The GOME Spectrometer
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 atmospheric trace constituents (see Figure 1). The GOME industrial management was funded by the European Space Agency (ESA) and the industrial consortium was led by Officine Gallileo. 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 June 2001. 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. Second generation GOME-2 are planned for the EUMETSAT MetOp 1, 2, and 3 platforms (2005-2020), extending the European atmospheric chemistry measurement series in the UV/VIS into the next two decades.
Figure 1. GOME aboard ERS-2 (courtesy of ESA).
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 2a and 2b. 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 -40ºC. Except for the scan mirror at the nadir view port,
all spectrometer parts are fixed and the spectra are recorded
simultaneously from 240 nm to 790 nm. The spectral resolution varies
between 0.2nm (UV, Channel 1) and 0.4 nm (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 (300-400
nm), 3 (400-600 nm), and 4 (600-800 nm), respectively. The PMDs
measure the amount of light at an instrument defined polarization
Figure 2a. Instrumental Setup of GOME (courtesy of ESA).
Figure 2b. Schematics of GOME Optics (Weber et al. 1998). 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 for wavelength calibration 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 various pointing geometries of the GOME scan mirror permit in addition to solar and earth nadir viewing, polar viewing (scan mirror angle of 45º), and lunar observations (scan mirror angle of about 80º) 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 795km. The maximum 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 10:30am. 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 3.
Figure 3. GOME Scan
Geometry in Nadir Viewing (schematic). Two successive scan
sequences are shown. Forward scan consists of East (E), Nadir (N), and
West (W) pixels and is followed by a backscan (B) (light
grey). Channel 1a (240-307 nm, after June 1998 240-283 nm) ground
coverage extends to that of ground pixels numbered 1-8 corresponding
to 12 sec integration time. During the commisioning phase saturation
of the diode arrays were found in cases of high reflectivity (cloud)
scenes. Before a co-adding patch was transferred to GOME in March
1996, integration time was shorten to 0.375 sec in channels 1B, 2, 3,
and 4. The ground coverage was then limited to the last quarter of the
individual pixel areas shown here.
Figure 4. Two Examples of Earthshine Spectra Recorded in September 1995 by GOME (Burrows et al. 1999). 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.
Figure 5. GOME Solar Spectrum from 22 July 1995 (Weber et al. 1998). 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.
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.
Burrows, J.P. et al., 1999: The Global Ozone Monitoring Experiment (GOME): Mission Concept and First Scientific Results, J. Atmos. Sci. 56, 151-175.
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.
Weber, M., J.P. Burrows and R.P. Cebula, GOME Solar UV/VIS Irradiance Measurements between 1995 and 1997 - First Results on Proxy Solar Activity Studies, Solar Physics 177, 63-77, 1998.
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