ABSTRACT

The options for SCIAMACHY in-flight calibration wereinvestigated. Using the sun, moon, and on-board whitelamp as calibration sources, it was shown that thesignals of these sources could be modelled as a functionof time and illumination geometry with an accuracy ofbetter than 1%, in some cases as good as 0.05%. Thisallows cross-calibration of the various observationmodes (ESM diffuser, ASM diffuser, limb smallaperture, sub-solar small aperture, moon, WLS) to asimilar or even better level. With the resulting internallyconsistent relative SCIAMACHY calibration, andknown external calibration sources, such as the moonalbedo, it will be possible to derive significantlyimproved radiometric calibration key data.

1. INTRODUCTION

SCIAMACHY is an imaging spectrograph described indetail in [1]. Before launch, the instrument wascharacterised under thermal vacuum conditions in orderto be able to produce calibrated nadir and limbradiances, solar irradiances over one of the on-boarddiffusers, and sun-normalised reflectances. Due topractical limitations the instrument was not tested for allviewing geometries it would encounter in orbit, nor wasit radiometrically calibrated for the self-calibratingobservation modes using direct sun viewing.

Now, after 5 years of in-orbit data have been collectedand SCIAMACHY seems capable to continueobservations for many years to come, it is possible toperform relative calibration of all predictable radianceand irradiance sources against each other, and thusobtain a fully internally consistent instrumentcalibration for any viewing geometry or observationmode.

SCIAMACHY is a complex instrument, with manyviewing modes (see [1] for a complete overview). Inorder to track instrument and component ageing, manyof these observation modes are used daily, weekly ormonthly for observations of the sun, the moon, and theinternal white light source (WLS) and spectral linesource (SLS) lamps. The time series of measurements ofstable light sources (mainly the sun and the WLS) revealthat the instrument changes in a predictable way. Withthe sun as a light source, illumination geometry changeswith orbital position and season, which results in veryrepeatable behaviour over the years. Superimposed

upon the illumination geometry effects are the effects ofageing of the scan mirrors, diffusers, and remainingoptical components, in particular the dichroic mirrorsand the detectors.

It turns out [2,3] that the instrument changes with timefor the common light path which is the part after theelevation scan mechanism (ESM). Most notable effectsare the ice layer on channels 7 and 8, detector damagedue to high energy particles in channels 6+, 7 and 8,localised quantum efficiency changes in channel 2, andslow shifting of the wavelength response of the dichroicmirrors used for the channel separation betweenchannels 3, 4, 5 and 6. In addition to these effectscommon to all measurements, there are signs that themirrors and diffusers are suffering from contaminationbuild-up, as also observed in GOME on ERS-2 [4].Properly chosen ratios of light path monitoringmeasurements [3] indicate that the ESM mirror isdegrading fastest, with reduced reflectivity in the UV.Also the azimuth scan mechanism (ASM) mirror isdegrading, but at a slower rate. The ESM and ASMdiffusers show minor signs of degradation.

The illumination geometry effects combined with theobserved degradation (which may be observation modedependent) are responsible for the bulk of the signalvariation observed over the 5 years of in-orbitcalibration and monitoring data. Under the assumptionthat the sun can be regarded as a constant light source, itis possible to create a model which describes signalvariation as a function of geometry angles and time.

This paper describes some preliminary results obtainedwith the simple ad hoc models describing signalvariability. It is shown that all time- and geometry-dependent variations can be modelled to 1% or muchbetter, and the implications for future in-flightradiometric calibration data are explored.

2. CALIBRATION DATA

The SCIAMACHY calibration plan only foresees in oneabsolutely calibrated irradiance product, which is thesun over the ASM mirror and ESM diffuser. A seconddiffuser was added at a very late stage in the project,since it turned out that the ESM diffuser displayedspectral features which significantly change behaviourwith changes in illumination geometry and wavelength.The addition of the diffuser on the back of the ASM hasproven to be a good choice, since several DOAS

Ralph Snel

SRON Netherlands Institute for Space ResearchSorbonnelaan 2, 3584 CA Utrecht, the Netherlands

Email: R.Snel@sron.nl

SCIAMACHY IN-FLIGHT CALIBRATION

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Proc. ‘Envisat Symposium 2007’, Montreux, Switzerland 23–27 April 2007 (ESA SP-636, July 2007)

products benefit from the reduced spectral featuresexperienced by this diffuser. Unfortunately, there iscurrently no accurate absolute or even relativecalibration of the ASM diffuser, so not all products canbenefit from this diffuser. However, if the signals of thesun over ESM diffuser, sun over ASM diffuser, and thesolar irradiance measured through the small aperturecan each be modelled as function of wavelength,illumination geometry and time, then either one can beused to normalise the earth radiance spectra with.

2.1. On-ground calibration data

The instrument calibration database contains the resultsof the extensive measurements performed beforelaunch. The parameters of relevance to this investigationare the bi-directional scattering distribution function(BSDF) of the ESM diffuser, and the illuminationgeometry dependence of the BSDF. The illuminationgeometry is determined for 3 solar elevation angles and4 solar azimuth angles, while the BSDF is measured at32 wavelength bands between 260 and 2500 nm, all fortwo perpendicular linear polarisation directions.Typically, at any given wavelength or solar azimuthangle, the BSDF varies by about 30% from solarelevation angles of 19.5 to 25 degrees.

In the data-processor the BSDF values at the discretegrid of solar azimuth, solar elevation and wavelength, isinterpolated to the actual in-flight geometry angles andwavelengths, and is then used to correct the measuredsun-over-diffuser signal.

2.2. In-flight calibration data

Currently, data analysis is done using the on-groundradiometric calibration data, which were obtained on alimited grid of wavelengths and viewing geometryangles. With the many years of in-orbit data available, itis possible to use these in-flight measurements to deriverelative radiometric calibration data which cancomplement the on-ground calibration.

The typical in-flight geometries for the sun over ESMdiffuser observations cover solar elevation angles of20.9 to 22.5 degrees. Over this approximate elevationrange, 240 equally spaced read-outs are performed foreach detector pixel wavelength, for each day. Thus, alarge number of observations at each wavelength isavailable, and any predictable behaviour can in principlebe modelled.

In addition to the sun over ESM diffuser measurements,there are daily sun over ASM diffuser measurements,and direct measurements of the sun through the smallaperture (thus eliminating the need for a diffuser) on anorbital to monthly time interval.

The direct sun measurements need the small aperture toprovide the reduction in light intensity by four orders of

magnitude in order not to saturate the detectors. Due tothe resulting low intensity it was not feasible to performon-ground characterisation of this observation mode tothe accuracy required. In addition to the change inradiometric response, the slit function of the instrumentis affected as well by the small aperture. However, withthe abundant measurements available since launch, it ispossible to accurately determine the ratio of the smallaperture versus the diffuser mode observations, which inprinciple allows for normalisation of the earth radiancespectra with any of the sun over diffuser or direct sunthrough small aperture observations.

3. IN-FLIGHT INSTRUMENT BEHAVIOUR

Since SCIAMACHY has reached the final configurationand all state definitions are optimised, instrumentresponse to a stable source is predictable. The WLS isalways observed at the same geometry, so any observedchange in signal is either the result of instrument changeor of lamp change. It is assumed for now that the lampis stable. The observed signal change for each detectorpixel can be well described with a smooth function oftime, describing instrument degradation, and smallerabrupt changes related to detector temperature changes.For the remainder of this paper detector temperaturechanges are ignored, which means that any temperatureeffects on the signal will end up in the scatter of thesignal.

For observations of the sun, the geometry of the sunviewing may affect the recorded signal in addition to theinstrument changes identified above. First of all, theannual variation of the sun-earth distance modulates thesignal with about 10% peak to peak. This effect iscorrected for using the known distance between the sunand the earth. For direct viewing of the sun, the pointingto the sun by the instrument is of great importance.Depending on the observation mode (pointing to suncentre, scanning over the sun, or performing a fastsweep over the sun) different signals are observed.

Direct sun measurements are performed in both sub-solar and limb geometry. For sub-solar geometry, thesun moves through the field of view as a result of theorbital motion, while the ESM can be used to track thesun in the perpendicular direction, and perform a scan orfast sweep over the solar image. This introduces apossible ESM angle dependence in addition to the timedependence of the observed signals. For limb geometry,the sun is imaged over the ASM mirror as well, whichgives full control over the pointing of the instrumentinstantaneous field of view relative to the sun. The limbmode thus introduces additional ASM angle dependenceto the signal. For both limb and sub-solar observationmodes through the small aperture the viewing direction(determined by ASM and ESM angles and timing) mustclosely match the solar azimuth and elevation angles.Failing to do so will result in no significant signal beingrecorded by the detectors.

When using the diffuser modes this implies limbgeometry for the illumination. One of the mirrors isreplaced by a diffuser, which is mounted on the backside of the mirror. The angle of incidence on the diffusercombined with the direction of the diffuser normal willdominate the recorded signal. In contrast to the directobservation modes of the sun, the diffuser modes willreturn a significant signal for most illumination andviewing geometries, with maximum signals expectednear specular reflection of off the diffuser. In practice,this means that in addition to time-, ASM angle, andESM angle dependence, the recorded signal depends onthe solar azimuth and elevation angles.

In the sections below, the signals are described with amodel which fits a polynomial function to the time-,solar azimuth, and solar elevation angle behaviour. Theoverall fit residual gives an indication of the stability ofthe signal and the goodness of the fit. Using the best fitpolynomial parameters, the signal for all but onedimension are corrected, revealing the isolatedbehaviour of the signal as a function of the dimension inquestion (time, solar azimuth angle, etc).

3.1 SunESM diffuserInitial investigations of the sun over the ASM diffuserand polynomial functions of solar zenith angle, ASMangle, time, and season, show that the residual signal isstable to a level of up to 0.05 % of the assumed constantsignal. For wavelengths with lower signal to noise ratio,known solar variability, or high detector temperaturesensitivity, this stability is worse (Fig. 1).

The light path used in these measurements is sun - ASMmirror - ESM diffuser - large aperture - Optical BenchModule (OBM)

The daily solar observation mode is with the ESMdiffuser. Signal variation is dominated by solar zenithangle effects (typically 3% RMS), and time effects(0.3% RMS at 500 nm). Fit residuals to the polynomialmodel are typically 0.06% RMS in the wavelengthrange 400-800 nm. For more details see Tab. 1.

The diffuser spectral features show up very clearly as afunction of wavelength and solar zenith angle (Fig. 2and 3), and are extremely repeatable over time. Thetypical amplitude of the spectral features is 0.5%, but itvaries with wavelength. The final daily ESM solarreference spectrum is an average of over 200 individualspectra, each with slightly different spectral features,which should average the total spectral features to alevel below the number mentioned above.

Figure 3. ESM diffuser spectral features as function ofpixel number for channel 3 (horizontally) and SZA(vertically). The average intensity change has beenremoved.

ASM diffuserWhen using the ASM diffuser for monitoring the sun,the signal variation is dominated by two effects: orbitalsolar zenith angle dependence, and seasonal solarazimuth dependence, each responsible for about 20%variability RMS (Fig. 4 and 5). One complicating matteris that, in order to achieve optimal signal levels, theazimuth scan angle has to be varied, which is done in 5discrete steps which are chosen depending on the solarazimuth angle. During each measurement, the ASM

Figure 1. Fit residuals (root mean square) of the sunover ESM diffuser signal, as a function of wavelength.

Figure 2. Solar zenith angle dependence of the sun overESM diffuser signals, for 3 wavelengths: 250 nm (top),500 nm (middle) and 1000 nm (bottom). The signals arenormalised to unity at the average SZA, and offset forclarity.

angle is scanned through 14 degrees in order to reducethe effect of the diffuser spectral features. The approachto describe signal dependencies as polynomials in time,solar azimuth and zenith angles results in fit residuals ofaround 0.5%. Split up per state ID the residuals reduceto about half that value.

The light path used in these measurements is: sun -ASM diffuser - ESM mirror - large aperture – OBM.

Small aperture limbThe small aperture (SA) is inserted into the light path toreduce the signal when observing the sun directly. Theoptical path is the same as for the limb radianceobservations, (with the exception of the large aperture):sun – ASM mirror – ESM mirror – SA – OBM.

The sun through small aperture in limb is quite stable,with signal variability being dominated at the shortwavelengths by throughput degradation (17 % RMS),and the remaining variability is around 0.3% each forsolar elevation and azimuth angle dependence.

Small aperture sub-solarThe sub-solar configuration uses only the ESM scanmirror to observe the sun, through the optical path: sun– ESM mirror – SA – OBM. Signal variability is onceagain largest for the shortest wavelengths due tothroughput degradation (12%), with some remainingvariability with solar elevation angle (approximately1%).

3.2 White Light Source

The WLS can be modelled to an accuracy of 0.09% withonly time dependence being described with apolynomial. For 250 nm, the variability of the signal is18%. The light path followed is: WLS – ESM mirror –large aperture – OBM. The observed degradation of theUV signal is marginally larger than the equivalentdegradation as seen in similar geometry using the sun,and degradation of the UV output of the lamp itself cannot be excluded.

Table 1. Accuracy with which the data could be modelled, and contributions of the individual components.

Stability with illumination geometry: Stability over time at:

Overall Solar elevation Solar azimuth 250 nm 500 nm 1000 nm

ESM diffuser 0.05% 3.0% 0.05% 3.7% 0.35% 0.5%

ASM diffuser 0.45% 25% 19% 13% 1.5% 0.8%

SA Subsolar 0.07% ~1% - 12% 0.7% 0.8%

SA Limb 0.35% 0.3% 0.3% 17% 1.2% 0.8%

WLS 0.09% - - 18% 0.9% 0.8%

Moon 1%

Figure 4. Solar zenith angle dependence of sun overASM diffuser signal.

Figure 5. Solar azimuth angle dependence of sun overASM diffuser signal.

3.3 Moon

The moon irradiance varies with the phase of the moon,and needs to be modelled with sufficient accuracybefore any comparison with the observed signal can beperformed. The ROLO project [5] observed the moonwith a dedicated ground-based observatory over a longenough time period to set up a parametric model of themoon capable of predicting the absolute lunar irradiance(integrated over the entire lunar disc) and albedo for anypoint in time and in 32 wavelength bands between 350and 2400 nm, with an accuracy of a few percent. TheROLO model was used to calculate the lunar irradiancesand albedos for the times at which the moon wasobserved by SCIAMACHY. Over a period of severalmonths dedicated measurements were performed whichobserved the moon over a large and dense spread in thetotal clear field of view for limb geometry. For theseseveral hundred orbits and many thousands of spectra,the deviation between the measurements and the model,taking into account the on-ground key data describingASM and ESM angle dependence, was typically 1%.Some systematic differences remain, which may beexplained with the differences between the geocentricgeometry used to calculate the lunar phase and libration,and the Envisat-centric geometry which should havebeen used. Since the observations under investigationonly covered a few months, degradation effects were notinvestigated.

The light path used for the lunar observations is: sun –moon – ASM mirror – ESM mirror – large aperture –OBM. This is identical to the limb radiance light pathfrom the moon onward.

The lunar albedo is modelled for only 32 wavelengthbands, but it is known that any spectral features in thelunar albedo are at least several hundred nm wide andthat the albedo is spectrally very smooth. Using anapollo moon soil laboratory reflectance spectrum as areference, the observed SCIAMACHY lunar albedo was

compared with the scaled moon soil spectrum, for bothASM and ESM diffuser based measured moon albedos.Fig. 6 shows the differences between the scaled moonsoil spectrum and the ESM diffuser based albedo (blackpoints) and for the ASM diffuser (blue points).Systematic differences are expected around 800 to 1000nm, where the moon albedo shows spectral featureswhich may differ from those of the moon soil sample,and below 300 nm where the soil sample spectrum wasnot measured. Additionally, spectral features due toinstrument degradation, in particular in channels 1 and2, below 400 nm, are likely to show up in this ratio.Other than that, the main spectral features expected inthe ratio are due to the characterisation of the neutraldensity filter used for the ESM diffuser measurements,residual polarisation features, and the intrinsic spectralfeatures of the ESM and ASM diffusers. The latter onesare expected to be particularly strong in channel 6,between 1100 and 1600 nm. The figure clearly showsdistinctive differences between the ESM and ASMdiffusers, with the spectral features of the ASM diffuserbeing smaller as expected from white light interferencetheory.

The moon observations are of such quality that the ratioof two spectra with different elevation angles revealinstrumental polarisation features as a result of thedifference in degree of polarisation of the reflected lightoff the ESM mirror at the different elevation angles. Fig.7 shows such a ratio (black points), and a scaled on-ground polarisation response (eta key data) with knowninstrumental polarisation features as a reference (blue).It is clear from the figure that the majority of thespectral structures in the channels 3 to 5 shown here arethe result of polarisation properties of the instrument.This allows the moon to be used as a source for in-flightverification of the polarisation response of theinstrument.

Figure 6. Spectral features in the ASM (blue) and ESM(black) diffusers, relative to the moon albedo.

Figure 7. Instrumental polarisation features observed inthe moon light (black curve) and scaled on-groundpolarisation measurements with known instrumentalpolarisation features (blue curve).

4. ON-GROUND VS IN-FLIGHT CALIBRATION

On-ground calibration is essential to characterise thoseinstrument parameters which will influence themeasurements in flight, but which can not readily bedetermined from such measurements. On-groundcalibration is also essential in order to provide aninstrument description valid from the first day in orbit.Due to practical limitations, probably all on-groundcalibration results will lack to some degree some subtleyet important features which will show up clearlyduring regular in-orbit observations. If the means exist,it should be considered to complement or even replaceon-ground calibration results with better onesdetermined from in-flight measurements.

SCIAMACHY has known issues with the radiometriccalibration, not the least being that the ASM diffuserwhich is known to be better from a spectral feature pointof view, is not radiometrically calibrated. It is shown inthis paper that many of the solar observation modes(including the sun using the moon as a diffuser) can bemodelled with good accuracy. This allows an internallyconsistent calibration of all observation modes againsteach other, which only leaves one wavelengthdependent external calibration factor needed to bring theinternal SCIAMACHY calibration to the same scale asthe rest of the world.

A tentative example of such an internal calibration is theratio of the small aperture and the ESM diffuser: duringon-ground calibration only the ESM diffuser was well-characterised, while the SA was never radiometricallycalibrated since the data obtained through it would onlybe used in a relative, self-calibrating way. Nevertheless,should it for some reason be desirable to use the smallaperture solar spectrum instead of the ESM diffuserspectrum, the correction factor would resemble that asshown in Fig. 8.

The on-board ESM diffuser was calibrated on-ground,

but the comparison with the moon has shown that in-flight calibration is possible under certain conditions,the most important ones being visibility of the moon bythe instrument, and known properties of the moon. Inparticular the predictability of the moon albedo overtime makes the moon an extremely valuable calibrationsource: using the moon it is possible to comparesatellites over time, even different satellites which havenever been in orbit simultaneously. Also the fact that themoon radiance, or integrated over the lunar disc, theirradiance, is very similar in spectral behaviour andintensity level to the earth radiances, makes the moon aneven better calibration source for earth observationsatellites with high demands on radiometric calibrationquality.

5. DISCUSSION AND CONCLUSIONS

It was shown that the solar, lunar and WLS observationmodes can be modelled to a level of 1% or better withsimple polynomial functions of time, solar azimuthangle, and solar elevation angle. The fact that the simplemodel already provides such a good description is aguarantee that more sophisticated models will do at leastas good, and that the instrument is predictable to such alevel. With such accurate descriptions of the signals, itis possible to determine instrument properties to at leastthe same accuracy, derived from the ratios of suchsignals or predictions. Any multiplicative factorscommon to both signals will cancel in the ratio, like thesolar spectrum or OBM degradation. This opens thedoor for novel in-flight calibration products, such as aradiometrically calibrated ASM diffuser, or even aradiometric calibration of the WLS against the sun.

In the near future, the investigations presented here willbe extended and lead to improved radiometriccalibration key data for SCIAMACHY, and likely tocompletely new calibration products, such as anabsolutely calibrated ASM diffuser.

References

1 Gottwald, M. (editor), (2006), SCIAMACHY - Monitoring the changing Earth's Atmosphere, DLR,Institut für Metodik der Fernerkundung (IMF).

2 Noël, S., Bramstedt, K., Bovensmann, H., Burrows, J.P., Gottwald, M., Krieg, E., (2007), SCIAMACHY degradation monitoring results, ENVISAT symposium 2007, Montreux.

3 SCIAMACHY Science Advisory Group – Calibration Subgroup, (2006), private communication.

4 Snel, R., (2000), In Orbit optical path degradation: GOME experience and SCHIAMACHY prediction, ERS-ENVISAT Symposium 2000, Gothenburg.

5 Kieffer, H.H., Stone, T.C., (2005), The Spectral Irradiance of the Moon, Astronom. J. 129,, pp 2887-2901.

Figure 8. Ratio of the throughput of the small apertureand the ESM diffuser.