SCIAMACHY IN-FLIGHT CALIBRATION

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1 SCIAMACHY IN-FLIGHT CALIBRATION Ralph Snel SRON Netherlands Institute for Space Research Sorbonnelaan 2, 3584 CA Utrecht, the Netherlands ABSTRACT The options for SCIAMACHY in-flight calibration were investigated. Using the sun, moon, and on-board white lamp as calibration sources, it was shown that the signals of these sources could be modelled as a function of time and illumination geometry with an accuracy of better than 1%, in some cases as good as 0.05%. This allows cross-calibration of the various observation modes (ESM diffuser, ASM diffuser, limb small aperture, sub-solar small aperture, moon, WLS) to a similar or even better level. With the resulting internally consistent relative SCIAMACHY calibration, and known external calibration sources, such as the moon albedo, it will be possible to derive significantly improved radiometric calibration key data. 1. INTRODUCTION SCIAMACHY is an imaging spectrograph described in detail in [1]. Before launch, the instrument was characterised under thermal vacuum conditions in order to be able to produce calibrated nadir and limb radiances, solar irradiances over one of the on-board diffusers, and sun-normalised reflectances. Due to practical limitations the instrument was not tested for all viewing geometries it would encounter in orbit, nor was it radiometrically calibrated for the self-calibrating observation modes using direct sun viewing. Now, after 5 years of in-orbit data have been collected and SCIAMACHY seems capable to continue observations for many years to come, it is possible to perform relative calibration of all predictable radiance and irradiance sources against each other, and thus obtain a fully internally consistent instrument calibration for any viewing geometry or observation mode. SCIAMACHY is a complex instrument, with many viewing modes (see [1] for a complete overview). In order to track instrument and component ageing, many of these observation modes are used daily, weekly or monthly for observations of the sun, the moon, and the internal white light source (WLS) and spectral line source (SLS) lamps. The time series of measurements of stable light sources (mainly the sun and the WLS) reveal that the instrument changes in a predictable way. With the sun as a light source, illumination geometry changes with orbital position and season, which results in very repeatable behaviour over the years. Superimposed upon the illumination geometry effects are the effects of ageing of the scan mirrors, diffusers, and remaining optical components, in particular the dichroic mirrors and the detectors. It turns out [2,3] that the instrument changes with time for the common light path which is the part after the elevation scan mechanism (ESM). Most notable effects are the ice layer on channels 7 and 8, detector damage due to high energy particles in channels 6+, 7 and 8, localised quantum efficiency changes in channel 2, and slow shifting of the wavelength response of the dichroic mirrors used for the channel separation between channels 3, 4, 5 and 6. In addition to these effects common to all measurements, there are signs that the mirrors and diffusers are suffering from contamination build-up, as also observed in GOME on ERS-2 [4]. Properly chosen ratios of light path monitoring measurements [3] indicate that the ESM mirror is degrading fastest, with reduced reflectivity in the UV. Also the azimuth scan mechanism (ASM) mirror is degrading, but at a slower rate. The ESM and ASM diffusers show minor signs of degradation. The illumination geometry effects combined with the observed degradation (which may be observation mode dependent) are responsible for the bulk of the signal variation observed over the 5 years of in-orbit calibration and monitoring data. Under the assumption that the sun can be regarded as a constant light source, it is possible to create a model which describes signal variation as a function of geometry angles and time. This paper describes some preliminary results obtained with the simple ad hoc models describing signal variability. It is shown that all time- and geometrydependent variations can be modelled to 1% or much better, and the implications for future in-flight radiometric calibration data are explored. 2. CALIBRATION DATA The SCIAMACHY calibration plan only foresees in one absolutely calibrated irradiance product, which is the sun over the ASM mirror and ESM diffuser. A second diffuser was added at a very late stage in the project, since it turned out that the ESM diffuser displayed spectral features which significantly change behaviour with changes in illumination geometry and wavelength. The addition of the diffuser on the back of the ASM has proven to be a good choice, since several DOAS Proc. Envisat Symposium 2007, Montreux, Switzerland April 2007 (ESA SP-636, July 2007)

2 products benefit from the reduced spectral features experienced by this diffuser. Unfortunately, there is currently no accurate absolute or even relative calibration of the ASM diffuser, so not all products can benefit from this diffuser. However, if the signals of the sun over ESM diffuser, sun over ASM diffuser, and the solar irradiance measured through the small aperture can each be modelled as function of wavelength, illumination geometry and time, then either one can be used to normalise the earth radiance spectra with On-ground calibration data The instrument calibration database contains the results of the extensive measurements performed before launch. The parameters of relevance to this investigation are the bi-directional scattering distribution function (BSDF) of the ESM diffuser, and the illumination geometry dependence of the BSDF. The illumination geometry is determined for 3 solar elevation angles and 4 solar azimuth angles, while the BSDF is measured at 32 wavelength bands between 260 and 2500 nm, all for two perpendicular linear polarisation directions. Typically, at any given wavelength or solar azimuth angle, the BSDF varies by about 30% from solar elevation angles of 19.5 to 25 degrees. In the data-processor the BSDF values at the discrete grid of solar azimuth, solar elevation and wavelength, is interpolated to the actual in-flight geometry angles and wavelengths, and is then used to correct the measured sun-over-diffuser signal In-flight calibration data Currently, data analysis is done using the on-ground radiometric calibration data, which were obtained on a limited grid of wavelengths and viewing geometry angles. With the many years of in-orbit data available, it is possible to use these in-flight measurements to derive relative radiometric calibration data which can complement the on-ground calibration. The typical in-flight geometries for the sun over ESM diffuser observations cover solar elevation angles of 20.9 to 22.5 degrees. Over this approximate elevation range, 240 equally spaced read-outs are performed for each detector pixel wavelength, for each day. Thus, a large number of observations at each wavelength is available, and any predictable behaviour can in principle be 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 small aperture (thus eliminating the need for a diffuser) on an orbital to monthly time interval. The direct sun measurements need the small aperture to provide the reduction in light intensity by four orders of magnitude in order not to saturate the detectors. Due to the resulting low intensity it was not feasible to perform on-ground characterisation of this observation mode to the accuracy required. In addition to the change in radiometric response, the slit function of the instrument is affected as well by the small aperture. However, with the abundant measurements available since launch, it is possible to accurately determine the ratio of the small aperture versus the diffuser mode observations, which in principle allows for normalisation of the earth radiance spectra with any of the sun over diffuser or direct sun through small aperture observations. 3. IN-FLIGHT INSTRUMENT BEHAVIOUR Since SCIAMACHY has reached the final configuration and all state definitions are optimised, instrument response to a stable source is predictable. The WLS is always observed at the same geometry, so any observed change in signal is either the result of instrument change or of lamp change. It is assumed for now that the lamp is stable. The observed signal change for each detector pixel can be well described with a smooth function of time, describing instrument degradation, and smaller abrupt changes related to detector temperature changes. For the remainder of this paper detector temperature changes are ignored, which means that any temperature effects on the signal will end up in the scatter of the signal. For observations of the sun, the geometry of the sun viewing may affect the recorded signal in addition to the instrument changes identified above. First of all, the annual variation of the sun-earth distance modulates the signal with about 10% peak to peak. This effect is corrected for using the known distance between the sun and the earth. For direct viewing of the sun, the pointing to the sun by the instrument is of great importance. Depending on the observation mode (pointing to sun centre, scanning over the sun, or performing a fast sweep over the sun) different signals are observed. Direct sun measurements are performed in both subsolar and limb geometry. For sub-solar geometry, the sun moves through the field of view as a result of the orbital motion, while the ESM can be used to track the sun in the perpendicular direction, and perform a scan or fast sweep over the solar image. This introduces a possible ESM angle dependence in addition to the time dependence of the observed signals. For limb geometry, the sun is imaged over the ASM mirror as well, which gives full control over the pointing of the instrument instantaneous field of view relative to the sun. The limb mode thus introduces additional ASM angle dependence to the signal. For both limb and sub-solar observation modes through the small aperture the viewing direction (determined by ASM and ESM angles and timing) must closely match the solar azimuth and elevation angles. Failing to do so will result in no significant signal being recorded by the detectors.

3 Figure 1. Fit residuals (root mean square) of the sun over ESM diffuser signal, as a function of wavelength. Figure 2. Solar zenith angle dependence of the sun over ESM diffuser signals, for 3 wavelengths: 250 nm (top), 500 nm (middle) and 1000 nm (bottom). The signals are normalised to unity at the average SZA, and offset for clarity. When using the diffuser modes this implies limb geometry for the illumination. One of the mirrors is replaced by a diffuser, which is mounted on the back side of the mirror. The angle of incidence on the diffuser combined with the direction of the diffuser normal will dominate the recorded signal. In contrast to the direct observation modes of the sun, the diffuser modes will return a significant signal for most illumination and viewing geometries, with maximum signals expected near specular reflection of off the diffuser. In practice, this means that in addition to time-, ASM angle, and ESM angle dependence, the recorded signal depends on the solar azimuth and elevation angles. In the sections below, the signals are described with a model which fits a polynomial function to the time-, solar azimuth, and solar elevation angle behaviour. The overall fit residual gives an indication of the stability of the signal and the goodness of the fit. Using the best fit polynomial parameters, the signal for all but one dimension are corrected, revealing the isolated behaviour of the signal as a function of the dimension in question (time, solar azimuth angle, etc). 3.1 Sun ESM diffuser Initial investigations of the sun over the ASM diffuser and polynomial functions of solar zenith angle, ASM angle, time, and season, show that the residual signal is stable to a level of up to 0.05 % of the assumed constant signal. For wavelengths with lower signal to noise ratio, known solar variability, or high detector temperature sensitivity, this stability is worse (Fig. 1). The light path used in these measurements is sun - ASM mirror - ESM diffuser - large aperture - Optical Bench Module (OBM) The daily solar observation mode is with the ESM diffuser. Signal variation is dominated by solar zenith angle effects (typically 3% RMS), and time effects (0.3% RMS at 500 nm). Fit residuals to the polynomial model are typically 0.06% RMS in the wavelength range nm. For more details see Tab. 1. The diffuser spectral features show up very clearly as a function of wavelength and solar zenith angle (Fig. 2 and 3), and are extremely repeatable over time. The typical amplitude of the spectral features is 0.5%, but it varies with wavelength. The final daily ESM solar reference spectrum is an average of over 200 individual spectra, each with slightly different spectral features, which should average the total spectral features to a level below the number mentioned above. Figure 3. ESM diffuser spectral features as function of pixel number for channel 3 (horizontally) and SZA (vertically). The average intensity change has been removed. ASM diffuser When using the ASM diffuser for monitoring the sun, the signal variation is dominated by two effects: orbital solar zenith angle dependence, and seasonal solar azimuth dependence, each responsible for about 20% variability RMS (Fig. 4 and 5). One complicating matter is that, in order to achieve optimal signal levels, the azimuth scan angle has to be varied, which is done in 5 discrete steps which are chosen depending on the solar azimuth angle. During each measurement, the ASM

4 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% % 0.9% 0.8% Moon 1% angle is scanned through 14 degrees in order to reduce the effect of the diffuser spectral features. The approach to describe signal dependencies as polynomials in time, solar azimuth and zenith angles results in fit residuals of around 0.5%. Split up per state ID the residuals reduce to about half that value. The light path used in these measurements is: sun - ASM diffuser - ESM mirror - large aperture OBM. Small aperture limb The small aperture (SA) is inserted into the light path to reduce the signal when observing the sun directly. The optical path is the same as for the limb radiance observations, (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 short wavelengths by throughput degradation (17 % RMS), and the remaining variability is around 0.3% each for solar elevation and azimuth angle dependence. Small aperture sub-solar The sub-solar configuration uses only the ESM scan mirror to observe the sun, through the optical path: sun ESM mirror SA OBM. Signal variability is once again largest for the shortest wavelengths due to throughput degradation (12%), with some remaining variability with solar elevation angle (approximately 1%). 3.2 White Light Source The WLS can be modelled to an accuracy of 0.09% with only time dependence being described with a polynomial. For 250 nm, the variability of the signal is 18%. The light path followed is: WLS ESM mirror large aperture OBM. The observed degradation of the UV signal is marginally larger than the equivalent degradation as seen in similar geometry using the sun, and degradation of the UV output of the lamp itself can not be excluded. Figure 4. Solar zenith angle dependence of sun over ASM diffuser signal. Figure 5. Solar azimuth angle dependence of sun over ASM diffuser signal.

5 Figure 6. Spectral features in the ASM (blue) and ESM (black) diffusers, relative to the moon albedo. 3.3 Moon The moon irradiance varies with the phase of the moon, and needs to be modelled with sufficient accuracy before any comparison with the observed signal can be performed. The ROLO project [5] observed the moon with a dedicated ground-based observatory over a long enough time period to set up a parametric model of the moon capable of predicting the absolute lunar irradiance (integrated over the entire lunar disc) and albedo for any point in time and in 32 wavelength bands between 350 and 2400 nm, with an accuracy of a few percent. The ROLO model was used to calculate the lunar irradiances and albedos for the times at which the moon was observed by SCIAMACHY. Over a period of several months dedicated measurements were performed which observed the moon over a large and dense spread in the total clear field of view for limb geometry. For these several hundred orbits and many thousands of spectra, the deviation between the measurements and the model, taking into account the on-ground key data describing ASM and ESM angle dependence, was typically 1%. Some systematic differences remain, which may be explained with the differences between the geocentric geometry used to calculate the lunar phase and libration, and the Envisat-centric geometry which should have been used. Since the observations under investigation only covered a few months, degradation effects were not investigated. 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 path from the moon onward. The lunar albedo is modelled for only 32 wavelength bands, but it is known that any spectral features in the lunar albedo are at least several hundred nm wide and that the albedo is spectrally very smooth. Using an apollo moon soil laboratory reflectance spectrum as a reference, the observed SCIAMACHY lunar albedo was Figure 7. Instrumental polarisation features observed in the moon light (black curve) and scaled on-ground polarisation measurements with known instrumental polarisation features (blue curve). compared with the scaled moon soil spectrum, for both ASM and ESM diffuser based measured moon albedos. Fig. 6 shows the differences between the scaled moon soil spectrum and the ESM diffuser based albedo (black points) and for the ASM diffuser (blue points). Systematic differences are expected around 800 to 1000 nm, where the moon albedo shows spectral features which may differ from those of the moon soil sample, and below 300 nm where the soil sample spectrum was not measured. Additionally, spectral features due to instrument degradation, in particular in channels 1 and 2, below 400 nm, are likely to show up in this ratio. Other than that, the main spectral features expected in the ratio are due to the characterisation of the neutral density filter used for the ESM diffuser measurements, residual polarisation features, and the intrinsic spectral features of the ESM and ASM diffusers. The latter ones are expected to be particularly strong in channel 6, between 1100 and 1600 nm. The figure clearly shows distinctive differences between the ESM and ASM diffusers, with the spectral features of the ASM diffuser being smaller as expected from white light interference theory. The moon observations are of such quality that the ratio of two spectra with different elevation angles reveal instrumental polarisation features as a result of the difference in degree of polarisation of the reflected light off the ESM mirror at the different elevation angles. Fig. 7 shows such a ratio (black points), and a scaled onground polarisation response (eta key data) with known instrumental polarisation features as a reference (blue). It is clear from the figure that the majority of the spectral structures in the channels 3 to 5 shown here are the result of polarisation properties of the instrument. This allows the moon to be used as a source for in-flight verification of the polarisation response of the instrument.

6 Figure 8. Ratio of the throughput of the small aperture and the ESM diffuser. 4. ON-GROUND VS IN-FLIGHT CALIBRATION On-ground calibration is essential to characterise those instrument parameters which will influence the measurements in flight, but which can not readily be determined from such measurements. On-ground calibration is also essential in order to provide an instrument description valid from the first day in orbit. Due to practical limitations, probably all on-ground calibration results will lack to some degree some subtle yet important features which will show up clearly during regular in-orbit observations. If the means exist, it should be considered to complement or even replace on-ground calibration results with better ones determined from in-flight measurements. SCIAMACHY has known issues with the radiometric calibration, not the least being that the ASM diffuser which is known to be better from a spectral feature point of view, is not radiometrically calibrated. It is shown in this paper that many of the solar observation modes (including the sun using the moon as a diffuser) can be modelled with good accuracy. This allows an internally consistent calibration of all observation modes against each other, which only leaves one wavelength dependent external calibration factor needed to bring the internal SCIAMACHY calibration to the same scale as the rest of the world. A tentative example of such an internal calibration is the ratio of the small aperture and the ESM diffuser: during on-ground calibration only the ESM diffuser was wellcharacterised, while the SA was never radiometrically calibrated since the data obtained through it would only be used in a relative, self-calibrating way. Nevertheless, should it for some reason be desirable to use the small aperture solar spectrum instead of the ESM diffuser spectrum, the correction factor would resemble that as shown in Fig. 8. The on-board ESM diffuser was calibrated on-ground, but the comparison with the moon has shown that inflight calibration is possible under certain conditions, the most important ones being visibility of the moon by the instrument, and known properties of the moon. In particular the predictability of the moon albedo over time makes the moon an extremely valuable calibration source: using the moon it is possible to compare satellites over time, even different satellites which have never been in orbit simultaneously. Also the fact that the moon radiance, or integrated over the lunar disc, the irradiance, is very similar in spectral behaviour and intensity level to the earth radiances, makes the moon an even better calibration source for earth observation satellites with high demands on radiometric calibration quality. 5. DISCUSSION AND CONCLUSIONS It was shown that the solar, lunar and WLS observation modes can be modelled to a level of 1% or better with simple polynomial functions of time, solar azimuth angle, and solar elevation angle. The fact that the simple model already provides such a good description is a guarantee that more sophisticated models will do at least as good, and that the instrument is predictable to such a level. With such accurate descriptions of the signals, it is possible to determine instrument properties to at least the same accuracy, derived from the ratios of such signals or predictions. Any multiplicative factors common to both signals will cancel in the ratio, like the solar spectrum or OBM degradation. This opens the door for novel in-flight calibration products, such as a radiometrically calibrated ASM diffuser, or even a radiometric calibration of the WLS against the sun. In the near future, the investigations presented here will be extended and lead to improved radiometric calibration key data for SCIAMACHY, and likely to completely new calibration products, such as an absolutely 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