GOME stands for Global Ozone Monitoring Experiment.
It is an instrument aboard the ERS-2 (European Remote Sensing) satellite,
launched by the European Space Agency (ESA) on 21 April 1995.
The picture on the right shows an artist's view of the GOME instrument on
board of ERS-2.
ESA uses four ground stations for downlinking ERS-2 data; they are located
in Kiruna (Sweden), Gatineau (Canada),
Prince Albert (Canada) and Maspalomas (Spain).
GOME is a spectrometer, which means that it measures Earthshine spectra,
that is: the sunlight which is reflected back into space by molecules in the
atmosphere and by the surface.
The instrument also measures the solar spectrum directly. The ratio between
the Earthshine and solar signal is a measure of the reflectivity of the
Earth's atmosphere and surface.
GOME measures the spectra in a wide wavelength range,
from the ultraviolet (UV; 240 nm), via the visible into the near-infrared
(790 nm), at high resolution (0.2-0.4 nm).
From the measurement (an example of the ultraviloet part of the measured
spectra is shown on the right) information can be extracted on ozone and
several other trace gas species in the atmosphere.
The spectral features around 330 nm, for example, are used to retrieve the ozone column: the total ozone concentration in a column of air between the Earth's surface and the top of the atmosphere. Information about the vertical distribution of the ozone is retrieved from the data in the range between 260 and 350 nm. The ozone columns are combined (by a technique called data assimilation) with a model of the transport of ozone in the atmosphere and the chemical reactions involving ozone. This combination of observations and model results in global ozone maps.
Ozone absorbs ultraviolet (UV) light, and the amount of harmful UV reaching the surface is directly related to the ozone column above. Based on the total ozone column maps, a clear-sky UV-index is computed.
Important for the analysis of the data of ozone and other species is information on clouds, which can be derived from the so-called oxygen-A band around 760 nm.
Monitoring ozone is the primary function of GOME. But the spectra it measures also give unique information on other atmospheric species, such as the total concentration of nitrogen-dioxide, bromine and chlorine species, formaldehyde, sulphur-dioxide and the abundance of aerosols (particles) in the atmosphere. See for example the products at the GOME page of the University of Bremen
Observing the world-wide concentration and distribution of ozone is important to monitor the evolution of the ozone layer, to derive the amount of UV, to provide ozone and UV forecasts, and to improve the weather forecasts. Furthermore, the data sets are of great importance to climate research, as changes in the concentration and distribution of ozone, nitrogen-dioxide and other trace gases reflect possible consequences of human activities on the Earth's atmosphere.
A wide range of important issues is studied based on the data sets provided
by instruments such as GOME: the trend in the ozone hole and a possible
recovery of the ozone layer in the future; the amount and global
distribution of air pollution; changes in the amount of human-related
emissions (such as fossil fuel and biomass burning) and natural emissions
(e.g. emissions from soils and vegetation, lightning); trends in
ozone in the lower atmosphere related to these changes in the atmospheric
composition; the relation between changes in ozone and the greenhouse
effect; etc.
To illustrate briefly how information is retrieved from spectra, consider
the retrieval of the total ozone column from the spectral features around
330 nm with a method named DOAS (Differential Optical Absoption
Spectroscopy), which uses the reflectivity, the ratio between the
Earthshine and solar spectrum. The figure on the right shows the
reflectivity in the relevant wavelength window for the same example as shown
above.
The specific absorption structures, their depth and width, in the
reflectivity are a measure for the amount of ozone along the lightpath
through the atmosphere. This value is then converted into the total ozone in
a vertical column of air above the ground pixel that GOME measured.
The latter step is a rather complex procedure and involves knowledge
the various effects that affect the absorption structures:
the viewing geometry (in what direction is GOME actually looking), the
position of the Sun in the sky, the temperature of the atmosphere,
information of clouds in the line of sight, etc.
ERS-2 flies in a sun-synchronous polar orbit with an inclination of 98°
at an altitude of 780 km. This results in an orbital period of about 100
minutes and a speed of the subsatellite point of 7 km/s and 14 orbits per
day. The satellite crosses the equator at a local time of 10.30 h at
the day side of the Earth, flying from North to South.
The full width of a normal GOME scanning swath is 960 km, which is divided in three ground pixels (named east, centre or nadir, and west, relative to viewing straight down), as the picture on the right illustrates. The scan measures 40 km in the direction of flight. The scanning width and the orbit specification combined means that GOME obtains global coverage in 3 days, as the figures below show, thus providing the opportunity to monitor relatively short-term processes that play a role in the Earth's atmosphere and are important for climate research.
Coverage after one, two and three days by GOME's scan |
To this end, a
GOME Fast Delivery Service
has been set up at the Royal Netherlands Meteorological Institute (KNMI),
with financial support of the European Space Agency.
The goal of this Fast Delivery Service is to provide total ozone columns,
vertical ozone profiles, assimilated ozone fields, ozone field and UV
radiation forecasts, and cloud information, to users via the World Wide Web
in near real time.
This service uses a part of the spectral data of each measurement, also used
by ESA to monitor the instrument's status and health.
The official release of GOME products, based on the full spectral date and
made with the GDP (GOME Data Processor) of the ESA Processing and Archiving
Facility at DLR (Deutches Zentrum für Luft- und Raumfahrt) in
Oberpfaffenhofen, Germany, are released on CD-ROMs and via the Internet
between two weeks and two months after observation.
Note that a temporary GDP processor is stationed at the Kiruna (one of the
four ground stations used for ERS-2 data) also provides near real time data,
but only for 10 out of 14 orbits.
Picture credits:
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ERS-2 pictures and coverage:
GOME design pages at ESA-ESRIN
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spectrum plots: made for this page
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GOFAP logo:
Fast Delivery service at KNMI
Authors: Jos van Geffen, Henk Eskes
Copyright © KNMI -- Last modified: 30 October 2001