GOME long-term performance analyses
GOME calibration and performance studies are being carried out by ESA in
cooperation with various institutes and companies, as listed below.
- Deutsches Zentrum für Luft und Raumfahrt, D (DLR)
- University of Bremen, D (IFE)
- Royal Netherlands Meteorological Institute (KNMI)
- Space Research Organization Netherlands (SRON)
- Smithsonian Centre for Astrophysics, MA, USA
GOME diffuser and dark signal trends (November 1999)The analysis on the
GOME diffuser plate stability and the detector array dark signal measurements
was updated for the period July 1995 - September 1999. During the period of
about ~4.5 years no significant changes of the diffuser plate could be detected.
The analysis of the dark signal measurements shows a linear increase of the
leakage current of about 13-14 % per year. See the document.
GOME Degradation files (September 1999)To be used when working with
specific options of the GOME Level 1 extraction software (gdp01_ex) Version 2.0.
Quantitative spectroscopy for the analysis of GOME data (Chance, Florence
1997)A prerequisite for successful retrieval of trace gas vertical columns
from GOME data is the availability of accurate quantitative spectroscopic and
light scattering knowledge. Quantitative spectroscopic knowledge concerns the
high-resolution temperature-dependent absorption cross section of trace gases.
Light scattering knowledge includes the Rayleigh scattering cross section and
phase function in the spectral interval 200-1000 nm; the Ring effect (inelastic
light scattering) cross section and phase function, as determined from
NO2 and O2 molecular parameters; the line parameters for
calculations of absorption in band A of molecular oxygen (used in the GOME cloud
filtering algorithm). Additionally the solar irradiance reference spectrum has
to be accurately known.
In the implementation of the GOME data processing,
all the above mentioned quantities, have been measured by means of the GOME
flight model, at a moderate spectral resolution. This implies that the
instrument response is known quite accurately and is supposed to be stable in
space conditions. Alternatively, they can be measured with a higher spectral
resolution and a high accuracy (in vacuum conditions), and then convolved to
GOME spectral resolution. Such determinations are being carried out. Data are
already available for solar irradiance, Rayleigh scattering, Ring effect and
NO2 absorption cross section (and have been implemented in the latest
GOME product Level-2, 2.0 version); investigations relative to O3,
O2, BrO and H2O are under way, whereas improvements,
relative to OClO, SO2, NO and O4 are needed.
GOME wavelength calibration using solar and atmospheric spectra (Caspar and
Chance, Florence 1997)GOME spectral calibration is based on the spectral
lines emitted by a PtNeCr lamp. These lines have turned out to be stable to
0.02-0.04 GOME pixel, when measured in orbit over a typical range of instrument
temperature variations. The temperature needed in the calibration procedure is
taken at the predisperser prism: it has been argued that this can introduce
additional uncertainties. Two alternative calibration methods are being
investigated, which make use of the Fraunhofer lines present in, both, solar
irradiance and earth radiance spectra. Both methods show an improved spectral
calibration accuracy < 0.002 nm (< 0.02 GOME pixel), in most of the GOME
spectral range, especially in the spectral regions where O3,
NO2, BrO and OClO gas concentrations are measured. Still additional
studies are needed before implementing any procedure at operational level.
(dataset: July 1995 - January 1997)
analysis of the GOME diffuser plate stability and of the detector array dark
signal measurements has been performed. No significant change of the diffuser
plate has been detected. From dark signal measurements analysis a linear
increase of the leakage current - of about 15% per year - has been observed.
Keywords: ESA European
Space Agency - Agence spatiale europeenne,
observation de la terre, earth observation,
satellite remote sensing,
teledetection, geophysique, altimetrie, radar,
chimique atmospherique, geophysics, altimetry, radar,