Manfred Gottwald

Nadir, limb, and occultation measurements with SCIAMACHY

Abstract The Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) is a contribution to the ENVISAT-1 satellite, which is to be launched in mid 2001. The SCIAMACHY instrument is designed to measure sunlight transmitted, reflected and scattered by the Earths atmosphere or surface simultaneously from the UV to the NIR spectral spectral region (240 – 2380 nm) in various viewing geometries. Inversion of the SCIAMACHY measurements will provide the amount and distributions of a large number of atmospheric constituents in the stratosphere and troposphere (O 3 , NO 2 , H 2 O, CO 2 , CH 4 , N 2 O, BrO, CO, O 2 , O 2 ( 1 Δ g ), NO, SO 2 , H 2 CO, (ClO,) and OClO). This paper concentrates on the characteristics of the SCIAMACHY mission. In particular, the measurement strategies for the different observational modes — nadir, limb, and both solar and lunar occultation — and their operational implementation are described.

The SCIAMACHY Calibration/Monitoring Concept and First Results

The Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) is a contribution to the ENVISAT satellite, which has been launched successfully in March 2002. SCIAMACHY determines the amount and distributions of a large number of atmospheric constituents by measuring Earthshine radiance and solar irradiance spectra simultaneously from the UV to the NIR in various viewing geometries. The scientific goals of the SCIAMACHY mission can only be reached by an instrument having a high spectral and radiometric accuracy and stability. For example, to determine global NO2 concentrations SCIAMACHY measurements need to be sensitive to differential spectral structures down to the order of 10−4. These requirements can only be met by an appropriate instrument design in combination with both on-ground and in-flight calibration and monitoring. This paper gives an overview on the SCIAMACHY calibration and monitoring concept and activities which are essential to ensure the high quality of SCIAMACHY data products throughout the instrument lifetime. Furthermore, first in-flight monitoring results are presented.

SCIAMACHY’s View of the Changing Earth’s Environment

Since August 2002 SCIAMACHY delivers a wealth of high-quality data permitting to study the status of the Earth’s atmosphere. Enhanced concentrations of greenhouse gases are identified as the major source of global warming and their atmospheric concentrations are increasing. SCIAMACHY monitors the most prominent species such as CO2, CH4 and water vapour, the latter including isotope variants. Further anthropogenic impacts on the troposphere occur by emission of reactive trace gases contributing to pollution and affecting air quality. With SCIAMACHY their global, regional and even local signatures can be detected. Long-term analyses document how the emissions of NO2, SO2, HCHO, CHOCHO and CO evolve with time. In addition, the halogen cycle of polar BrO and IO, both of natural origin, is studied. The stratosphere is the layer where public interest in the Earth’s atmosphere has begun to grow with the detection of the ozone hole in the mid-1980s. Until the mid-1990s a steady decrease has been observed in the ozone abundance. The most striking feature is the massive loss of stratospheric ozone over Antarctica during each southern spring. In order to detect possible signs of recovery, SCIAMACHY contributes to the continuous monitoring of the ozone layer, the ozone hole, Polar Stratospheric Clouds (PSC) and species impacting the ozone chemistry such as NO2, OClO and BrO. A much more poorly explored region is the mesosphere and lower thermosphere, which forms the transition between interplanetary space and the terrestrial atmosphere. This region is dominated by extraterrestrial impacts as well as couplings to the lower atmosphere. With SCIAMACHY’s limb viewing capabilities Noctilucent Clouds (NLC) are studied providing insight into generation and depletion mechanisms. At times of strong solar activity, SCIAMACHY measurements reveal how the chemistry of the upper atmosphere is disturbed. By analysis of emission lines in SCIAMACHY spectra the composition of the thermosphere above 100 km can be studied. SCIAMACHY is the first instrument to globally observe the metal layers in the upper mesosphere/lower thermosphere (MLT) region. When applying appropriate retrieval techniques it is meanwhile possible to derive vegetation information over land and phytoplankton characteristics in the oceans from SCIAMACHY data. Finally SCIAMACHY even has proven useful in planetary science by measuring spectra of our solar system neighbour Venus.

The SCIAMACHY instrument on ENVISAT: first performance monitoring results

Since March 2002 the spectrometer SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) is orbiting the Earth aboard the new European environmental satellite ENVISAT. SCIAMACHY will measure during the next years concentrations and distributions of atmospheric trace gases (O/sub 3/, BrO, OClO, ClO, SO/sub 2/, H/sub 2/CO, NO/sub 2/, CO, CO/sub 2/, CH/sub 4/, H/sub 2/O, N/sub 2/O), clouds, and aerosols. To assure the quality of these data products at any time during the whole mission a detailed knowledge of the instruments status and behavior is mandatory. To achieve this a comprehensive monitoring concept has been developed and implemented. This paper presents selected results of performance monitoring activities from the commissioning phase and from early nominal operations of the instrument, showing that SCIAMACHY is in good shape.

SCIAMACHY In-Orbit Operations and Performance

Since the launch in early 2002 SCIAMACHY has successfully operated in low-Earth orbit for more than 8 years. For the first several months a challenging Commissioning Phase programme was executed. It successively brought SCIAMACHY into full operation mode and verified the instrument’s functional capabilities. In early August 2002 quasi-routine measurements executing nominal mission scenarios could start. In January 2003 the routine operations phase commenced. Since then SCIAMACHY is kept under strict configuration control. Because of the harsh space environment the instrument is subject to degradation, both optically and thermally. The optical performance is described by the throughput which is a measure for how optical components in a light path age with time. It also includes characterisation of optical imperfections such as scan angle dependence, channel 7 light leak and spatial stray light. Illustrating the thermal performance includes decontaminations, used to tackle the ice layers in channels 7 and 8 and configuration of the thermal control systems to respond to degradation. Finally the improvement of the line-of-sight performance by determination of mispointing angles achieved the best possible pointing knowledge. This was especially needed for the retrieval of accurate limb data products. The current excellent status of SCIAMACHY is a prerequisite for successfully accomplishing the intended ENVISAT mission extension until 2013.

Nine years of atmospheric remote sensing with sciamachy - atmospheric parameters and data products

The SCIAMACHY instrument on-board ENVISAT measures since 2002 trace gas constituents of the atmosphere in nadir, limb and occultation configuration. It is an imaging spectrometer with q spectral range from the UV/VIS to SWIR (212 nm – 2384nm). In this paper we describe shortly the current status of the operational processing chains from Level 0-1b and Level 1b-2 that deliver Earth radiances, solar irradiance, trace gas total columns and profiles as well as cloud characteristics on an orbital basis. An outlook for future operational products is also given.

Nine years of atmospheric remote sensing with SCIAMACHY - instrument performance

SCIAMACHY on ENVISAT has meanwhile explored the Earths atmosphere for more than 9 years. All subsystems perform well which is a precondition for maintaining a high optical and operational performance. Even the unavoidable in-orbit degradation due to the harsh space environment is lower than expected and can be largely compensated by sophisticated calibration and monitoring means. Because of the excellent status of both the platform and the instruments the ENVISAT mission has been extended until the end of 2013. Associated with this extension was a change of the orbit in late October 2010 and reconfiguring SCIAMACHY for achieving successful operations in the coming years.

ENVISAT – SCIAMACHY’s Host

ENVISAT is Europe’s ambitious Earth Observation enterprise to study the many facets of the Earth system. It carries ten remote sensing instruments with SCIAMACHY, MIPAS and GOMOS forming the atmospheric chemistry mission. SCIAMACHY, although provided by national space agencies to ESA, is an integral part of the payload. Orbit and attitude of ENVISAT determine the framework of SCIAMACHY’s observing capabilities. As a polar, sun-synchronous satellite, ENVISAT provides a stable platform for orbiting the Earth every 100 min. All instruments share the available on-board resources, particularly on-board data handling capabilities. The ENVISAT ground segment consists of the Flight Operation Segment for platform and instrument control and of the Payload Data Segment for measurement data acquisition, processing, archiving and dissemination. The SCIAMACHY data processing occurs at the LRAC and the D-PAC, depending on whether the data is of type level 0, 1b or 2. Access to SCIAMACHY data follows the general ENVISAT data policy with the exception that the instrument providing agencies receive a separate copy of such data.

SCIAMACHY on ENVISAT: 4 Years in Space: A Status Report

SCIAMACHY on ENVISAT has now successfully operated more than 4 years in space. The current in-flight performance of the instrument shows an excellent status with only a few minor in-flight non-conformances. A large volume of high quality scientific data has been acquired with results being retrieved covering many aspects of the changing Earths atmosphere. In this paper a brief overview of SCIAMACHY is given with special emphasis on operations and performance aspects.