Wavelength calibration of spectra measured by
the Global Ozone Monitoring Experiment:
variations along orbits and in time

J.H.G.M. van Geffen
Applied Optics  43, 695-706 (2004).

1. Introduction

The Global Ozone Monitoring Experiment (GOME) is a nadir-viewing ultraviolet and visible spectrometer aboard the second European Remote Sensing satellite (ERS-2). GOME measures spectra in four spectral channels between 240 and 790 nm (see Table 1) of sunlight scattered in the atmosphere and reflected by the surface of the Earth. Once a day a direct solar spectrum is measured in these four channels for radiometric calibration. ERS-2 was launched by the European Space Agency (ESA) in 1995 and orbits Earth at an altitude of about 790 km in a sun-synchronous polar orbit. Each orbit takes about 100 minutes and ERS-2 completes 14 orbits per day. The satellite crosses the equator at 10:30 a.m. local time when going north to south.
Table 1 Wavelength ranges of the four detector channels of the GOME instrument, the approximate detector pixel size and the spectral resolution of GOME for each channel. The last column shows the number of data points (detector pixels) for each channel in GDP level-1 spectra.

channel wavelength range pixel size resolution #points
1 237 - 315 nm 0.11 nm 0.17 nm   695
2 313 - 405 nm 0.12 nm 0.17 nm   841
3 407 - 608 nm 0.22 nm 0.30 nm 1024
4 599 - 794 nm 0.22 nm 0.35 nm 1024
The largest off-nadir scan angle of GOME's scan mirror is about 30°, giving a full swath width of around 960 km at ground level and global coverage in three days. In the direction of flight each ground pixels measures 40 km. GOME's scan mirror performs a 4.5-second forward scan from east to west, with respect to the direction of flight, followed by a 1.5-second backscan from west to east. The forward scan is divided into three 1.5-second scan pixels: east, centre and west, where the centre (or: nadir) pixel is directly below the satellite.

Detector channels 1 and 2 (Table 1) are for earthshine spectra divided into two bands ('a' and 'b'); channels 3 and 4 not. The wavelength of the division point between bands 1a and 1b has been changed once since the launch of ERS-2: initially the division was around 307 nm, since June 1998 it is around 283 nm. Band 1a has an integration time of 12 seconds, in order to improve the signal-to-noise ratio, as the spectral signal for these short wavelengths is relatively weak due to strong absorption by ozone. The eight times larger integration time of band 1a thus comprises two full scans, each with three forward and one backscan ground pixel, which implies that a measurement corresponds to a ground pixel of about 960 by 100 km. For more information on the GOME instrument, the reader is referred to Ref. 1-3 and references therein.

In order to facilitate a near-real time delivery (that is: within 3 to 6 hours after observation) of ozone data, a GOME Fast Delivery Service (FDS) is set up at the Royal Netherlands Meteorological Institute (KNMI), in collaboration with ESA. The FDS provides total ozone columns, vertical ozone profiles, assimilated ozone fields, UV index forecasts, and cloud data via the World Wide Web (http://www.knmi.nl/gome_fd/). For this service, KNMI receives parts of the raw (level-0) GOME spectra via ftp: the so-called Extracted GOME Instrument header (EGOI) data, used by ESA to monitor the status of the GOME instrument.[4,5]

The raw data in these EGOIs contains the spectral measurements of nine selected wavelength windows, listed in Table 2, providing enough data to retrieve ozone columns and vertical ozone profiles with sufficient accuracy. EGOI window 6 is the "ozone DOAS window" (Differential Optical Absorption Spectroscopy) from which the total ozone column can be retrieved.[4,6] Windows 1 through 6 are used for the retrieval of ozone profiles.[7] The measurements in windows 7 and 8 are used for the correction of the polarisation sensitivity of the instrument. Window 9 contains the oxygen-A band, which is used for the retrieval of cloud fraction and cloud top height. In the FDS, the latter is done with the Fast Retrieval Scheme for Cloud Observables[8,9] (FRESCO).

Table 2 Definition of the nine EGOI windows since June 1998, used for the near-real time FDS at KNMI as outlined in the Introduction. The fourth column lists the number of data points (detector pixels) in each window, which totals to 445. Apart from these nine windows, the EGOI data contain all instrument health parameters, such as system temperatures, and polarisation information.

window wavelength range width #points band
1 272.16 - 275.91 nm   3.75 nm   35 1a
2 282.93 - 285.55 nm   2.62 nm   25 1b
3 292.51 - 302.96 nm 10.45 nm   98 1b
4 305.31 - 307.87 nm   2.56 nm   25 1b
5 311.92 - 314.46 nm   2.54 nm   25 1b
6 323.13 - 336.22 nm 13.09 nm 116 2b
7 351.64 - 352.76 nm   1.12 nm   11 2b
8 372.32 - 373.42 nm   1.10 nm   11 2b
9 758.00 - 778.50 nm 20.50 nm   99 4  
A number of steps has to be performed to convert the raw level-0 data in the EGOI files into spectral level-1 data.[10] One of these steps is a wavelength calibration, which attributes a wavelength value to each detector pixels using a high-resolution solar spectrum as reference spectrum. The wavelength calibration method has been designed specifically for the FDS, i.e. for the nine EGOI windows listed in Table 2, to provide spectra that are sufficiently accurately calibrated to retrieve ozone columns and vertical ozone profiles. The method is, however, not restricted to these windows: it can in principle be used for any high-resolution spectrometer in any wavelength window within an available reference spectrum.

The method of this wavelength calibration is described in detail by Van Geffen and Van Oss,[11] together with a discussion of the accuracy and some examples. Section 2 of the present paper briefly describes the wavelength calibration, in particular the aspects relevant for the subjects of the remainder of the paper: the variation of the results of the calibration of earthshine spectra -- the change in wavelength assigned to a detector pixel with respect to an arbitrary initial wavelength guess -- along GOME's orbits and from orbit to orbit.

Section 3 describes the data used in this analysis. Investigated is whether the variation shows a correlation with one of the instrument temperatures. Due to a temperature dependence of some of the optical components of the instrument, namely, there are variations in instrument temperatures along an orbit and from orbit to orbit, as discussed in Section 4. The orbital variation of the calibration results and the correlation with instrument temperature variations are presented in Section 5.

Apart from a variation in the calibration results along and between successive orbits, there is also a variation in time over a much longer period. Due to the orbital variation of the earthshine spectrum results, these spectra are not suited for a study of year-to-year variations. Hence, measured solar spectra covering a period of almost six years are used for this study, as described in Section 6. Some concluding remarks are given in Section 7.

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created: 16 October 2003
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