IN-FLIGHT CALIBRATION OF GOME-2 LEVEL-1 DATA USING THE OZONE MONITORING INSTRUMENT

Marcel Dobber(1), Piet Stammes(1), Pieternel Levelt(1) , Quintus Kleipool(1) , Robert Voors(1)

(1)Royal Netherlands Meteorological Institute, PO Box 201, 3730 AE De Bilt, The Netherlands

Email: dobber @ knmi.nl

ABSTRACT We propose to use Ozone Monitoring Instrument (OMI) level-1 measurement data, calibration experience and calibration algorithms to verify and validate the calibration and the accuracy of the GOME-2 in-flight measurement data and level-1 radiance and irradiance data products. Areas of attention include the radiometric calibration (radiance and irradiance, including the irradiance goniometry calibration), the spectral calibration, the spectral stray light calibration, especially below 300 nm, line of sight and field of view calibrations of GOME-2. This paper briefly describes the status of the OMI calibration as well as the algorithms and methods that can be employed to verify and validate the accuracy of the GOME-2 in-flight calibration.

1. INTRODUCTION The Ozone Monitoring Instrument (OMI) was launched on board the EOS-Aura satellite on 15 July 2004. The primary objective of OMI is to obtain global measurements at high spatial and spectral resolution of a number of trace gases in both the troposphere and stratosphere. Using these measurements, science questions on the recovery of the ozone layer, the depletion of ozone at the poles, tropospheric air pollution and climate change will be addressed. In order to meet the science objectives, measurements are needed that combine both a good spatial resolution of 13×24 km2 and daily global coverage. This is realized by implementation of a unique optical design of the telescope system and the use of CCD detectors, which enables an instantaneous field of view of 115 degrees, corresponding to a 2600 km broad swath on the Earth’s surface, while at the same time the desired spatial resolution is obtained. This spatial resolution is required to optimize the probability of observing cloud-free ground pixels, which is important for obtaining the best tropospheric trace gas amounts and to enable OMI to monitor tropospheric pollution phenomena, like biomass burning and industrial pollution, on urban or regional scale. In flight the optical bench is operated at 264 K. The CCD detectors are warmed up to their operational

temperatures of about 265 K with separate active heaters. This is done in a closed-loop feedback system, with which an in-flight temperature stability of about 10 mK is obtained.

Table 1: OMI instrument properties. Spectral range UV1: 264-311 nm

UV2: 307-383 nm VIS: 349 – 504 nm

Spectral sampling UV1: 0.33 nm / px UV2: 0.14 nm / px VIS: 0.21 nm / px

Spectral resolution (FWHM) UV1: 1.9 px = 0.63 nm UV2: 3.0 px = 0.42 nm VIS: 3.0 px = 0.63 nm

Telescope swath IFOV 115 degrees (2600 km on the ground)

Telescope flight IFOV 1.0 degrees (12 km on the ground)

Ground pixel size at nadir, global mode (electronic binning factor 8)

UV1: 13 km x 48 km UV2: 13 km x 24 km VIS: 13 km x 24 km

Ground pixel size at nadir, spatial zoom-in mode (electronic binning factor 4)

UV1: 13 km x 24 km UV2: 13 km x 12 km VIS: 13 km x 12 km

Silicon CCD detectors 780 x 576 (spectral x spatial) pixels

CCD detector shielding 10 kg, about 29 mm thick aluminum

Operational CCD temperature

UV: 265.07 K VIS: 264.99 K

In-orbit CCD temperature excursion

UV and VIS: ±10 mK (stabilized)

Operational optical bench temperature

264 K

In-orbit optical bench temperature excursion

±300 mK

Duty cycle 60 minutes on daylight side (Earth and sun measurements) 10-30 minutes on eclipse side (calibration measurements)

Average data rate 0.8 Mbps Power 66 W Mass 65 kg Size 50 cm x 40 cm x 35 cm Orbit Polar, sun-synchronous

Average altitude: 705 km (438 mi) Orbit period: 98 minutes 53 seconds Ascending node local time: 1:42 PM

The telescope, consisting of a primary convex telescope mirror, a polarisation scrambler and a

Proceedings of the 1st EPS/MetOp RAO Workshop, 15-17 May 2006, ESRIN, Frascati, Italy (ESA SP-618, August 2006)

secondary convex telescope mirror, images the Earth light onto the spectrometer's entrance slit (44 mm long, 0.3 mm wide). The polarisation scrambler makes the OMI instrument insensitive to the polarisation state of the incident light. The instrument has separate UV and VIS optical channels each equipped with a CCD detector. A number of OMI instrument parameters are listed in table 1. The UV channel is optically divided in a wavelength range below 311 nm (UV1) and one above 307 nm (UV2). Both are imaged on different regions of the same CCD detector. This has been done to suppress spectral stray light and to optimize the instrument optical and electronic settings for the wavelength range below 311 nm separately, because in that wavelength range the Earth fluxes decrease by 3-4 orders of magnitude as a result of absorption by ozone in the Hartley-Huggins bands. Besides the Earth view optical path, the instrument also has a separate sun measurement port, that can be closed when not looking at the sun. Sunlight illuminates one of three reflectance diffusers, which are mounted on a caroussel mechanism. A folding mirror, located on another mechanism, reflects the sun light to the polarisation scrambler and the remainder of the optical system, while blocking the Earth light. The remainder of the optical system, including the secondary telescope mirror and the entrance slit of the spectrometer, is exactly the same for the sun and Earth viewing modes. The final spectral dispersion in the optical channels is achieved using reflective gratings. OMI is also equipped with a white light source, which illuminates the entire entrance slit via a transmission diffuser. The white light source is used mainly for detector calibration purposes. More details about the OMI instrument and its on-ground and in-flight calibration can be found elsewhere [1]-[4]. For remote sensing instruments like OMI and GOME-2 a good on-ground calibration delivering reliable calibration key data for 0-1 data processing as well as a good and continuous in-flight calibration are essential to meet the required accuracies of the target scientific data products, especially when the data is to be compared to and to become part of long-term ozone trend records. Intercomparison between Earth observing satellite instruments, in this case GOME-2 and OMI, is an important part of this effort. The GOME-2 and OMI instrument have many similarities. The measurement methods and instrument principles are in many cases very similar. Both instruments have telescopes, an entrance slit and gratings for spectral dispersion. Both instruments are equipped with on-board white light sources (WLS) for optical and detector characterisation. Both instruments observe the sun and are equipped with similar types of reflection

diffusers. The OMI spectral range (264-504 nm) is included in the GOME-2 spectral range (240-800 nm). There are also differences. OMI measures all viewing angles at the same time using CCD detectors, GOME-2 is using a scanning mirror and linear array detectors. OMI is equipped with a polarisation scrambler, whereas GOME-2 is equipped with Polarisation Measurement Devices (PMDs) to characterise the polarisation state of the incident light. GOME-2 has an on-board spectral line source (SLS) for spectral calibration, OMI doesn't and relies solely on the solar Fraunhofer lines for the spectral calibration. GOME-2 will show an in-flight etalonning effect on the detectors, whereas this effect is absent for OMI. Using and investigating the similarities and differences between GOME-2 and OMI in order to verify and validate the accuracies of the in-flight 0-1 calibrations of both instruments is the subject of this paper.

2. RADIOMETRIC CALIBRATION During the on-ground calibration of the OMI instrument the spectral slit functions as a function of wavelength and viewing direction have been calibrated accurately using a method and experimental measurement setup designed for OMI [5], [6]. Using these accurately calibrated spectral slit functions it is possible to convolve a literature high-resolution solar spectrum and compare the result with the irradiance as measured by OMI. The result is shown in Fig. 1.

Figure 1. Ratio of measured irradiance from orbit

2465 (31 December 2004) over high-resolution solar reference spectrum convolved with OMI spectral slit

functions. Any deviations from unity may be caused by:

1) Errors in the OMI radiometric irradiance calibration.

2) Errors in the OMI wavelength calibration. 3) Errors in the determined OMI spectral slit

functions.

4) Errors in the high-resolution solar reference spectrum.

All possibilities were carefully investigated and changes were made to all of the above four parameters. The final result is shown in figure 1. This figure shows that the calibration of the OMI irradiance is sufficiently understood. At the same time we have obtained a high-resolution solar reference spectrum. The spectral slit functions have also been calibrated for the GOME-2 instrument using the same setup as used for OMI. This implies that the exercise performed for OMI can also be performed for GOME-2 and the two instruments can be cross-calibrated. This will lead to a better understanding of the GOME-2 irradiance radiometric calibration, the spectral slit functions and the spectral calibration. By investigating the orbital (elevation angle) and the seasonal (azimuth angle) dependences of the measured GOME-2 irradiance in the same way as we have successfully done for OMI allows for an accurate verification and possible improvement of the available irradiance goniometry calibration parameters. For OMI the irradiance goniometry, which is a function of wavelength, viewing direction, azimuth angle and elevation angle has been calibrated to an accuracy of about 0.3% using in-flight irradiance measurement data. The optical stability of the GOME-2 instrument and the stability of the detector optical etalonning effect can be examined using the WLS measurement results. OMI is equipped with a similar type WLS. Finally, a new type of on-board reflectance diffuser has been developed for the OMI instrument (the quartz volume diffuser), because the existing reflective aluminium diffusers were shown to introduce spatial and spectral features with too large amplitudes in the irradiance spectra. These aluminium features would be unacceptable for accurate DOAS retrievals. The volume diffuser and aluminium diffusers were extensively studied both on the ground and in flight for OMI. The same exercise can be performed for the volume diffuser on board of the GOME-2 instrument. Once the irradiance radiometric calibration is properly understood the attention can be directed more towards the radiance radiometric calibration and the calibration of the Earth reflectances, that are closely related to the accuracy of the instrument BSDF calibration of the GOME-2 instrument.

3. SPECTRAL CALIBRATION Looking at the spectral calibration the GOME-2 and OMI instruments are very similar and similar algorithms and methods can be applied to both instruments. However, there are also a number of important differences that do not prevent an accurate comparison of the spectral calibrations of both instruments, but that will need to be considered

carefully in the comparison. First, GOME-2 is equipped with a spectral line source (SLS), whereas OMI is not and relies solely on the spectral calibration from the solar Fraunhofer lines and from the absorption lines originating in the Earth's atmosphere. Second, GOME-2 is a scanning instrument, whereas OMI uses CCD detectors to capture all viewing directions instantaneously. This implies that OMI can measure very accurately when e.g. clouds are entering the field of view in the flight direction. In such cases the entrance slit of the instrument is illuminated inhomogeneously, which introduces spectral shifts. In OMI we have been able to correct such spectral shifts in the Earth measurement data (magnitude of shifts is typically 0.4-0.5 pixels) using the signal change of the so-called small-pixel column data, that are available at a higher frequency than the normal image read-outs. For OMI we have shown that a correlation exists between the change in small-pixel column data and the spectral shifts. Fig. 2 shown a representative example of such a correlation. With this correction the achieved accuracy for the spectral calibration for OMI is about 0.02 pixel in UV1 and 0.01 pixel in UV2 and VIS (see Fig. 3).

Figure 2. Correlation for earth shine spectra between

wavelength shifts close to cloud transitions and gradients in small pixel column readouts for the VIS

channel. This effect is attributed to partial slit illumination at inhomogeneous ground scenes and is

corrected in the 0-1 data processor using a correlation as shown. In the figure the correlation coefficient is

0.96, the slope is 0.92. For GOME-2 two things can be investigated. It can be investigated what the accuracy of the spectral calibration will be when the solar spectrum is used as input and when the OMI algorithm is applied. The differences with the results from the internal SLS can be investigated. Second, it can be examined to what extent GOME-2 is sensitive to ground scene variations in the flight direction for Earth data. If the instrument is scanning, some smearing of the effect would be present, inherently limiting the accuracy of the spectral calibration. If GOME-2 is not scanning (e.g. nadir

pointing measurements) the effects would be roughly the same as observed for OMI. For GOME-2 the PMD signals or other signals with higher readout frequencies can be used instead of the small-pixel column data used in OMI.

Figure 3. Difference between the wavelength

calibration and assignment in the level-1b data product, for 1 day of data (4 September 2005) for the

VIS channel. The RMS of the difference is 0.008 pixels. The UV2 channel gives very similar results.

4. SPECTRAL STRAY LIGHT CALIBRATION For OMI the spectral stray light in all channels has been investigated carefully, most notably in the UV1 channel (wavelengths < 300 nm), because the spectral stray light is largest as compared to the useful signal, that is 3-4 orders of magnitude lower than for wavelengths above 310 nm as a result of ozone absorption. For OMI the spectral stray light is also dependent on the viewing direction, which further complicates matters. Below 300 nm the surface of the Earth is not visible, because all observed light is reflected off the higher layers in the atmosphere. Signatures of clouds or other high-intensity scenes as function of time or viewing direction in channel UV1 (OMI) or 1 (GOME-2) may be an indication for imperfect spectral stray light correction below 300 nm. Another method for investigating the accuracy of additive correction algorithms such as spectral stray light and multiplicative correction algorithms is to examine the signals in the channel overlap regions for which it is known that the incident light fluxes are the same at the same wavelengths.

5. CONCLUSIONS We propose to use in-flight level-1 measurement data, calibration experience and calibration algorithms from the Ozone Monitoring Instrument (OMI) to verify and validate the calibration and the accuracy of the GOME-

2 in-flight measurement data and level-1 radiance and irradiance data products. In this paper a number of topics and methods have been described in detail, including the radiometric irradiance calibration (including the irradiance goniometry), the spectral calibration and the spectral stray light calibration. We feel that with this effort the accuracy of the GOME-2 in-flight calibration will be improved and that the GOME-2 and OMI instruments will be cross-calibrated, the first step in order to couple existing series of data products (e.g. total ozone).

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