GOME Instrument Description

Introduction. On April 21, 1995, the European Space Agency (ESA) launched the Global Ozone Monitoring Experiment (GOME) aboard the second European Remote Sensing satellite (ERS-2) . GOME is the first European passive remote sensing instrument operating in the ultraviolet, visible, and near infrared wavelength regions whose primary objective is the determination of the amounts and distributions of trace atmospheric constituents. The instrument was proposed as a precursor to the Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) to be launched on the ENVISAT-1 platform (1st Environmental Satellite) in 2000. GOME is a small scale version of SCIAMACHY observing the atmosphere in nadir sounding only and having only four spectral channels, as opposed to eight channels for SCIAMACHY. The GOME industrial management was funded by the European Space Agency (ESA) and the industrial consortium was led by Officine Gallileo.

During the commission phase of GOME, which lasted from April 1995 until July 1996, a limited amount of data were processed at the Data Processing and Archiving Facility (DPAF) at the DLR Oberpfaffenhofen. The major objective during this phase was the validation of the radiometric accuracy of the GOME solar irradiance and earthshine radiance and validation of trace gas and cloud data products. End of June 1996 nominal operation of the GOME processing chain, providing continuous calibrated data products, commenced at the GOME DPAF. This page attempts to provide a brief overview of the GOME instrumental design, operations, and instrument calibration. For further details the readers are referred to ESA's ERS homepage, where information on other instruments aboard ERS-2 and on the public distribution of calibrated ERS-2 data products can be obtained.

Instrument Details. The GOME instrument is a double monochromator which combines a predisperser prism and a holographic grating in each of the four optical channels as dispersing elements. A schematic diagram of the GOME optical layout is shown in Figures 1a and 1b. The irradiance and radiance spectra are recorded with four linear Reticon Si-diode arrays with 1024 spectral elements each. Peltier elements attached to the diode arrays and connected to passive deep space radiators cool the detectors to about -40 C. Except for the scan mirror at the nadir view port, all spectrometer parts are fixed and the spectra are recorded simultaneously from 240nm to 790nm. The spectral resolution varies between 0.2nm (UV, Channel 1) and 0.4nm (VIS, channel 4). Part of the light which reaches the predisperser prism is branched out and recorded with three broadband polarization monitoring devices (PMD), which approximately cover the spectral range in channels 2 (300-400nm), 3 (400-600nm), and 4 (600-800nm), respectively. The PMDs measure the amount of light at an instrument defined polarization angle.

Figure 1a. Instrumental Setup of GOME (courtesy of ESA).

Figure 1b. Schematics of GOME Optics. The GOME instrument is a four channel spectrometer. Adjacent to the spectrometer is a calibration unit housing a Pt/Cr/Ne hollow cathode discharge lamp and the fore optics for solar viewing. Not shown is an additional mirror which directs the lamp light to the solar diffuser plate for diffuser reflectivity measurements.

The calibration unit adjacent to the spectrometer part consists of the sun view port and a compartment housing a Pt/Ne/Cr hollow cathode discharge lamp. The solar radiation is attenuated by a mesh (20% transmission) and directed via a diffuser plate (wet-sanded Al plate with Cr/Al coating) on to the entrance slit of the spectrometer. When no solar measurements are carried out and during nadir and calibration lamp measurements, a protective shutter is placed in front of the solar view port in order to avoid unnecessary UV exposure and to prevent straylight from entering the instrument. The calibration unit becomes optically coupled to the spectrometer by proper positioning the scan mirror.

The various pointing geometries of the GOME scan mirror permit in addition to solar and earth nadir viewing, polar viewing (scan mirror angle of 45 deg), and lunar observations (scan mirror angle of about 85 deg) at selected times during a year.

Global Earthshine Measurements. The ERS-2 satellite moves in a retrograde, sun-synchronous, near polar orbit at a height of about 785km. The maximal scan width in the nadir viewing is 960km and global coverage is achieved within three days (after 43 Orbits). The local crossing time at the equator is 10:30UTC. An across track scan sequence consists of four ground pixel types called East, Nadir, West, and Backscan with 1.5 sec integration time each as indicated in Figure 2.

Figure 2. GOME Scan Geometry in Nadir Viewing (courtesy of ESA). The acute reader may already have noticed that the actual scanning is reversed (mirror image around the nadir direction), i.e. the scan direction goes from east to west, followed by the backscan going into the opposite direction.

Since the light levels in the UV channel (channel 1) is decreasing rapidly with descending wavelength, the diode array has been separated into two sections with different integration times. Channel 1a (240-307nm) requires an integration time of 12 sec (covering eight ground pixels), while in channel 1b (307-316 nm) the spectrum is integrated for 1.5sec. Each spectrum integrated over 1.5sec covers an area of maximal 40X320km² on the ground. At high solar zenith angles while crossing the polar regions higher integration times are selected for all channels. The PMDs are readout sequentially every 93.75msec, which means that for a nominal East-West scan (4.5sec) forty-eight PMD measurements are collected for each wavelength region. Using the maximum possible scanwidth of 960km, each PMD covers an area of 40X20km² on surface. This makes the PMD measurements useful for detecting rapid changes in the observed surface reflectivities and cloudiness during scanning. Table 1 summarizes the GOME/ERS-2 and its measurement capabiities.

Table 1. Instrumental Details on GOME/ERS2.

Two examples of sun-normalized earthshine spectra under different cloudiness conditions are shown in Figure 3.

Figure 3. Two Examples of Earthshine Spectra Recorded in September 1995 by GOME. The top curve is a reflectivity spectrum for a cloudy scene (cloud cover fraction of 1.0) from 7 September; the bottom a spectrum under clear sky condition (4 September). Both spectra were measured in the North Atlantic region (48N, 30W). Reflectivity is here defined as R=(I/F)(pi/cos SZA), where I is the backscattered radiation, F, the directly measured solar irradiance, and SZA, the solar zenith angle. The absorption of the O2 A-band at 760nm is used to determine the fractional cloud cover.

Solar Irradiance. Once a day (every fourteenth orbit) GOME solar irradiance measurements are carried out when the ERS-2 satellite crosses the terminator in the North polar region coming from the night side. Since GOME is not equipped to actively track the sun, viewing of the full solar disc is only possible for a time span of about 50 sec. Integration times are 0.75 sec for all channels, except for the UV channel, where the integration time is doubled. A mean solar spectrum is constructed from the series of measurements during full disc solar viewing. Figure 4 shows a calibrated mean solar spectrum measured by GOME in 1995.

Figure 4. GOME Solar Spectrum from 22 July 1995. Selected prominent Fraunhofer lines are shown. Asterisks mark instrumental artifacts due to the changing transmission characteristics of the anti-reflection coating on the channel 3 beamsplitter (450nm) and due to a Wood anomaly in the Channel 4 holographic grating (700nm). The overlap regions of the four optical GOME channels are at 315nm, 405nm, and 600nm.

Internal Calibration Sequences. A typical GOME orbit lasts about 100 min, half of which are spent in the night side of the earth. During this phase GOME carries out several sequences of dark current and LED measurements (pixel-to-pixel gain). Various temperature sensors spread over the entire focal plane assembly monitor the in-orbit temperature variations.

Once a month, the internal calibration lamp is switched on over an entire orbit. During this sequence a series of lamp measurements with and without the solar diffuser permits the investigation of long term degradation of the diffuser and an update in the wavelength calibration of the diode arrays, respectively. Prior to launch, the spectral irradiance of the GOME flight model was calibrated by the Dutch firm TPD using a 1000 Watt FEL lamp, which in turn was referenced to an absolute standard at NIST. The absolute accuracy of the NIST standard is quoted to be 1 to 3% in the range 250-340nm. Spectral radiance of GOME was calibrated by placing a Spectralon diffuser plate between the FEL lamp and the nadir scan mirror. In cooperation with NASA Goddard Space Flight Center and ST System Co., Boulder, CO, the Spectralon diffuser plate was compared with the NASA integrating sphere, which served as a radiance standard to calibrate the SBUV/2 and SSBUV instruments. The agreement between the two standards is within 1%. The radiance response was determined as a function of the scan mirror angle.

Suggested Reading:

Burrows, J.P. et al., 1999: The Global Ozone Monitoring Experiment (GOME): Mission Concept and First Scientific Results, J. Atmos. Sci. 56, 151-175. (postscript preprint 9.8Mb and compressed preprint 1.5Mb available).

ERSCONF, 1997: Proceedings of the 3rd ERS Conference , ESA Special Publication 414, Vol. II, Florence, Italy, 17-21 March 1997.

GOMESCIENCE, 1993: GOME Interim Science Report, ESA Special Publication 1151, ESA/ESTEC, Nordwijk, The Netherlands.

GOMEMANUAL, 1995: GOME Users Manual, ESA Special Publication 1182, ESA/ESTEC, Nordwijk, The Netherlands.

GOMEVAL, 1996: Geophysical Validation Campaign Workshop Proceedings, ESA WPP-108, ESA/ESTEC, Nordwijk, The Netherlands.

see University of Bremen GOME papers