: 915 - 95(i nm 0.72 / 0.89 nm in UVVISS (min / max typical) 0.12 / 0.14 nm in NIR (min / max typical) 0.314 nm in UV, 0.312 nm in VIS 0.0465 nm in !RI, 0.0571 nm in IR2 UVIS: 85'X, in 2.8 / 4.7 samples monochromatic (min / max typ) NIR: 85'X, in 3. 15 / 3.85 samples monochromatic (min / max typ) 0.04 nm in UWIS 0.007 nm in IR1 0.008 nm in IR2 0.04 nm in UWIS 0.011 nm in !RI 0.014 nm in IR2 0.017 / 0.20 nm in UVVIS (dark/ bright limb typical) 0.0.05 / 0.05 nm in IR I(dark / bright limb typical) 0.06 / 0.0 I .83 nm in IR2 (dark / bright limb typical) Tangental to earth limb within ±2 degrees (worst case) <0.0 I nm in UVVIS <0.002 nm in IR I <0.002 nm in IR2 0.04 nm in UWIS 0.02 nm in !RI 0.02 nm in IR2 sparat.ion Band Target (star) Background Separation - 4 Binned Lines 2 - 14 3 - 19 I - 4 20 arcsec UVIS 11-74arcsec 16- I00 a resec 5.3-21 arcsec 20 arcsec !Rl I IR2 11-74 arcsec 16- 100 a resec 5.3-21 arcscc < 1, 3, 7, 7% for UV/VIS/IR/ PHOT respectively UV : 0.27 - 0.48% depending on gain / limb VIS : 0.36 - 0.5% depending on gain / limb !Rl : <0.33% IR2: <0.34% Table 4.6 Instrument performance summary (continued on next paqc} 57 Requirement description Requirement Integration Time 0.5 sec ± 0.5 msec The time during which the detectors are collecting < 10 msec photo electrons. < 5 msec Dead time < 100 µsec Dead time between consecutive integration's (transfer time) Start time of record precision Datation accuracy PHOTOMETERS The spectral bands 470-420 nm and 650- 700 nm Defined by the full-width half-maximum (FWHM)of the spectral response function Photometer instataneous FOV > 60 arcsec vertically The photometer pixel FOV (each photometer > 10 arcsec horizontally sees 14 pixels in the vertical direction and 5 pixels in the horizontal direction) Photometer spatial rejection > 95% of spatial LSF within 26 arcsec The target energy in percent collected by the 14 x 5 photometer pixels Sampling frequency 1 kHz The rate at which the detectors are transmitting < 0.1 msec measurement values. < 5 msec Dead time Dead time between consecutive integration's (transfer time) Start time of record precision Datation accuracy Linearity response 1% Maximum deviation of the response w.r.t. perfect linearity of any spectral element for a variation of any specified flux within a dynamic -ratio of 1 to I0, i.e. local linearity over each decade. Gain setting dependant POINTING REQUIREMENTS Pointing stability Better than 40 microradian peak-peak Peak to peak LOS jitter during one integration = FWHM of the dynamic LSF Number of occulations per orbit 45 on average, i.e. approximately 920000 occulations during the 4-year mission Detectable stars number of stars No Spec Assuming use of SATU-2 Loss of tracking probability <10°;(,in dark limb, <20% in bright limb Probability of losing star tracking due to other reasons than a) programmed tracking duration exceeded, b) edge o·-total FOV exceeded, c) edge of tracking FOV exceeded. Valid for specified visual star magnitude range. Angular coverage Azimuth angle with respect to the flight direction -10 deg. to +90 deg. Elevation angle with respect to the flight direction 62 deg. to 68 deg. Tracking FOV Maximum possible LOS range of the pointing 7 deg. azimuth function during a single occulation 5 deg. elevation 58 Actual Performance 0.498 sec ± 50 microseconds < 2 msec < 3 msec < 100 µsec 466-528 nm and 644- 705 nm > arcsec vertically / spatial direction > 50 arcsec horizontally / spectral direction > 95'Yoof spatial LSF within 26 arcsec 1 kHz < 0.1 msec < 5 msec Phot-1 (blue): <2.8°/c,measured Real performance is likely below 0.5% Phot-2 (red): <0.51'X1 Better than 40 microradian peak-peak 45 on average, i.e. approximately 920000 occulations during the 4-year mission Bright limb: 180 stars (typical) Dark limb: 1450 stars (typical) <1% in dark limb, <20% in bright limb -11 deg. to +91 deg. (typical) 61. 7 deg. to 69 deg. (typical) 7.4 deg. azimuth Table 4.6 Instrument performance summary 6.5 deg. elevation (continued) 59 5. Characterisation and Calibration 5.1 Introduction the calibration and monitoring of parameters related to spectral The main advantage of the GOMOS assignment and for precise knowledge operating principle is that of being of the dispersion. inherently self-calibrating as far as its principal low-levelproduct, the Furthermore, due to the method used atmospheric transmission spectrum, to achieve the ozone vertical density is concerned. This is defined as the profiles from the transmission ratio between the measured spectrum spectra, the instrument's spectral of the star without atmospheric resolution needs to be known. The absorption (i.e. outside the Earth's spectral resolution is measured and atmosphere) and the measured reflected in the spectral rejection spectrum of the star with atmospheric parameter and the instrument line• absorption. This means that the spread-function (LSF). instrument is self-calibrating, assuming that the instrument As far as the self-calibrating nature of response function does not change the spectrometer is concerned, the during one occultation of typically radiometric response, transmission 30 seconds (which is most unlikely). and instrument gain are not important. since any long term However GOMOSis, as far as the star response changes are cancelled when signal is concerned, a slitless calculating the transmission. spectrometer, so any difference in the However, the radiometric response is pointing-error history between the two of interest for the photometer data spectra will result in a small relative and for the spectrometer when used spectral shift between the two. as a photometer. Because of this the two spectra are initially spectrally aligned using the pointing history data from the star 5.2 Definition of Calibration acquisition and the tracking unit versus Characterisation (SATU).This procedure ensures that the self-calibration remains valid. Calibration is the process of quantitatively defining the system Nevertheless, transmission spectra response with respect to known, may, due to long-term thermal controlled signal inputs. The calibration distortion and the "settling" of the parameters are the instrument optical bench, be shifted with respect parameters which are expected to to each other in the long term. The change and which therefore require related spectral shift is not reflected continuous scrutiny and monitoring. in the SATUdata and this calls for GOMOShas special observation 61 modes, which will be used for in-flight 5.4 Ground Support monitoring of several calibration Equip~e• quantities and which are also used during the on-ground calibration. Several items of equipment have been developed to support the instrument The characterisation parameters are on-ground characterisation and determined by design or calibration: measurement at unit level and are assumed to be fixed. Monitoring of • source-pack these parameters is still necessary, as optical bench with a a drift could indicate changes in the collection of optical spectral instrument's functioning. sources. • collimator 5.3 Characterisation and 30 cm aperture collimator for Calibration Database simulation of star and background signal The GOMOS characterisation and calibration database defines 35 • calphot parameters, of which some will be absolute calibrated photometer for used for science data correction and determination of collimator some for instrument performance output flux monitoring. The main calibration and characterisation parameters are listed The source-pack consist of a in Table 5-1. transportable optical table with a number of spectral lamps (Ne, Hg, All the data presented in the following K/ Ar and Kl and a HeNe laser. A discussion relate to the flight model Jobin-Yvon monochromator is fed (FM)data and have been measured in with a xenon lamp flux and is used as ambient conditions (20xC and a medium resolution adjustable ambient pressure in the laboratory). spectral source (FWHMDl=O.l nm). unless otherwise stated. The main parameters related to the The collimator simulates a star with spectrometer are listed below: adjustable spectral and photometrical characteristics (motorised star focus) • spectrometer wavelength ranges A background signal can be (50% transmission edges) superimposed on the star signal to simulate the Earth limb. The • spectrometer wavelength assembly is vacuum compatible. It dispersion (nm/pixel or nm/mm) will be extensively used during ambient. as well as thermal vacuum, • spectrometer static line spread tests. function (spectral and spatial) The calibration photometer measures • spectrometer radiometric the input flux to the collimator. The uniformity specified accuracy is ± 10%. The detector is a Hamamatsu photo• • spectrometer linearity multiplier tube with a InGaAs(Cs) photocathode. The spectral range is • spectrometer and photometer specified as 160 to 1040 nm. radiometric sensitivity (DN/ph/s/nm/cm2) 62 Parameter characterised Comment Spectrometer wavelength ranges Spectral band 50% transmission edges vs. CCD position Spectrometer wavelength dispersion For each CCD a dispersion look-up table Spectrometer radiometric sensitivity Spectral sensitivity (ph/s/cm2/nm/DN) Spectrometer spectral uniformity Pixel to pixel CCD uniformity maps Spectrometer linearity Detection chain response to full exposure range Spectrometer IR vignetting Instrument response variation at encl of pointing range clue to vignetting from optomcchanical cover Spectrometer static line spread function Semi-static (optical and detector MTF)line (spectral and spatial) spread function. Also contains dynamic term from mirror control loop Spectrometer direction of spectral Spectrum rotation vs. pointing direction dispersion Spectrometer dark current CCD dark signal maps Spectrometer dark current temperature CCD dark current doubling temperature dependency Photometer spectral bands Spectral band 50% transmission edges Photometer radiometric sensitivity Photometer sensitivity (ph/s/cm2 /ON) Photometer response uniformity CCD uniformity pixel map Photometer linearity Photometer response to a linear increasing nux Photometer dark current CCD dark signal maps Photometer dark current tempera! ure CCD dark current doubling temperature dependency SATU radiometric sensitivity Photometric response of SATU Instrument-external Sun stray light CCD stray light maps clue to sun outside response but close to line of sight (sun angle > 40 degrees) Instrument-external Earth stray light CCD stray light maps due to Earth limb response outside but close to line or sight Instrument internal stray light response Internal scattering CCD maps SATU pointing shift with respect to SATU pointing shift due to limb gradient background SATU output versus wavelength shift Pointing data (SATU)conversion factors Polarisation sensitivity Sensitivity to polarisation of extended source Steering mirror reflectance vs. angle of For correction of photometer response incidence 5.5 Wavelength Ranges, assignment taken from the calibration Table 5-1 Main Assignment and Dispersion database that includes. for each CCD. Calibration and Characterisation a wavelength look-up table. Parameters The GOMOS ozone retrieval algorithm fits the wavelength assignment of the Even though the environmental tests pixels in the transmission spectrum. have shown that the wavelength It uses a first guess of the wavelength assignment is very stable. it is likely 63 Band Measurement Specification Half-maximum [nm] Half-maximum [nm] UV 242 - 382 250 - 375 Visible 385 - 693 405 - 675 IRl 749 - 777 756 - 773 IR2 916 - 956 926 - 952 Phot 2 'Blue' 468 - 528 470 - 520 Phot 1 'Red' 644 - 703 650 - 700 Table 5-2 Spectrometer/ to change due to the relatively harsh with the specifications and, since the Photometer spectral launch environment. Consequently GOMOS transmitted wavelengths can band-edges. the measurements show that the on-ground wavelength assignment be programmed, this allows for the the UV band is slightly cannot be assumed to be useful for shift adjustment (but not extension) shifted toward the shorter in-orbit operation. An on-ground of spectral bands. wavelength compared with the specification calibration is nevertheless important (250-375 nm), and also to assess the effect of the launch and Spectral dispersion is unlikely to that the visible band is will be useful for initial in-orbit significantly change due to the launch extended 18 nm toward the longer wavelength instrument programming. environment or during the lifetime of with a half-maximum at the instrument. The spectral 693 nm (specification The wavelength range of each CCD is assignments and dispersion were 405-675 nm). The photometer bands are determined from a white spectrum deduced from measurements by both slightly larger than covering the complete band. The several monochromatic lines using a the specification measurements in Table 5.2 list the monochromator from Jobin-Yvon. half-maximum values at the band Table 5-3 lists the barycentre spot• edges. The measurement in the positions of each wavelength, for each infrared band (IR2)is somewhat CCD. The final spectral assignment degraded by the strong variations in calibration, was performed in thermal the transmission of the fibre used to vacuum using spectral lamps, with the couple the source to the collimator, to same results as during the ambient illuminate GOMOS. The IR2 band test presented below. edge accuracy is consequently about ±1 nm. From the spatial position column, it can also be seen that the spectrum Table 5-3. Spectrometer dispersion (from FM From the table it can also be seen is slightly rotated (of the order of ambient test) that all spectral bands are compliant 0.1°). This small rotation, together Band Wavelength Spectral position Spatial position Dispersion Mean dispersion (nm) (pixel number) (pixel number) (nm/pixel) (nm/mm) UV 280 558.06 79.99 0.314 15.70 ± 0.05 UV 370 844.82 79.46 0.313 Visible 435 963.04 78.47 0.312 15.58 ± 0.05 Visible 675 192.45 79.01 0.310 IRl 761 654.39 83.47 0.0466 2.32 ± 0.02 IRl 771 870.25 82.93 0.0464 IR2 931 556.47 83.74 0.0574 2.85 ± 0.02 IR2 951 907.75 84.06 0.0569 64 with the spatial line-spread-function symmetric integral of the line spread (or spatial rejection), is the main function. along either the spatial or contributor to the star-band binning the spectral direction containing width (presently set at 7 lines). 85% of the LSF area (or energy of the spectral line observed). The The spatial/spectral position spatial rejection value is accuracy was calculated to 0.2 pixel. particularly important for setting the The corresponding UV/visible band-binning width (or number of dispersion accuracy is 0.001 nm/ rows per binned line). pixel and the IRl /IR2 dispersion accuracy 0.0005 nm/pixel. The The LSF has been measured at 18 corresponding value in nm/mm is wavelengths from 250 nm to 756 nm, found by dividing by the spectral and with model predictions direction pixel size (0.02 mm or performed for thrce wavelengths in 20 µm). the IR2 band from 926 to 956 nm. Three measured spectral LSFs are presented here in terms of the spectral rejection (85%) and spectral 5.6 Spectrometer Line resolution (FWHM)as measured Spread Function during the instrument thermal vacuum tests. The specified and The spectrometer line spread function measured spectral rejection and (LSF)is a measure of the instrument's resolution are given in Table 5-4. ability to resolve fine spatial or spectral details in the scene or the 5.6.1 Contributors to systematic spectrum. Knowledge of the LSF is errors important in the GOMOS product algorithm because the reference There are three main contributors to transmission is based on an ideal the instrument LSF and the total instrument with higher resolution instrument LSF is the convolution of than the instrument can achieve. the three: Prior to correlating the measured Table 5-4. Spectrometer transmission with the reference. the • The detector modulation transfer spectral rejection (85%) reference spectrum needs to be Junction (MTF)is a measure of the and spectral resolution (FWHM).The value in convoluted with the LSF and re• ability of the detector to resolve the pixels is found by sampled to reflect the real image focused onto the focal plane dividing by the spectral instrument resolution. The LSF is (the CCD). The primary contributor sampling width (UVIS = 0.31 nm/pixel, specified in terms of the rejection, to the detector MTF is the pixel IRI = 0.05 nm/pixel and defined as the width of the cross-talk, i.e. the fact that IR2 = 0.06 nm/pixel) Spectral Rejection Spectral Resolution Wavelength Specification Measured Specification Measured (nm) [nm] [nm] [nm] [nm] 254 1.4 1.48 1.2 0.89 375 1.4 0.89 1.2 0.74 405 1.4 0.87 1.2 0.72 675 1.4 0.95 1.2 0.75 756 0.19 0.18 0.2 0.12 926 0.19 0.18 0.2 0.14 65 components are. in addition to the 1000 --334.1 nm Spectral control-loop noise, the acoustic 0.900 and vibration environment coming 0.800 ----334.1 nm Spatial from the test equipment (vacuum 0.700 pumps, air-conditioning and other 0.600 ambient noise-sources). During the 0.500 on-ground measurements the 0.400 dynamic contribution proved to be 0.300 much smaller than first 0.200 anticipated, partly due to the high 0.100 stiffness of the pointing system, 0.000 and can be expected to be a minor 20 30 40 50 60 70 80 contributor to the total LSF. Sampling (2.5 µm/step spectral & 3.3674 mm/step spatial) Fig. 5-1 illustrates the static line spread function (convolution of detector MTF and optical LSF) as measured using a point- like Fig. 5-1 Static line photons falling on one pixel caused monochromatic source of spread function a response in neighbouring pixels. 334.1 nm. (Engineering Model Data). The full width half maximum (FWHM)of the • The optical line spread function is 5.6.2 Measurement method two curves is 50.5 micron the ability of the optical system to spectrally (2.5 pixels or 0.8 nm) and 53.9 micron resolve the spatial information of The final LSF calibration spatially the scene. The major contributors measurement was performed in (2 pixels) are design, defocus and thermal vacuum with the instrument non-alignment due to thermal in spatial spread monitoring mode distortion. As far as optical design and with the pointing loop activated. is concerned, the telescope The advantage of this set-up was that aperture shape and imaging the instrument was in the same grating have a large influence on configuration and environment (apart the defocussing, as well as the from the lg gravitational factor) as it imaging of a curved spectrum onto will be in orbit. a flat detector. Other less significant contributors include Even though the acoustic or vibration straylight and particle environment will not be flight contamination of the optical representative, the pointing loop surfaces. The static line spread control will effectively filter all function is in principle the mechanical noise below 5 Hz, while convolution of the MTF and the high frequency noise will be kept to a optical LSF. minimum, as the complete test set-up will be mounted on a seismic optical • The dynamic line spread function is bench. As for long term effects such the response of the detection as moisture release, gravity, micro• system to a vibration of the optical setting and thermal effects, these are axis. The main contributors are not expected to influence the results, electronic noise in the steering• due to the relatively short mirror control-loop and mechanical measurement time. micro-vibrations from other moving mechanisms on Envisat such as The test consists of the construction of reaction wheels and coolers. the spot-shape curve by displacement During the on-ground of a point-like spot (monochromatic) on calibration the dynamic the CCD. The flux received over the 66 Fig. 5-2 Line Spread Function measurement --~ Spectrometer Static Line Spread Function principle. When 1.000 measuring the spatial O') 0.900 c LSF the spot is moved e o.eoo Cl. along the row (across the : 0.700 column) in steps of 2.5 0:: 0.600 micrometre (4 micron on ~ 0.500 the SATU). For each step• ,,...°' 0.400 setting the 'course' LSF ~ 0.300 is read out through the ••• 0.200 column. For each position Q z 0.100 of the position a new LSF plot is created. From the 20 30 40 50 60 resulting set a 'high Sampling resolution' LSF is created I I I I I I I I I pl p2 p3 pl p2 p3 pl p2 p3 pl H I~ ~I Scan step Pixel size 4 micron 20 x 27 micron columns (or the lines) is measured as remove the sky (scattered sunlight) the spot is moved in 2.5-micron steps signal. First. the signals in the three (1/5 of a pixel) across the spectrometer bands (star band. band of the sky CCD. The spot is moved on the above and below the star) are corrected spectrometer CCD, not by physically for non-uniformity of the CCD. moving the source (30 cm aperture Second, the sky band signals are collimator with a 10 micron pinhole at interpolated from the two sky bands to the focal plane), but by shifting the the star band, where it will be SATUtracking-window position subtracted from the star band signal. between consecutive measurements. The pointing control reacts to the Due to pointing jitter during the changed position-information by occultation, a shift of the spectrum moving the steering front mirror such from one position on the CCD to that the spot moves accordingly on the another will give an apparent change spectrometer. The line spread function in the signal, due to the non• measurement principle is shown in uniformity. Jitter correction therefore Fig. 5-2. needs both the CCD uniformity map and the pointing history from the star-tracker. 5. 7 Spectrometer Radiometric Uniformity The CCD uniformity maps are created during the on-ground calibration The uniformity of the response of the using an extended uniform source. A individual pixels of a CCD must be corresponding in-orbit calibration will known accurately, in particular for be difficult to achieve with the same the scattered sunlight correction and accuracy because of the lack of pointing jitter correction. uniform calibration targets in orbit. For example, the Earth's limb is not One important reason for calibrating sufficiently uniform. the spectrometer radiometric uniformity comes from the need to The on-ground uniformity calibration 67 will be performed in a thermally• 5. 7 .2 Measurement of the controlled vacuum environment with uniformity map the instrument in its 'uniformity monitoring' mode, using the The uniformity was measured using a collimator and a polychromatic and white screen and a QTH lamp. The uniform source. The uniformity has uniformity of the white screen was already been measured in ambient much better than 0.5%. A high flux conditions using a quartz lamp and a (and hence small error-contribution, white screen (as part of the external due to a large SNR)was ensured. The Earth stray light test). The data results of these measurements are presented here are the result of these given in Table 5-5. measurements made in ambient conditions. 5.8 Spectrometer Linearity The uniformity is not likely to change due to the launch environment, as it The GOMOS detection chain, from the is largely dependent on the CCD CCD output amplifier to the quantum efficiency uniformity. analogue-to-digital converter, exhibits However, tests with earlier CCDs of a very small but measurable non• different manufacture showed linearity and is specified to be better changes in uniformity on time-scales than 1% over any 10:1 dynamic of days to months. range. It will largely depend on the amplifier gain setting and will 5. 7 .1 Definition of radiometric increase with the electronic gain. uniformity A specific instrument mode for the The uniformity of spectral response is calibration of linearity has been the measure of the relative difference implemented and allows the user to between the average sky band observe a stable source using a response and the average target programmable integration time from response to a uniform illumination. 0.25 to 10 sec. The measurements are performed with the instrument in its Using the linearity mode with a stable 'occultation' mode. The widths of the extended source, the linearity was upper and lower sky bands are set to measured while illuminating each CCD the same value as the star band. T is over a wide area. Each measurement the average signal measured for a was averaged spatially over 200 pixels. binned pixel in the star-target band The performance has been tested over and B1 and B2 the signal in the a large dynamic range (widerthan the upper and lower sky band. Then, the star specified range). Different gains radiometric uniformity is defined as: have been used to measure both CCD and ADC saturation levels. T- Bl uniformity. . = ---B 68 Band Uniformity Uniformity Accuracy specification measured UV mean 1 % mean 1.5% < 0.5?1o max 3.5°1<> VIS mean 1 % mean 1% < 0.3% max 4.5% IRl mean 3 % mean 2% < 0.5% max 7.5% IR2 mean 9 % mean 13% < 2lYcJ max 44% 5.9 Instrument Radiometric • instrument in tracking Table 5-5. Uniformity Sensitivity ('occultation ') mode values from ambient measurements using white screen and QTH lamp. The radiometric sensitivity of the • collimator with monochromator The high values in the spectrometer, photometer and SATU illuminating the instrument IR1 and IR2 bands are due to interference in the (Star Acquisition and Tracking Unit) aperture using a 11ourine• thinned CCD substrate. represent the conversion factors tungstcnhyclrogcn-molybclenum This is a typical feature between the instrument flux in spectral source (e.g. 2.5 nm at of thinned CCDs photons/s/cm2 /nm at the instrument 500 nm) input and that at the instrument output in terms of digital number • 500 acquisitions recorded for each (ON).The spectrometer sensitivity is of the four UVIS detection module shown in Fig. 5-3. gains Characterisation was performed Each acquisition set was recorded under ambient conditions. as part of for the UV-visiblechain for all 4 gains. the last performance test prior to while the IR spectrometer was Table 5-6. Synthesis of linearity performance: delivery of the instrument. The recorded for the lowest gain (i.e. The values indicated are measurement at ambient has the operation gain). Also the corresponding the worst case non• advantage of being more accurate photometer and SATU(star-tracker) linearity error over any 10:1 range (values for than if measured in vacuum but will signals were recorded. gain 8 are over the full nevertheless be verified against the range). Dark cells indicate thermal vacuum signal-to-noise The source calibration accuracy is that the measurement was not performed, because measurements. In view of this, the listed in Table 5-7. which also lists they are not used in calibration database will contain the spectral sensitivity as measured normal operation of the values from the ambient during the ambient measurement instrument. Note that, during nominal operation measurements as well as those from campaign. It represents the best for spectrometer IR1/IR2, the vacuum. knowledge of the instrument. only gain 1 will be used The radiometric sensitivity will decrease throughout the life of GOMOS as the optical Channel Gain 1 Gain 2 Gain 4 Gain 8 transmission is reduced, due to UV 0.27 % 0.45<% 0.30 % 0.48 % contamination VIS 0.36 69 0.0450,.------·------ones presented below. The values 1 I I I I I I I I I I 6..1 presented in Table 5-8 are based on a o.o4oot- - UVIS/IR Gain 1-:--- Phot~2 ·.----~-;- IR1 --- nominal instrument temperature N"' High target 'E 0.0350 _L -~------_L_ J (CCD temperature of about 12°C). o I Also, with the SNR model, the 0E 0.0300 ------·-r------I ---,---I ------·i------I -,----I ------.I performance presented is that c I I I I I expected (typical) at the end of life 'en ------~------__:___ ------_:_ I IR2 -~- ~ 0.0250 I I I I :.c (i.e. after 4 years in-orbit). Fig. 5-4 0. : : • Phot-1 : : z 0.0200 ------~------;------;· ------;------~ shows the ratio of the measured and 8...... specified signal-to-noise ratio. Z' 0.0150 ------·r----I - ---....--I --,-I ------.--I ------,I 's I I :;:: "iii c 0•0100+------L---I. r--.----'--I UVIS --'-I·~------'--.I ------·.I (J) (/) 0.0050 ------,-----I - ----,------I ----,-I ------,--I ------,I 5.11 In-flight Calibration I I I I I I I I I I I 0.0000+-----+----+----+-----+----t 5.11.1 Introduction 0 200 400 600 800 1000 The GOMOS measurements are in Wavelength [nm] principle self-calibrating as far as the atmospheric transmission measurements are concerned. The Fig. 5-3 Spectrometer 5.10 Instrument Signal-to• transmission product is the ratio sensitivity (or gain) in Noise Ratio between two spectra, the star DN/ph/s/nm/cm2• Note that the measured signal was spectrum outside the atmosphere first corrected for non• The signal-to-noise (SNR)was and the attenuated spectrum, uniformity before measured as part of the noise acquired closely in time. It can be calculating the gain. The curves presented are for characterisation and is defined for assumed that during one occultation UVIS and IR detection each binned pixel (7 lines wide) in sequence the change in the chain gain setting 1 with several wavelength bands. The SNR is instrument's response function will a high target flux defined as ratio between the be small, so that long-term (background subtracted) star flux and instrument drifts cancel out and can Table 5-7. Total radiometric sensitivity the standard deviation of the total be ignored. accuracy is given by measured signal over 500 adding up all error acquisitions. Regarding the higher level products contributors. The UV• visible (spectrometer A) is (geophysical parameters). the calibrated for all four The SNR for GOMOS is specified for transmission product has to be gain-settings using the each wavelength band for a set of flux transformed by a number of low target flux. In addition to the error cases. The SNR measurements were numerical procedures, each one made listed, the instrument conducted in thermal vacuum and on the basis of specific assumptions. signal-to-noise ratio must performed at cold. nominal and hot More importantly, the inversion also be considered, although the effect will be operational temperature in order to algorithm is a nonlinear process, much lower than the two validate the instrument's SNR models. meaning that a small change in the main contributors listed. The source used was a white xenon transmission could have a large The high and low flux levels are defined in lamp. The fluxes used during these nonlinear impact on vertical density Table 5-8 measurements were different than the products. Radiometric sensitivity accuracy UVIS IRl IR2 Gain 1 Gain8 LT HT LT HT Collimator source calibration [%] 3.9 5 6 4.4 8 6.3 Dark image subtraction error [%] 1 1 1 Total (linear sum) [%] 4.9 6 7 5.4 9 7.3 70 SNR EOL Spectral band Star Flux Sky Flux Typical Specification phls!nmlcm2 ph!slnmlcm2/nsr 250-300 nm 1100 0 19.0 12 (target) 310-375 nm 1100 0 30.6 13 LOW 405-530 nm 1000 0 38.4 23 TARGET 540-600 nm 1000 0 28. l 12 NO LIMB 610-675 nm 700 0 20.3 12 (LTINL) 756-773 nm 1000 0 14.5 6 926-952 nm 900 0 6.7 3 250-300 nm 60000 0 248.8 150 310-375 nm 60000 0 339.3 190 HIGH 405-530 nm 60000 0 405.1 190 TARGET 540-600 mn 50000 0 364.1 190 NO LIMB 610-675 nm 40000 0 260.5 150 (HTINL) 756-773 nm 30000 0 143.4 100 926-952 nm 20000 0 73.3 40 250-300 nm 60000 120 185. l 20 310-375 nm 60000 10000 113.7 20 HIGH 405-530 nm 60000 10000 139.0 30 TARGET 540-600 nm 50000 3000 177.1 35 BRIGHT 610-675 nm 40000 1500 134.3 35 LIMB 756-773 nm 30000 0 83.6 70 (IMEL) 926-952 nm 20000 0 43.9 30 Table 5-8. Instrument Signal-to-Noise Ratio predicted at end of life 1o T------r ------:------r ------:----.----.·r------:------: (4 years) 9 +-~------· ------:------· ------:-- -- .-- -- .-· ------:------: I I I I I I I ....------•••I I I I 8 --6-Ratio LTNL ~--- .-- -- .-·------~------~ -a-Ratio HTNL ! ! l l 7 ~ Ratlo. HTBL ,~"''I'""' I"~"''''""""''~"""""",,, , .....~, 'I I I I I 6 ~------,\..~ ------..L..----- •.....--- L -~ L J I I I I I I I I I I I ~--- .•.••••r• .•.•••..•.•..••r •••••••••••••••••r• --- . 5 I I I I ' ' ' 4 """·------~ ------~------~------r-•I •.•.• ..••..... •rI •••••••.••••••••••••I,..•••••••••.••---..,I 3~-··--···-·r··-·-·----r------r -- -- ...... •....- 2 Fig. 5-4 GOMOSSignal to --~ ----==~------~------! Noise Ratio for high and ~.:: I I I ~I I l: I low target irradiance with 1+------t ------j------t ------j-- -- .-- -- .+------j------~ dark sky and high target I I I I I I I I I I I irradiance with brigh sky o.L_~~~~-+-~~t---~--i-~~-+-~---jr--~~ radiance. As can be seen the signal to noise 250 350 450 550 650 750 850 950 performance is more than a factor of two above the Wavelength [nm] specification and for high targets stars in bright close to 10 71 Calibration is the process of b) Linearity Mode quantitatively defining the system response to known, controlled, signal A stable star target is acquired, inputs. GOMOS has several special locked on to and tracked outside thr observation modes, which are atmosphere several times using intended to enable the in-flight different integration times (0.25 to determination of several calibration 10 sec). This mode is only possible quantities. This section considers how by exploiting the binned operation these quantities can be monitored and will thus generate the same from the normal flow of occultation spectrometer data as in occultation data and from the use of GOMOS mode, but with the signal level special operation modes. varying by a factor of 40. The dynamic range can be further a) Occultation Mode extended by using several stars of different brightness, such that their The occultation mode provides several 1:40 dynamic ranges overlap. possibilities for in-flight calibration: A linearity curve with integrated flux versus instrument output can • The reference spectra (from each then be generated. It will in all occultation) can be used both for likelihood be necessary to use long-term wavelength calibration observations of the same star in and absolute radiometric successive orbits. calibration. c) Uniformity Mode • The background spectra, particularly at high tangent heights An extended uniform target (the Earth (only Rayleigh scattering). can be limb is a reasonable good proxy) is used for long-term wavelength and imaged and a pixel map is obtained. relative radiometric calibration. The purpose is to take both binned and unbinned pictures to confirm • The stellar spectra (with a tangent (and update) flat field correction, to height above 40 km) can be check for bad pixels and to confirm examined by using cross• binning operation. correlation to check for the shift and broadening determined from d) Spatial Spread Mode the SATUdata. The objective of this mode is to • All the data can be examined for perform periodic monitoring of the statistical consistency. instrument line spread function (which includes satellite jitter). It is also important to note that in A star (preferably with sharp addition to the conventional star spectral features, such as Sirius) tracking there are other possibilities: is tracked above the atmosphere and a whole picture is taken to • fictitious star tracking, which compare with the binned version. follows the inertial trajectory of a In this way the instrument function hypothetical star, (spatial and spectral directions). including the effects of satellite • offset tracking, which follows a real jitter, can be observed directly star, but the SATUtarget point is and compared with the binned offset so that the star remains out version. of the field of view of the slit. 72 5.11.2 The GOMOS calibration b) Spectral Characteristics database By comparing stellar Fraunhofer lines During the development of the with ground-based high-resolution algorithm for determining GOMOS spectra, one can verify the spectral level 1b data products (consisting point-spread-function and the mainly of transmission and stellar absolute wavelength scale of the spectra), a number of correction spectrometers. as well as the parameters were identified. These correspondence between the SATU include parameters such as, star positions and the wavelength measurements of instrument gain assignment of pixels. Such (radiometric units per instrument verification might be made digital output), linearity correction, systematically for all occultations at biases. spectral wavelength assignment Level lb. and spectral dispersion. Using the instrument modes explained above. c) Straylight these parameters are computed using the CALEXcalibration processor (an Measurements of straylight, external to integral part of the GOMOS the spectrometer. are almost Instrument Engineering Calibration impossible to conduct during ground Facility, IECF),which updates the calibration. as it is very difficult to calibration database. This database is construct a stray-light environment on an important input to the geophysical the ground that simulates that in product processor and will need to be space. In order to generate look-up periodically updated. tables for data processing. a model has to be constructed after launch, using In order to achieve this, an in-orbit as input the measured light with solar calibration plan is being developed spectral features derived from detector and will be executed and optimised rows dedicated to the observation of during the Envisat commissioning the sky background (when observing phase. The primary objective is to above the atmosphere). together with selectively update the complete on• the solar elevation and azimuth angles ground calibration data base during and the angles with the terminator on the commissioning phase. Of course, the Earth's surface. this will have to be performed in a highly interactive way, due to the The presence of straylight internal to difficulty of balancing, on one hand the spectrometer is easy to detect at clearly defined and accurate places and times when one would calibrations made on ground and, on otherwise expect no light (e.g. in the the other relatively uncontrolled in• ultraviolet at low altitude, or from orbit calibrations. stars which have no ultraviolet in their spectra). It can also be detected a) Verification of A/D converter if any filling of Fraunhofer lines is observed (comparing equivalent widths A histogram of the outputs from any of ground-based high-resolution one CCD, accumulated over several spectra with those obtained in flight). occultations, will be used to indicate At night, light from stars, spilling over deviations in the converter's transfer into detector rows dedicated to the function. for example if one particular observation of sky background also value n is under-represented with determines internal straylight, when respect to values n-1 and n+ 1. sufficient care is taken to exclude any other astronomical source. 73 d) Flat Fielding are heavily smoothed. This allows confirmation that pre-flight Unlike stellar spectra, the spectral measurements of pixel response non• spread function of measurements of uniformity are still correct. Because the bright sky in the upper and lower the spectra are not smoothed over a background bands is determined by very large number of pixels, the exact the slit. Because the width of the values of any changes in pixel spectrometer slit is equivalent to response non- uniformity may have to many pixels, spectra of the bright sky be deduced by iteration. 74 6. Mission Planning 6.1 Introduction 90 - The advantage of the technique of - _,,,..- <-, 60 stellar occultation - as opposed to , / solar occultation - is the large number // I. of possible occultations per orbit. Stars 30 . I I are not evenly distributed in the sky, ,, i,, I and this may lead occasionally, -0"' 2 I r depending on the season, to poor ·~ 0 '\ coverage in certain regions and to an .' 1 \ oversupply of possible occultations in -30 ' ~ r. other regions. In the latter case, \ / decisions have to be made as to which \ »>: \ ~·--..-== star should be chosen among various -60 alternatives. In fact, the number of star targets that GOMOSis able to -90 track and measure is so large that -180 -120 -60 0 60 120 180 selection of the optimal target will be Longitude required most of the time . Since the quality of the retrieval of mode or the wait mode is selected. The Fig. 6-1 Possible GOMOSdata products depends on second level concerns mission planning occulations during one orbit in January. The star parameters such as brightness inside the occultation and monitoring green and red lines and temperature, different occultations modes. The third and lowest level indicate the location and are of different merit for the various concerns the determination of low level horizontal extension of the occulations. A GOMOSdata products. The purpose of instrument parameters, which will selection performed for the GOMOSmission planning is to use eventually be broadcast to the satellite the mission objective the freedom of selection to draw up and the instrument. In this chapter "stratospheric ozone monitoring" selects the measurement programmes that fulfil mission planning inside the occultation occulations marked in scientific user requirements in an mode is discussed. GOMOSmission red. The green occulations optimal way while satisfying the planning has previously been described are those not selected. The lines show the path of constraints of the GOMOS instrument by KyrOla and Tamminen, 1999. the tangent point from and the Envisat mission. the moment of star acquisition to the loss of the star signal in the The GOMOSmission planning can be 6.2 Motivation lower stratosphere. Long divided into three levels.The highest (oblique)occulations show levelis the mode selection level.Here Depending on the time of year there a significant horizontal extension while short decisions are made as to whether, will be 150-250 stars that are bright occulations are very during a given mission planning period, enough for GOMOSto track and localised the monitoring mode, the occultation which will be in its field of view at 75 specific times during an orbit. The number of available occultations during one orbit is shown in the Q) 200 z geographical map in Fig. 6-1. The ~ .«1 monthly variation in the number of ~ occultations available is shown in ~ 150 Fig. 6-2. ~ ::; Since the possible star occultations 8100 0 often overlap each other in time, as 0 shown in Fig. 6-3, not all the possible ~ E occultations can be observed. Instead, z:l a number of alternative occultation sequences can be generated, each with different characteristics, for 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec example with respect to the quality of Month ozone retrieval. This presents an opportunity to find an optimal sequence to satisfy the accuracy Fig. 6-2 Monthly requirements of a specific mission variations of the number objective. of occulations available for GOMOS.(This preliminary estimate as well as subsequent results are published by the FMI 6.3 Elements of the GOMOS LIMBO-simulator) Mission Planning In order to carry out the GOMOS Fig. 6-3 An example of a mission optimisation the different 200 s. period from the GOMOSmission objectives have to be Envisat orbit during accurately quantified. A phrase like March. Occulations are indicated by arrows in 'stratospheric ozone monitoring' as a time. The vertical scientific objective cannot be used as position indicates the such in the mathematical mission merit value of the occulation with respect to optimisation procedure. In order to ozone monitoring quantify the objectives, specific sets of mission planing criteria are used, to Availableoccultations which are assigned the values required to fulfil the mission objectives. These values constitute the .. .. data requirements. The mission planning tool is a routine which uses the list of available stars, along with the data requirements and instrumental constraints, to generate a measurement programme which satisfies the mission objectives in an .• •. optimum way. The main elements of the GOMOSmission planning are Low merit '--~--'-~--'~~-'-~--'~~-'--~--'-~~-'-~-'-~~-'-~~ depicted in Fig. 6-4. They are 5200 5220 5240 5260 5280 5300 5320 5340 5360 5380 5400 discussed in more detail in Time(s) subsequent sections. 76 6.4 Criteria characterising mission objectives Mission Objectives The mission objectives are presented in Chapter 2 and summarised in Fig. 2-10. The most important mission objectives are stratospheric ozone monitoring and mesospheric ozone monitoring. Each mission objective can be characterised by a set of criteria. Mission Planning Tool The realisation of a mission objective depends on the fulfilment of the MP Available corresponding set of criteria by which Optimiser the objective is characterised. The mission planning criteria fall into three groups according to their scope. Mission Plan 1. Star-specific criteria: the accuracy of the retrieved geophysical parameter (e.g. ozone monitoring' is ozone accuracy with the Fig. 6-4 Main elements of profile, temperature profile, etc.) target requirement of 2%,and the the GOMOS mission planning (MP) for each constituent and altitude threshold requirement of 5%. A range measurement fulfilling the target requirement is considered to be of 2. Occultation-specific criteria: highest scientific value. A measure• ment fulfilling the threshold occultation condition (dark limb, requirement is considered to be of bright limb, terminator significant importance. A star measurement) would not be prioritised if it does not fulfil the threshold requirement. obliqueness (directly related to the The reason for using a target duration of the occultation) requirement, rather than forcing the algorithm to find the absolute best local coverage star for the given criterion, is to provide flexibility for fulfilling other 3. Occultation sequence-specific criteria as well. criteria The relative importance of each global coverage criterion is set by the priority assigned to it. According to Table 6-1, the As an example, Table 6-1 shows the ozone accuracy criterion has the data requirements for the highest priority, i.e. is more important stratospheric ozone monitoring than the other criteria in the mission mission objective. objective (aerosol accuracy, air density accuracy etc.). In general, each criterion is defined by its target and threshold requirement. The global coverage criterion is For example, one criterion of the incorporated through a global coverage mission objective 'stratospheric ozone parameter, as described later. 77 Table 6-1 Data requirements for the stratospheric ozone Stratospheric ozone monitoring with global coverage monitoring mission objective. The first value in the requirement Criterion Target and Threshold Priority column indicates the Requirement accuracy, which is still (xl and x2 values) considered as the best accuracy and the second value is the least 03, line density accuracy 0.5%, 5% 1 acceptable accuracy. The 20-50 priority column weights km the different criteria H20, vertical profile accuracy 30%, 70% 2 against each other with 20-35 km respect to the mission air density, line density accuracy 1%, 2% objective 2 20-40 km aerosol, line density accuracy 5%, 40% 2 15-20 km The data requirements listed in Tables the mission objective. They enable the 6-1 and 6-2 take into consideration merit of an occultation to be the estimated product accuracies of determined with respect to the GOMOSand will be reviewed after mission objective. The value m of the analyses of the true product merit function (0$ms 1) is equal to one accuracies, which will be estimated if the criterion is perfectly satisfied by during the first six months of the the occultation and zero if it is not Envisat mission, the commissioning satisfied at all. phase. The tables reflect only which altitude range and which product Each mission planning criterion accuracy will be the driver for the star (accuracy, local coverage, duration, selection. Each occultation will, in and diurnal condition) has its own general, cover the whole altitude range merit function, which is derived from about 130 km down to the from the data requirements. For tropopause region and will also be simplicity, each criterion is used to produce the other data characterised by the following generic products aside from those listed in the merit function with five free tables. parameters (see Equation 6.1). The shape of the function is 6.5 Mission Planning Tool monotonic, either ascending or descending, with a maximum equal to Merit functions are mathematical one. The parameters x1 and .x:z functions which quantify the mission determine the width of the transition planning criteria and indicate their region from the target values (x1) for relative importance with respect to the associated criterion (merit near 1) b - b 2 1 m(x) = b1 + ( ( < + x2) lx1 + x2I) (6.1) l+exp- x- 2 I H 78 Table 6-2. Data Chemistry of the lower and middle stratosphere requirements for chemistry in the lower stratosphere Criterion Target and Threshold Priority Requirement (xl and x2 values) 03' line density accuracy 2°/c,,10°/c, 1 20-35 km N02, line density accuracy 10%, 30c% 2 20-35 km N03, line density accuracy 20%, 40c% 2 25-35 km H20, vertical profile accuracy 30%, 50%, 2 20-35 km aerosol, line density accuracy 5%, 40% 2 15-20 km to the threshold value (A'.2). (See the However, not all have the same example given in Table 6-1 and merit priority; ozone has the highest function examples discussed below). priority, while aerosol has the lowest. The parameters b 1 and b2 are the best and worse possible merit values and For a priority- I criterion (e.g. criterion associated with the parameters x1 and '03 accuracy' of objective X2' respectively. b , is always equal to 'stratospheric ozone monitoring') the one, while b2 is set to reflect the parameter b2 is set to O;for a priority- priority of the criterion, as explained 2 criterion (e.g. 'aerosol accuracy') it later. The parameter His a scaling is set to 0.3. The respective values of parameter, which determines the slope b2 have been determined after between b 1 and b; It has to be assessing the relative importance of selected so that m(x1)=b1 and m(X2)=b2. the different criteria for the respective An appropriate value is H=lO. mission objectives. An example is Mission planning would be a nearly shown in Fig. 6-5. In this example, trivial exercise if only one requirement ozone has the highest priority and at a time had to be considered, e.g. target and threshold accuracy ozone accuracy. Instead, occultation requirements of [x,, x2] = [Sc%,10%]. sequences have to be planned for Aerosol has priority 3 (b2 = 0.5) and mission objectives containing more target and threshold accuracy than one criterion (for example as requirements of [x1, x2] = [20%, given in Tables 6-1 and 6-2). Within 100%i]. one mission objective different criteria can have different priorities. For The principle of how a merit function example, the mission objective attaches a specific value to an 'stratospheric chemistry (lower and occultation is illustrated middle stratosphere)' necessitates schematically in Fig. 6-6. The total measurements of ozone, N02, N03, merit of each occultation is obtained H20 and aerosol, each with their by combining the merit values from respective target and threshold the various criteria (accuracy, requirements, as listed in Table 6-2. coverage, duration, and diurnal 79 The merit of a sequence of occultations can then be obtained simply by adding all the merits 1.0 -.. ' . associated with the occultations in " \ ·, aerosol the sequence: 0.8 \ ' . \ -, . B= ~M (6.2) 0.6 \ ' <,priority 3 ' .:!: \ N03 !.. ----- Q) 0.4 \ The sum extends over all the E <, priority 2 ._ .• - .• - - .• - - - .. ~ .• ..;....;. ,. .. ~ .. ------occultations in the sequence. This does not yet incorporate the global 0.2 coverage merit, which cannot be derived from individual occultations. 0.0 This is due to the fact that the global 0 20 40 60 80 100 coverage merit is not intrinsic to the parameters associated with the accuracy I% occultation but rather depends on the distribution of tangent point locations. The goal is to seek a uniform distribution of occultations over the globe. By the very nature of Fig. 6-5 Example for condition). Among the several possible polar orbits, an occultation can generic merit functions ways of combining them the following usually be repeated once each orbit at was chosen: the same latitude, thus achieving uniform coverage. This means the question reduces to one of identifying M=Vm···········m (6.2) a uniform distribution of occultation 1 n merit with latitude. The concept of entropy is used for measuring the This approach avoids prioritising a star latitudinal distribution of the if a mission planning criterion of first occultations. The global coverage priority is not fulfilled, even if criteria merit is denoted by S (irrespective of of lower priority are perfectly fulfilled. how it is calculated). Data requirements for a mission criterion Meritvalue m Dataon availableocculations Fig. 6-6 The principle of the merit function 80 Table 6-3 Enhancement of Method Number of stars Merit M measurement programme by the optimised selection Available occultations 182 46.6 Occultations selected on first-come basis 44 9.3 Best ozone merit (a=O) 41 17.8 Best global coverage (a=l) 41 12.7 Global coverage and ozone merit (a=0.5) 41 17.3 The total merit of an occultation selected according to their entrance sequence, F, is thus obtained by into the instrument's field of view as combining the sequence merit Mand the satellite progresses along the orbit the global coverage merit S: ('blind selection'); (2) stars were selected in order to maximise the F = (1 - a)B + aS (6.4) ozone accuracy merit M; (3) stars were selected to maximise the global coverage merit S; (4) stars were where B is as defined in Equation selected to maximise the total merit F (6.3). Through the parameter a, the with a=0.5. relative emphasis of the merit of the occultation sequence Band the global The results show that by using a coverage merit Scan be adjusted. The technique of optimal selection, the parameter a is set to zero for mission total merit can be doubled compared objectives which do not include the to a 'blind selection'. global coverage criterion. Fig. 6-7 gives an example of how the The total merit F can now be latitude distribution can change calculated for every possible depending on whether (a) the merit occultation sequence. The objective of sum (parameter Bin Equation 6.4) or the optimisation algorithm is to find (b) the global coverage (parameter Sin the sequence with the highest total Equation 6.4) is maximised. merit F. This task is more difficult than it may appear because the number of possible sequences 6.7 Summary increases in an explosive way as the duration of the mission planning The primary mission objective of window increases. A special technique GOMOS is to monitor ozone in the has been developed to handle this stratosphere on a global scale. The problem. multitude of suitable stars makes it possible to seek an optimal selection of occultations in order to maximise 6.6 Results the global coverage, the accuracy of the retrieved geophysical parameters, Table 6.3 gives an example of how the as well as other requirements. An use of an optimising algorithm leads example of the global coverage to a more efficient measurement obtainable with GOMOSusing an programme for GOMOS. Here, the optimal selection of stars is shown in focus is on how to obtain the best Fig. 6-8. The mission planning also possible ozone measurement accuracy provides the opportunity to consider and global coverage. Four cases were other mission objectives in a manner chosen for comparison: (1) stars were such that the interference with the 81 main objective is minimal. In 1.6.--,----,-----,----,.---,.----.,---,---.----.-...,Selected occultations by optimizing merit sum summary it can be said that a considerable enhancement of the 1.4 Merit sum= 17.77 41 occultations GOMOS mission can be achieved 1.2 using the optimising mission planning Q) 1 technique described here. An ::l 04 and the planning of observation periods in which to focus on 0.2 particular mission objectives. -60 -60 -40 -20 0 20 40 60 Latitude (a) Selected occultations by optimizing global coverage 1.6.---,----,---.,----,----,.---..,.---,----.----.-· Merit sum= 12.86 41 occultations 1.2 Q) 1 ::l (ij > 0.8 ~ ~0.6 0.4 0.2 -60 -60 -40 -20 0 20 40 60 Latitude b Fig. 6-7 The upper plot (a) shows an occulation sequence selection with 90 an optimised total merit, disregarding the global coverage. The second plot (b) emphasises the global 60 coverage. This leads to a much more even distribution of occulations over latitude, but to a 30 lower merit sum "O"' .3 , I , I / ___ Fig. 6-8 Full global :g 0 ' I , I coverage is achieved with ..J 15 consecutive orbits. / / This figure shows the /I / \ /I /I / \ --30: l \ \ \ \ \ II I I selected occulations for / / / / / . / / / / / // / / / the mission objective "stratospheric ozone monitoring." The lines -60r," ---" ---" ---" ---" ---" ---" "~ -,~~~-..,. --.:;"'-'.---"' ---"' ---" show the path of the tangent point from the moment of star acquisition to the loss of the star signal in the -9al--~- -180 -120 -60 0 60 120 180 lower stratosphere 82 7. Scientific Data Processing and Data Products 7.1 Introduction The concept of the transmission function is central to both the The GOMOSdata processing consists GOMOSinstrument concept and the of two major steps. The raw data (Level GOMOSdata processing scheme. 0 data) are processed in the Level0-1b Transmission provides in principle processing, creating the Level 1b data, information that is free of calibration which are the localised calibrated factors. The property of self• transmission spectra and photometer calibration is the most important fluxes and the background spectra. advantage of any occultation From this, the Level 1b-2 processing instrument. calculates the Level2 data products consisting of the atmospheric The main inputs to the GOMOS constituent profiles, which are the processing are the spectrometer vertical and line density profiles of 03, (UV-visibleand IR) and photometer N02, N03, 02, H20, air density, data plus the occultation and aerosols, temperature and turbulence. photometer recording times and Furthermore, the Level 1b-2 processing auxiliary data (e.g. satellite location, outputs the recomputed transmission mirror position). spectra, corrected for scintillation and dilution effects. The tangent height The GOMOSdata retrieval processes range of the Level2 data products is are not totally independent of external 15-100 km, the elevation range is +62° data. In particular, meteorological to +68° and azimuth range is +90° to (ECMWF)data will be used for ray +190° (withrespect to flight direction). tracing calculations and temperature• dependent cross sections. This The link between the constituent meteorological information can densities and the spectra measured by optionally be replaced by the the spectrometers is the horizontal atmospheric information retrieved transmission function. From the from GOMOSmeasurements during measurements it can be expressed as an iterative retrieval procedure. the ratio between a stellar spectrum observed through the atmosphere and one measured outside the atmosphere. 7.2 Level 0-1b Processing If the measurement is made through the limb illuminated by the Sun, the The main goal of the GOMOS Level scattered sunlight must first be 0-1b processing is to estimate a set of eliminated from the occulted spectrum. horizontal transmission functions Moon,auroral, and other emissions T0bJA.) using data measured by the may also add background radiation to GOMOSspectrometers. This so-called the spectrum. full transmission function will serve 83 as basic data for the Level 1b-2 they will cancel out. Otherwise processing. corrections have to be used in order to remove the distorting effects. In an idealised setting the Level 0-1b processing would be a series of simple Besides the instrumental effects there operations. For each stellar exposure are a few atmospheric phenomena one could calculate the atmospheric which obscure the identification of transmission as a ratio between the the transmission function. These are counts Nace measured through the the following: atmosphere at a given tangent altitude and the reference stellar • radiation generated in the counts Nrcf measured above the atmosphere and entering the FOV atmosphere: of the instrument. for the specific wavelength ranges of GOMOS the N following sources are expected: DCC T (A) =JV-. (7.1) - solar or lunar light scattered obs re} one or more times, - auroral lights, In the real world, the following - natural emissions (continuum instrumental effects will have to be and lines). taken into account in particular, straylight causing deviations from an ideal cosmic rays situation: Solar light scattered in the • optical deformations from atmosphere will dominate over the instrument input aperture onto the stellar light when GOMOS is looking detector surface at the daylight side of the Earth - spreading of the image in the (bright limb) whereas it can be spatial dimension, ignored in the night side (dark limb). - changes in the orientation of the In the bright limb condition the solar optical axis. background must be estimated and non-perfect efficiency of the separated from the stellar signal, in detector order that the occultation processing accumulation of dark charges and can be continued. The solar signal the addition of read-out noise removed will, however, constitute an electronic gain in the amplifiers important data product in itself. All these are connected with the Taking all these effects into account characteristics of the instrument. If makes forward modelling a relatively any of these effects act in the same complicated task. After several way on the occultation signal and on simplifications the signal counts N (for the reference signal and can be a binned pixel column m during one represented as multiplicative effects, integration time) can be calculated as: 84 Table 7-1 Pre-processing Process Content in level lb Datation Alldata willbe tagged with time. Wavelengthassignment The small oscillations in the GOMOSand the spacecraft pointing make the image of the star wander on the detector surface. This will change the wavelengthsthat each detector pixelwill see. The information from the star pointing system willmake it possible to correct this perturbation. Geolocation The needed geolocationfor measurements. Calculated from orbit data, earth geoidmodel and atmospheric model. Fig. 7-1 Level 0-lb data processing: data providers, main processing modules, Here F represents all the elements in external data suppliers the amplification chain, Q is the quantum efficiency for pixels, Wis the instrument function, Tatm is the on-ground atmospheric transmission and J0 star characterisation the original star spectrum. The remaining terms are due to the background radiation, dark current and read-out noise. The sum goes over the binned pixels. Level 1 data are distinguished into Level 1a and Level 1b. The Level 1a data are Level 0 data, assorted and filtered by some low-level quality checks. The first data product of interest to the user is the Level 1b Level lh processing product. chain Level 1b processing consists of several steps, listed in Tables 7-1 and 7-2. The data input and output are summarised in Fig. 7-1. The processing can be divided into pre• processing common to all data and the processing of data from various sub-instruments. During preprocessing of the data, the wavelength assignment and the notes: • <111\ilur~dutuhases and Incl 0 product arc made uvuilublc at orhit level calculation of the geolocation are D pro1.:cssing and level lh products arc performed at ucculution level carried out. In the wavelength assignment a nominal assignment is LJ off-line produced but there is also a step where the possible wavelength shift is 85 Table 7-2 Level lb processing for Process name Content spectrometer Saturated samples Saturated samples are screened, flagged and removed from processing. Bad pixels CCD pixels that do not respond normally will be flagged and removed. Instead of removing, new data will be generated for bad pixels. Cosmic rays CCD pixels hit by cosmic rays will be temporary or permanently damaged. The abnormal values in the spectra can be detected with some probability and data from these pixels will be replaced using median filtering. Dark charge correction CCD detectors generate dark charge electrons. By ground and in-flight measurements the average amount of dark charge can be estimated on a pixel• by-pixel basis and this average can be removed from the measured signals. Optionally, dark current can be estimated from GOMOS measurements. Internal and external Straylight may potentially disturb the measurements straylight and it will be removed from signals as completely as possible. Vignetting This operation is applied only for spectrometer B. Central background The two-dimensional nature of the CCD detector estimation means that we can estimate background signal relatively accurately. Star signal At this stage the background contribution is subtracted from the central signal leaving only the star occulation signal. The flat field (modified by scintillations) correction is also applied. Slit correction This will correct the optical transmission modifications if the image of the star is shifted near the edge of the slit in the focal plane. Reference star spectrum The measurements above a predefined altitude level are interpreted to be samples representative of the original star spectrum. This means that all absorptive and scattering effects are considered to be negligible. From a set of n measurements the reference spectrum is defined as an average. This reference spectrum will be one of the side products of the GOMOS mission. Transmission This is the final step in the level 1b processing of the spectrometers. The ratio of two quantities are determined and this ratio is interpreted to represent the full transmission function for this measurement. An estimate for the corresponding variance vector is also calculated. 86 determined. This is based on the analysis of the star tracker data. Reference star Spectrum Original Spectrum and Spectrum Measured by GOMOS During the determination of the geolocation, ray tracing calculation is performed. The atmospheric data j 1500 ~ needed for this calculation come from 'u 1000 I meteorological data or from ~ st.tr spectrum 1!1•,:;_isur•:d ijy COMOS atmospheric models. -a~ 500 There is a distinction between the 300 400 500 600 700 near real time (NRT) processing and the off-line processing. NRT Absolute Error Relative Error processing uses the ECMWF forecast 100 10 for the lower part of the atmosphere -E .F 50 ;!'- and the atmospheric model MSIS90 !i ' "<> g for the upper part. In off-line I 0 UJ~ ! .a: processing the meteorological analysis 0 0 ~ .c -50 a: -5 replaces the meteorological forecast. a. It must be remembered that GOMOS -100 -10 itself will measure atmospheric 200 300 400 500 600 700 200 300 400 500 600 700 characteristics such as neutral Wavakmgth I nm Wavelength I nm density and temperature but these are produced only during Level 1b-2 processing. Therefore some initial atmospheric data are needed to start The flux in the two photometers Fig. 7-2 Example of a star the retrieval process. during one occultation sequence is spectrum: the original spectrum and the spectrum shown in Fig. 7-3. An example of as measured by GOMOS. The instrument-specific Level 0-1 b the major Level 1 product, the The two lower plots show data processing also accounts for atmospheric transmission spectra, is the absolute and relative statistical errors of instrumental effects. These include shown in Fig. 7-4. measurement. the star the identification of saturated magnitude is 2 and the star samples, the bad pixel processing and The reference star spectrum is temperature is 6400 K the elimination of cosmic ray effects. obtained by averaging the first five Very important steps concern the spectra obtained during the estimation and removal of dark occultation. It will be used to current and the atmospheric calculate the atmospheric background signal from the central transmittance and, as a mission CCD-band, which is the detection spin-off, can contribute to creating a area for the stellar signal. Both the stellar data base. dark current and the background can be estimated from the pixels on which The full transmission spectra are no stellar signal falls. The dark obtained by dividing each spectrum by current can also be estimated from the reference star spectrum. It is calibration data. described as 'full' because it is the actual measured transmission, not 7.3 Level lb Data Product corrected for refraction effects (dilution, scintillation, chromatic Table 7-3 lists the main level 1b refraction) or for variable point-spread• products from GOMOS. function. Each spectrum is resampled on the wavelength pixel grid of the A typical spectrum of a star with reference spectrum before calculating magnitude 2 is shown in Fig. 7-2. the transmission. The spectrum 87 This signal was subtracted from the measured stellar signal during the •0·•0:1 occultation in order to get the pure 1 ' '.'. t(, ~ stellar signal without background 4 ~,,] ~ 0 ' l ii l (atmospheric straylight) '""'J #bh.J contamination. '°"''[ ---- -•------·----~-----·-----·. __, j ,,. i:; 11:1 21 14 rr 0 .I b 9 '2 1:, 'ti L' 24 JI .;0 JJ --·--:~"'~" (•>) The photometer flux data (given in electrons) are used to quantify and correct for atmospheric scintillation. Further Level 1b data are the wavelength assignment, the 11! calculated geolocation and limb _ ~~'.:1-~':'rf!.:_?_~~ro_----- r---- -1 spectra measured with the two extra bands (above and below the star image) of the CCD 1'/',1111~_1/_ :_"_._·,r1~~~1111,l~\~'\~;11ij11t_'N)_11.~1/~r'1,_·1\1.~,H1_"•1__A'._1J spectrometers. .J II 11 I' I' 1'11 ~· - I I I 'l 1onoL ------·------·---·--···------·-····· ----·--·--··-··-·- All products issued from Level 1b are accompanied by error estimates. These estimates assume Gaussian error statistics and disregard modelling errors. Fig. 7-3 Photometric flux. The small inhomogenities covariances are computed from the on the atmospheric analysis of the signal-to-noise ratio. temperature field cause 7.4 Level 1b-2 Processing individual, originally parallel light rays to The central background estimate is the travel along slightly estimated background contribution to GOMOS Level 0-1b data processing differing routes. When the total signal in the central band. produces a set of the full these rays emerge from the atmosphere they will create a light field having a structure which varies rapidly in space. GOMOS will see this as a field Product Unit Potential application fluctuating rapidly in time. Although a nuisance Reference star electrons Astronomy for the stellar signal analysis, the fluctuating spectrum GOMOS processing field can be used for a GOMOS calibration high resolution analysis of the atmospheric temperature field Geolocated and dimensionless GOMOS level 2 calibrated transmission spectra Geolocated electrons GOMOS higher order product development calibrated limb spectra Photometer flux electrons GOMOS level 2 GOMOS higher order Table 7-3 Main level lb product development products 88 transmission functions for the then be converted to vertical profiles occultation. Before they can be used of the constituents. The third method to retrieve trace gas densities two starts from the transmissions from all physical effects must be considered. tangent heights and inverts the These are refractive dilution and constituent densities in one step. scintillation modulation. After these Details are given in Sihvola, 1994. have been eliminated the reduced transmission function is obtained. The present GOMOSprocessing For an occultation measurement the scheme relies on the first method, i.e. reduced horizontal transmission spectral inversion followed by vertical function (often expressed as inversion. This method has been extinction) is definitely an important chosen for two reasons. The first is a scientific product. significant reduction in the amount of data, which must be handled at a Modelling the reduced transmission time. The 'spectral first' approach by using Beer's law and applying contributes to the fast reduction in spectral inversion methods produces size of data. In order to take into tangential column densities of account the coupling effects between different contributors to the spectral and vertical problems, the extinction. As external data processing makes use of an iterative Fig. 7-4 Transmission absorption and scattering cross• approach. The second reason is spectra at several tangent sections for the constituents which related to the observation geometry of heights. affect the signals are required. The GOMOS.The result of the spectral Blue = 50 km, green = 34 km, red = 21 km). The atmospheric temperature is an inversion is atmospheric column vertical transmission important parameter for the densities along the line-of-sight of the function describes the calculation of temperature-dependent instrument (line density), i.e. the data important atmospheric transparency, which is of cross-section. In the vertical inversion product corresponds to the crucial importance for the knowledge of the measurement observation geometry. The line biosphere geometry is used to invert the line densities into vertical profiles of the constituents. Transmittance Spectra for Different Tangent Altitudes Three basic inversion schemes have Star Temperature: 6400K, Star Magnitude: 2 .Spectrometer A (ljV-vi!;ible) been considered. All assume spherical symmetry of the atmosphere and 1.0 evaluate individual, rather than several consecutive, occultations. 0.5 0.0 In the first scheme, transmission data from every tangent height are inverted 200 300 400 500 600 700 to line densities for different constituents (spectral inversion). For Spectrometer 81 (near IR) Spectrometer 82 (near IR) 1.5 1.5 i every constituent, the collection of I line densities at successive tangent 1.0 1.0 f !~¥~~~1,ii11~~¥~,M1,~M1. heights is converted to vertical 0.5 [ . ' I ' densities. By dividing the atmosphere 0.5 into discrete layers the well known 0.0 0.0 'onion peeling' method can be used. -0.5 -0.5 The second scheme first calculates 755 760 765 770 775 925 933 941 949 957 the vertical profile of the wavelength• Wavelength I run Wavelength I nm dependent extinction. Using the cross-sections the vertical profile can 89 densities might also be more useful retrieved. The treatment is divided intc for assimilation of GOMOS two parts depending on whether it measurements into atmospheric concerns the data from the UV-visible models, if the model can simulate spectrometer or the IR spectrometer. GOMOS observation geometry. This division is necessary because the forward calculation of the IR Further information on the level 1-2 spectrometer wavelength region data is processing can be found in Kyrola, computationally demanding and is 1999. done beforehand by using a large 'look• up' table. The UV-visiblespectrometer 7.4.1 Preparation of the inversion .wavelength range allows a relatively process fast calculation. The two data sets are inverted separately but information The Level 1b product, which is input exchange occurs at a later stage. to the Level 1b-2 processing, is processed in such a way that it The UV-visible spectrometer data contains only the effects of inversion may be done in a nonlinear atmospheric extinction (absorption or a linear way. The nonlinear method and scattering) and refraction. It can is the most favourable approach as it be assumed that the full transmission retains the original normal can be written as a product: distribution of the transmission data noise. In the linear case the T T T T logarithmic function distorts the noise atm abs scin ref (7.3) which may cause bias if not corrected. Details can be found in The refractive dilution Tref can be KyroUi et al., 1993. estimated from the ray tracing calculation. The scintillation The observed transmission can be modulation Tscin will be estimated by modelled as (see equation 7.4 below): using data from the fast photometers. If these effects are not removed from where L represents the collection of the transmission they will increase light trajectories l where each the errors in the aerosol and neutral trajectory depends on wavelength density retrievals. There is also an (chromatic refraction) and time optional stage for chromatic refraction (movement of satellite during the correction (the second possibility integration time), and Tis the optical resides inside the spectral inversion). depth: 7.4.2 The spectral inversion T(il.,l(il.,t)) = LI p/s)(J'(i\., T(s))ds module J sel(A,t) The purpose of this module is to (7.5) extract information about the atmosphere using the spectral Equation (7.4) is approximated into a dimension of the transmission data. still more practical form by moving From these data the horizontal column time and wavelength integration into densities of the constituents are the optical depth function and also T (il.L)=~JJw (il.i1.')e-T(A',1(A,1))dil.'dt (7.4) abs ' t:,. t gen ' at 90 making an approximate expansion of an iteration loop from the vertical the chromatic refraction. inversion (see below) back to the spectral inversion. Various methods are available for the introduction of cross-sections into the Absorption and scattering cross• calculation. These can be absolute or sections constitute the heart of the differential. The global spectral GOMOS Level 2 spectral inversion. inversion scheme uses the whole The Rayleigh cross-section is quite spectral range whereas in the step-by• accurately known and many of the step method a single constituent is absorptive cross-sections have retrieved from a restricted wavelength been measured to adequate range. accurately. An important fact is that many of the absorptive cross-sections The temperature dependence of the depend on temperature, which adds a cross-sections makes the spectral complexity to the spectral inversion. inversion of Equation (7.5) impossible The aerosol cross-sections are less without further approximation. The well known. In GOMOS processing straightforward method is to use the either a simple Angstrom law is tangent (or any other fixed) point assumed or the aerosol cross-section temperature which decouples spectral is taken to be a polynomial of and spatial parts. In the GOMOS wavelengths where the polynomial processing this is done for the first coefficients are unknowns. round of spectral inversion but the approximation is improved by The nonlinear inversion problem exploiting the 'effective' cross-section defined by Equations (7.4) - (7.8) is concept (see below). The optical depth solved for the UV-visible spectrometer is written as measurements by the well-known Levenberg-Marquardt algorithm. The algorithm provides estimates for T = 2: N Jfl( ,\) (7.6) the line densities and their errors. J J j A priori trace gas profiles are presently not being used for the where retrieval. N(l) = IP (s)ds (7.7) The IR spectrometer measures the J J' J SE /(,\.f) density of 02 in the 756- 773 nm band and H20 in the 926-952 nm band. The calculation of the is the column density and the transmission function is 'effective' cross-section is computationally very demanding. Therefore, a different retrieval algorithm is applied, which uses pre• ,1;: P ( ) ( calculated reference transmission ". (A,l(A,t)) =sc/(A.t)I j S 91 7 .4.3 The vertical inversion (z - z)p(z) + (z- z)p(z ) J I J J _J- 1 module p ( z ) = z - z j- 1 i The column densities produced by spectral inversion are inherent (7.9; quantities, which should vary rather smoothly as a function of height. The problem can be transformed into Therefore the Level 2 processing a matrix equation: includes an optional smoothing stage before the vertical inversion. Ap = N (7.10) In this module the set of horizontal where A is (a relative complicated) column densities is converted to kernel matrix. The line densities are vertical profiles. First a general layer the result of an integration in time structure is generated for the and should be adapted to the atmosphere and a coverage vector is 'instantaneous' expression that has calculated for every constituent. This been derived (because the linear is a collection of tangent heights at system is only valid for instantaneous which the spectral inversion module line densities). To pass from has been shown to be able to produce integrated to instantaneous data a column densities for the constituent Taylor expansion is used: in question. 2 N.= N +(Lit/ iJ N For the vertical inversion two J . - J (7.11.1 J 2 approaches have been considered. 4 at2 The first solution uses a 'generalised onion peeling' method, employing the well known 'onion peeling' method for These contributions can be the case where the number of layers summarised into matrix form and the is equal to the number of vertical inversion consists of solving measurements. In cases where more the following linear system: measurements are available than layers, a least-squares solution is N= Kp (7.12) first used inside each layer and then the ordinary 'onion peeling' method is used. The second solution, the This matrix equation is solved by so-called polynomial inversion, is an standard methods. The covariance attempt to include the variation of matrix will also be produced. the constituent profiles inside each layer in the solution. The polynomial 7 .4.4 Level 1b-2 processing loops method with a linear variation of the constituent densities inside each In the vertical inversion, density layer will be used in the GOMOS profiles and a new temperature profile ground segment. The stepwise linear are produced. Initially, temperature profiles will be taken to be profiles given by ECMWFand MSIS90 continuous. model are used. GOMOS data provide several possible sources for a new In the method used in the GOMOS temperature profile, most obviously ground segment the local densities the Rayleigh scattering from the UV• are approximated by a linear function visible spectrometer and the 02 data of altitude between two consecutive from the IR spectrometer. Neutral GOMOS measurements: density can be derived from Rayleigh 92 scattering using the ideal gas law and Fig. 7-5 GOMOS Level assuming hydrostatic equilibrium. lb-2 processing GO MOS architecture Similarly, neutral density can be Level lb products (after Kyrola, 1999) derived from the 02 data assuming a constant mixing ratio and temperature according to the ideal Rapid gas law. These two estimates must be Ii.ii fluctuations assimilated together with weights process mg according to their reliabilities (covariances). These new data are Cross-sections used, after a quality check, in the data base calculation of the effective cross• sections and the spectral and vertical inversions are carried out again. It UV-vis has been shown that this iterative spectrometer process over spectral and vertical inversions improves the final results New cross- significantly. It is therefore routinely computation applied with one or two iterations. A second iteration loop is available to repeat the ray-tracing calculation with the newly retrieved GOMOS atmosphere followedby another retrieval iteration. Fig. 7-5 depicts the GOMOS Levcl2 Level 1b-2 processing architecture. products Rayleighextinction and 02 absorption are not the only possibilities for generating temperature from GOMOS measurements. Small-scale vertical structures in the atmosphere induce scintillations in the signal of the blue also contain error estimates, which and red photometers, which are slightly are calculated throughout the shifted in time due to the chromatic processing chain and propagated to differencein the refractive bending of the final data products. the light. The computation of the time delay between the two signals can be The line densities are the results of used to determine the total bending the spectral inversion procedure. angle of the light and, using the Abel These line densities may be directly integral, the vertical profileof index of used, for example in data assimilation refraction, which is proportional to systems, to create higher-order data atmospheric density. A high vertical products. resolution (100-200 metres) temperature profilecan be derived from the density The vertical density profiles are the profile,using the hydrostatic equation. results of the vertical inversion. Vertical profiles derived from long• lasting (oblique) occultations have to 7.5 Level 2 Data Products be used with care as they have a large geographical extent. Occultations in The main Level 2 data products are polar regions can easily extend listed in Table 7-4. The data products through several local time zones. 93 Table 7-4 Main level 2 GOMOSproducts Product Unit Potential application 03, N02, N03, H20, air 1 molec/cm2 Higherorder GOMOSproducts horizontal column densities Monitoringof trends Aerosolextinction and poly- 1 molec/km Aerosolresearch nomial extinction coefficients 1 molec/(km nm") Atmosphericchemistry research 03, N02, N03, H20, 02, air 1 molec/cm3 Atmosphericchemistry research vertical density profiles Monitoringof trends GOMOSproduces profiles K GOMOSprocessing for T and density 1 molec/cm1 Atmosphericresearch Turbulence product: High K Atmosphericdynamics research resolution T and density 1 molec/cm1 Meteoproduct Meteorologicalapplications The turbulence product (high- are more suitable for 02 and H20 resolu tion temperature and density measurements in the IR. The vertical profiles) is derived basically accuracy also depends strongly on from the time delay of the signal the visual magnitude of the star between the peaks of the red and blue used. The precision of the retrieval fast photometer. also depends on straylight, i.e. dark limb or bright limb condition. The Meteoproduct consists of The expected precisions of extracted profiles with reduced spatial retrieved ozone profiles are shown resolution for near-real-time in Fig. 7-6. The magnitude and dissemination to meteorological users. star temperature limits of the GOMOS target stars are determined using an instrumental technique 7.6 Characteristics of GOMOS (star tracker). This has been shown to give useful retrieval accuracies Products for ozone. For other trace gases the useful limits are much narrower. Regarding the data products from GOMOS some special features of the 2. Retrieval coverage and accuracy is measurement principle (stellar geographically uneven: this is a occultation) and the instrument itself consequence of the points must be kept in mind. The following discussed above, the limited are the most important: number of stars and their uneven distribution. Another contributing 1. Accuracy of the retrieved product factor is that measurements in depends strongly on the star used daylight give lower accuracy than in occultation:For ozone, to take night-time measurements. This an important example, hot stars means that for ozone a quite good provide good accuracy for ozone at global coverage can be achieved high altitudes whereas cool stars but for other minor gases the 94 coverage will be more sporadic. .,/pnuence or St~r J;.mperalurr on 03 Local ~ensity Retriev_at o:Jlar Temper1alure on O'iColumn Depsity Retrie~val This is in contrast to instruments 71t;J1uence measuring atmospheric emission, - - - Ill.firt•mperalure 3300K - - - Illar lamperatuni 3300K e.g. MIPAS. - ll'lar l. maanituda3 4. The vertical resolution of the GOMOS measurements can be described by weighting functions, -----.•.--::.- -; -~---..:~:.:.::.:: as shown in Fig. 1-2. Most of the signal comes from near the tangent 10 altitude. The tangent altitude itself En-or I" Error I II: moves about 1.7 km during the Influence of Limb Condition on o~ Column Denett Retrieval occultation (in the aft direction). For long oblique occultations brl1htlimb brlchlllmb movement of the tangent point is considerably smaller but the horizontal component of the movement is correspondingly larger. Error/Ir: Error I Ir: 5. The horizontal extent of the measurement varies from occultation to occultation: the direct Fig. 7-6 Preliminary occultation ray covers about 15 estimate of the precision of ozone local density and degrees in latitude and during very ozone line (or line of sight oblique occultation the tangent column) density point moves considerably in the retrievals. Upper plots: influence of star direction of the latitude. temperature; middle plots: influence of star magnitude; lower plots: influence of limb condition (stray light) 95 8. GOMOS Validation The validation of GOMOS data observations will help confirm products will start during the that corrections for solar stray-light commissioning phase of Envisat and and vignetting are accurate. continue through the whole lifetime of although the effects of variations in the instrument. The Envisat viewing angle must be taken into validation is largely dependent on the account. Principal Investigators of the accepted proposals, who will contribute to the 8.1.2 Comparison of temperature validation of Envisat data products, derivations submitted in response to the 1998 Envisat 'Announcement of Atmospheric temperature will be Opportunity' (AO). derived from GOMOS measurements in the following three ways: Envisat is scheduled for launch in mid- 2001 and a validation plan has • from the 02 profile density. been established to provide early results during the six month • from the Rayleigh extinction of Commissioning Phase, as well as stellar light, longer-term validation during the mission lifetime. A validation team • from the scintillations observed has been set up by ESA, which with the two photometers. consists mostly of the Principal Investigators of accepted AO Verification of the consistency of these proposals. First validation results will methods is important for the be analysed at the end of the evaluation of each algorithm and for Commissioning Phase, before Envisat evaluating error budgets. products are released to a wider public. Theoretically. it could also be possible to derive temperature information from the following: 8.1 Internal Consistency of GOMOS Measurements • from the Rayleigh scattering of 8.1.1 Measurements at the same sunlight into the background place bands. There will be certain occultation • from the 0:1 temperature• geometries in which the same dependent cross-section in the geographic location can be observed ultraviolet. on successive orbits using different stars. Comparison of the different • from the curve of mean dilution. 97 However, these information sources 8.1.4 Self-consistency check of will not be used routinely for N02, N03 and 03 temperature information as the effect of the temperature on these The self-consistency of GOMOS N02. parameters is not as significant as N03, 03 measurements can be on the first three listed above. It checked by using the reaction might also be possible to retrieve equations: temperature via refractivity measurements, which would (8.1) require very accurate knowledge of mirror pointing and spacecraft attitude. (8.2) 8.1.3 Comparison of ozone derivations hence Ozone absorbs at both ultraviolet and visible wavelengths. Values will be retrieved separately in both parts of the spectrum from both stellar and bright-limb measurements and (8.2.) from both absolute and differential (DOAS)spectra. All eight values will where k. and k., are temperature be of high accuracy in the middle dependent. stratosphere (30 km), so a meaningful comparison between In the lower stratosphere, N03 goes values will be possible. Differences in rapidly to steady state: absolute values will indicate inconsistencies between the d [NOi] = o ultraviolet and visible cross-sections, (8.4) or errors in aerosol retrievals which dt differ in their effects in the UV and in the visible, or errors in internal Hence a consistency check in the straylight which again differ between lower stratosphere is to see that the UV and the visible. Differences in differential stellar values will point to (8.5) errors in the spectral transfer function - the ultraviolet bands have much finer structure than the visible In the upper stratosphere, Equation - so that errors in assumed spectral (8.3) has an equilibrium time of resolution will affect the ozone several hours. So consistency of retrieved from ultraviolet spectra GOMOS measurements with Equation more than the ozone from visible (8.3) might be checked explicitly. spectra. Differences between stellar Unfortunately the signal-to-noise and bright-sky values will point at ratio of the N03 measurement (based errors in the radiative transfer code on a single occultation) is poor, and used to calculate bright-sky paths several occultations at successive and radiance. Differences between times during the night at the same the absolute DOAS stellar values will location will be rare. Hence Equation point at errors in the aerosol (8.3) can only be checked in an characterisation deduced from the averaged sense, or via an assimilation absolute spectra. scheme. 98 No other consistency checks are the commissioning phase, in fact obvious from GOMOS data alone. during the whole lifetime of Envisat, albeit at a reduced level. The larger data set that is so obtained will allow 8.2 Geophysical Validation the statistically significant identification of errors that could not 8.2.1 Introduction previously be distinguished from random noise. In addition, the large Validation is the process of assessing set of regular, continuous data points the quality of data products by so obtained, will contribute to long• independent means. The main term instrument performance operational GOMOS products to be analyses and modelling. validated are in particular the vertical profiles of 03• N02, N03, 02, H20, 8.2.3 Independent sources of temperature and aerosol extinction correlative data coefficients and spectral parameters. Non-operational data products must Before scientific investigations of also be validated, but this has a lower trends and of agreements with model priority. predictions are carried out using GOMOS measurements, it is essential The GOMOS data product accuracy to conduct campaigns to confirm the will vary with altitude. occultation validity of the atmospheric geometry, temperature and visible parameters measured by GOMOSby magnitude of the occulted star and comparison with measurements of the the spectrum of the star. This means same parameters from other sensors. that validation results obtained by Different classes of observation observing the occultations of a techniques will be used for this specific star cannot be considered validation. representative of all stars. A common problem for all validation 8.2.2 Campaign activities measurements will be to measure the variables as simultaneously as It is planned to devote the first six possible in space and time with months after the launch of Envisat to GOMOS. Furthermore, the vertical the commissioning of the resolution and horizontal smearing of instruments. During this phase the the measurements will vary markedly functionality of the instruments and between validating instruments. Two the whole observing system will be strategies will be applied to overcome tested, calibrations will be performed this problem: and the first data products will be produced. The validation of the 1. The GOMOS mission planning has geophysical products will start as to try to make GOMOS soon as the Level 0-1b processing has measurements as close as possible been successfully tested and Level 2 to those of the validating products can be produced with instruments. This will be easiest preliminary confidence. After the for ground-based instruments, commissioning phase the mission will which are observing 24 hours a start its operational mode. day, but it will be more difficult for balloon campaigns. ESA is coordinating the geophysical validation activities for Envisat. These 2. The GOMOS data could be activities will last beyond the end of assimilated into atmospheric 99 chemistry-transport models, which individually, the comparison of the data can calculate (via their products can only be used for atmospheric models) the state of consistency checks and will be valuable the atmosphere on the exact for error exploration, but cannot really location and time of the validating validate GOMOS products. instrument. b} Other spaceborne instruments a] Other instruments on Envisat Comparison of products from different The instruments MIPAS and satellite instruments is particularly SCIAMACHYwill each measure, useful for global validation, since a amongst other parameters, 03, N02, large range of observation parameters aerosol and temperature. MERIS and (latitude, longitude, solar zenith angle. AATSRmay also provide relevant cloud presence etc.) can be covered in information, for example measures of a short time. Table 8.1 lists satellite aerosol properties. Comparisons with sensors that produce data products GOMOS data will start during the that will be useful for GOMOS commissioning phase. validation. In general, the measurements will Of these instruments, those with good neither take place simultaneously nor vertical resolution and with 03, N02 be collocated. SCIAMACHYwill or aerosol products of comparable measure only during the day, while accuracy to GOMOS are POAM-IIIand the best GOMOS measurements are SAGE-3 (03, N02, aerosol). HIRDLS at night. Collocated measurements of (03, N02) and MLS (03, BrO). GOMOS and MIPAScan only be Validation activities with these achieved when GOMOS acquires stars instruments will be continued as long in the same direction as the MIPAS as possible. field-of-view. c] Instruments on aircraft Table 8-1 Satellite Since none of the three Envisat sensors available for comparison with GOMOS chemistry instruments, GOMOS, MIPAS Aircraft provide more control (than (aer = aerosol) and SCIAMACHY,are yet validated any other platform) on the selection of Spacecraft Agency I Date Instrument Spectral Technique GOMOS Country region gases measured ADEOS-2 NASDA 1999 ILAS-2 IR, near IR solar 03, N02, aer, occultation 02 ODIN Sweden 1999 OSIRIS UV-vis limb sunlight 03, N02, aer S'.VIR sub-mm limb emission 03 SPOT-4 CNES/NRL 1998 POAM-III UV-vis solar 03, N02, aer occultation ERS-2 ESA 1995 GOME UV-vis nadir sunlight 03.N02 METEOR RKA 2000 SAGE-3 UV-vis solar 03, N02, aer occultation EOS-CHEM NASA 2003 HIRDLS IR limb emission 03, N02, aer EOS-MLS sub-mm limb emission 03, Bro 100 air volume that is observed by the The Wyoming aerosol sonde, which instruments they carry. The flight provides the size-spectrum of number path can even be flexible enough to density. can be used for GOMOS make in-situ and remote sensing stratospheric aerosol measurement instruments on board sample the validations. Meteorological same air mass during a single flight. radiosondes provide air pressure and temperature as a function of altitude. Although less suitable for inexpensive From these quantities, using the routine operation, such platforms known altitude dependence of the offer advantages during campaigns, oxygen volume mixing ratio (VMR).it since observation strategies can be is possible to calculate the 02 profile, adapted at short notice, even during to be used for validation of GOMOS the course of a measurement. 02. In addition. the temperature values can be directly compared to Payloads can be installed on various temperature values from GOMOS. aircraft that have been adapted for atmospheric measurements. Aircraft Balloon-borne remote sensors can also foreseen to participate in the provide good vertical resolution, though validation campaigns include the this varies from sensor to sensor. In Russian M-55 Geophysica. the NASA particular, instruments that can ER-2 and DC-8. the German DLR measure during ascent have excellent Falcon and Transall and the Dutch vertical resolution. and for this reason Cessna Citation. SAOZ-balloons may be the only practicable system to validate GOMOS d) Balloon-borne measurements N02 measurements. In fact they have a better vertical resolution than GOMOS. Balloon-borne in-situ sensors provide Lillesensors BALLADand BOCCAD vertical profiles of very good vertical measure aerosol phase functions. resolution throughout the stratosphere, as required for the The AMONballoon-borne sensor is validation of GOMOS. However. particularly relevant as it exploits the careful planning is needed to ensure stellar occultation method similarly to collocation of a stellar occultation GOMOS. This is essential for the with a balloon flight. validation of N0:1 profiles and of night-time profiles of OClO, as these Regular ozone sonde balloon flights cannot be measured by any other should be a prime target for ozone current sensors. During an validation up to 30 km altitude. A occultation. the balloon is much chemiluminescent N02 sensor is closer to each tangent point than available in Japan, but is large and Envisat. Thus errors in the refraction heavy. Although a small balloon• calculation induce a much smaller borne N02 sonde has been developed error in tangent altitude in AMON in Canada. it is as yet untested in its than in GOMOS. stratospheric version. Balloon-borne frost point hygrometers have been Balloon-borne in-situ instruments will flown by French and British groups. be limited to the 11ightaltitude of the and would provide important balloons in the lower stratosphere. contributions to the validation of Upper stratospheric and mesospheric GOMOS H20 measurements. A new GOMOS measurements can only be low-cost frost-point hygrometer is validated by remote sensing being developed in the UK. instruments. 101 e) Ground-based instruments • lidars (NDSCnetwork), measuring aerosols, temperature and 03: Several techniques provide profiles are measured with stratospheric measurements, which excellent vertical resolution. Clear can be used for comparison, with the skies are needed and limitation that these techniques - measurements must usually be except lidars - are generally very made at night. Aerosol extinction limited in vertical resolution: and backscatter can be retrieved independently on lidars with • spectroscopy of sunlight scattered Raman channels. from the zenith sky (SAOZnetwork and others, measuring 03, N02, Most of these ground-based OClO, BrO): these measurements instruments (and many sondes) are are of total column, plus profiles now grouped in the international of N02 and 03 (using Umkehr• Network for the Detection of Dobson measurements). Stratospheric Change (NDSC).The NDSC observation sites cover a wide • spectroscopy of stars in the UV• range of latitudes and longitudes. visible and of the Sun in the UV Several instruments record data every (Dobson and Brewer net works): possible day. For those instruments these measure 03 to high that measure continuously, no accuracy. This is now the planning effort is required (except for internationally accepted standard. fast processing) for comparisons with GOMOS. The GOMOS mission • microwave spectro-radiorneters planning has only to consider their observing thermal emission from location for specific localised the atmosphere (NDSCnetwork) occultations. measuring 03: profiles are measured day and night, although NDSC holds frequent inter• witha coarse vertical resolution comparison campaigns, which compared to GOMOS. The lowest focus on particular gases or part of the stratosphere (below instrument types. Inter-comparison about 12 km) can usually not be with NDSC measurements observed. guarantees a comprehensive validation. because: • infrared spectrometers observing the Sun (NDSCnetwork), • NDSC instruments use techniques measuring 03 and N02 as well as and algorithms that are different many non-GOMOS gases: profiles from GOMOS, are now being measured with the same coarse vertical resolution as • Most NDSC instrument scientists microwave sensors. but with an are independent of GOMOS; a opposite, upper altitude limit of high, uniform level of data quality about 35 km, imposed by the is guaranteed through strict, width of the infrared lines network-wide quality control becoming Doppler- rather than procedures pressure-broadened. Both heterodyne and Fourier-transform 8.2.4 Analyses using models spectrometers are used. Clear skies and dry atmospheres are a) Temperature analyses needed. All GOMOS temperature retrievals will 102 be compared with the output of specified from the wind fields and the ECMWF,allowing the determination temperatures from meteorological of the bias, error and domains of analyses. The transport calculations excellence. ECMWF analyses include are decoupled from the chemical data from radiosondes and calculations. Trajectory models. in temperature-sounding satellites which transport and chemistry are assimilated into the forecast from the also decoupled, can also be used for previous analysis, so they are assimilation. inherently collocated. Assimilation could be a helpful tool By the time of Envisat, a more for comparing GOMOS with other complete assimilation research instruments. The simplest application product for numerical weather could be to use the assimilation for prediction (NWP)may be available, interpolating measurements in time which includes more stratospheric and space in order to match GOMOS levels. This might be a useful tool to with other instruments. More compare Envisat measurements with sophisticated applications could atmospheric models and NWP include the assimilation of analyses. observations from several different instruments, excluding GOMOS, in b) Interpolation using chemistry order to compare with individual data assimilation models GOMOS constituent profiles. Perhaps one of the most ambitious Data assimilation is a technique for applications would be to derive combining observations and species. which are not directly theoretical models. Model quantities measured, but retrieved from the are forced towards observations. model chemistry, driven by the without using forcing gradients (in assimilation of measured species time or space) that are unreasonable data. However. this would require according to the physics or chemistry thoroughly validated assimilation of the model. In principle, the forced tools and chemistry models. values are the weighted mean of the observations and the model 8.2.5 Activities of the Envisat predictions. The assimilation mode validation team requires the tuning of the weights according to the perceived errors of For the coordination of the Envisat measurement and model. The validation activities. validation teams procedure has been successfully used have been established with ESA in meteorology for more than a convenors. For the three chemistry decade. but chemical assimilation is instruments, GOMOS. MIPASand hardly beyond an experimental stage. SCIAMACHY,the Atmospheric Chemistry Validation Team (ACVT) In order to assimilate chemistry has been created. The teams will have measurements, a general circulation the following tasks: model (GCM)can be used for the longer-lived products (03, total • review and update the validation reactive nitrogen). but it becomes plan, more difficult to insert other gases with shorter photochemical lifetimes. • perform validation rehearsal. It eventually becomes imperative is to use a chemistry-transport model • generate calibration/validation (CTM),where the transport of gases is requirements, 103 • perform correlative measurements production of higher order GOMOS or provide services with products, such as stratospheric ozone atmospheric models (e.g. data maps, total ozone maps and assimilation), turbulence products. The use of data assimilation techniques is being • perform validation analysis, studied. • report results. As for any other new instrument, there is a clear need for in-flight A data storage facility for the calibration and validation. The validating instruments will be necessity, principles and methods of established at the Norsk Institutt for calibration and validation for GOMOS Luftforskning (NILU)in Norway. NILU have been explained. GOMOS is a will provide access to correlative unique instrument, hence measurements from sensors onboard investigation of its error sources is satellites, aircraft, balloons (and ships a task that cannot fully rely on for other Envisat instruments), as well routine methods, and will require a as from ground-based instruments coordinated effort by many (and underwater devices for other scientists and engineers. Since Envisat instruments) and numerical GOMOS measurements are models, such as that of ECMWF. sensitive to a variety of observation parameters (including stellar properties) it is important to 8.3 Discussion and acquire many coincident observations with independent instruments, in Conclusions order to arrive at an assessment of GOMOS end-to-end performance that GOMOS uses stellar occultation for is representative under most measurements of the stratospheric conditions. composition. This technique is new and therefore particular care must be Both calibration and validation must taken in the selection of the data continue beyond the commissioning retrieval techniques. Amongst the phase, since the statistical large variety of different retrieval significance of calibration and techniques successfully employed for validation results will increase with other satellite instruments, a method the number of measurements, and has been chosen that accounts in new systematic errors may develop particular for the limb-sounding during the instrument lifetime. The geometry utitilising basically the calibration and validation results will 'onion peeling' method. have direct consequences on the evolution of the data-processing In addition to the official Level 1b and algorithms used in the ground Level 2 GOMOS data product, segment. scientists are working on the 104 9. Concluding Remarks This report presents an overview of where also OClO and BrO are GOMOS, one of the three atmospheric possibly detectable. Air density will be composition sounding instruments derived from Rayleigh scattering and onboard Envisat. Envisat will be 02 measurements. Water vapour and Europe's largest Earth observation 02 will be measured in the near mission to date, to be launched in the infrared. Temperature profiles will be summer of 2001. retrieved from 02 density measurements and Rayleigh Envisat will fly in a polar orbit at a extinction and, with very high vertical mean altitude of about 800 km with a resolution, from the analysis of the repeat cycle of 35 days. The local scintillation signal measured with two equator crossing time in the descent fast photometers. mode will be 10 am. Flight operations will be controlled from ESOC, GOMOSwill exploit the stellar Darmstadt, Germany. ESRIN, occultation technique to measure line Frascati, Italy, will provide the densities and vertical profiles of ozone interface to the users. More and other atmospheric constituents information can be found in ESA with a vertical resolution of 1.7 km or 1998a and on the Envisat web page better and an accuracy for ozone The primary objective of GOMOS is Most of the time there is more than the monitoring of stratospheric and one observable star in the mesospheric ozone for trend instrument's field of view, which observations. GOMOSwill be able to necessitates a selection of the star detect linear ozone trends of the order sequences with the highest scientific of 0. 1% per year, which would value. A concept using merit contribute to the long-term functions and entropy has been monitoring of the ozone layer. The developed to find the best compromise instrument will measure ozone in the between geophysical product UV and visible frequency band at 250 accuracies and global coverage for to 675 nm (Huggins band and alternative star observation Chappuis band). As secondary sequences. Different mission objectives, GOMOSwill also observe objectives can be mathematically N02 and N03, important species described and the optimal within the nitrogen chemistry family. combination of occultation sequences N02, N03 and aerosol extinction will can be derived. This approach makes be observed in the UV-visible band, it easier to address the various 105 mission objectives, with minimal the climate and ozone problem in interference to the main objective. A particular. Human activities are considerable enhancement of the causing damage directly to the GOMOS mission will be possible environment and have significant using the optimising mission long-term impacts in particular planning. related to climate. These concerns are reflected in the list of priority issues Data processing mainly involves identified by the Inter-Governmental calculating the atmospheric Panel on Climate Change (IPCC) transmittances and geophysical which includes topics such as the parameters. The retrieved geophysical sources, sinks and concentrations of parameters are column densities greenhouse gases, the Earth's along the line-of-sight and vertical radiation balance, ecosystem profiles. Both the transmittance dynamics and the role of aerosols. spectra and the geophysical data products will be made available to the In partial response to this, the users. Member States of the European Space Agency have not only approved the The GOMOS data products will Envisat mission, but, in addition, undergo extensive validation activities have supported the formulation of a with other satellite, ground-based and long-term strategy for Earth air-borne instruments, also exploiting Observation, which envisages a series data assimilation techniques. of research and/ or demonstration missions called the Earth Explorer Looking further to the future it is Missions. 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Organization, Global Ozone Research and Monitoring Project - Report N°. 44, Geneva, 1998. 109 European Spa1e Agen1y Agen1e spatiale europeenne Contact: ESA Publications Division C/o ESTEC,PO Box 299, 2200 AGNoordwijk,The Netherlands Tel.(31) 71 565 3400 - Fax {31) 71 565 5433