Günter Lichtenberg

SCIAMACHY on ENVISAT: in-flight optical performance and first results

The Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) is a contribution to the ENVISAT-1 satellite, which has been launched in March 2002. The SCIAMACHY instrument measures sunlight transmitted, reflected and scattered by the Earths atmosphere or surface simultaneously from the UV to the SWIR spectral region (214 - 2380 nm) in nadir, limb, and occultation viewing geometry. SCIAMACHY allows the characterisation of the composition of the Earth atmosphere from the ground to the mesosphere. This paper gives an overview of the SCIAMACHY instrument and its in-flight detector, spectral and radiometric performance. Furthermore first results on trace gas retrieval from limb and nadir measurement mode will be summarised.

In-flight calibration of the SCIAMACHY solar irradiance spectrum

Abstract ESAs new Earth observation satellite ENVISAT was launched successfully on 2002-02-28. Among nine other instruments it is carrying the new imaging spectrometer SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Chartography). This paper covers part of the calibration and validation efforts during the first seven months of SCIAMACHY in orbit with special emphasis on the application of the internal white light source for radiometric calibration. The solar irradiance spectrum measured by SCIAMACHY is presented and compared with a solar spectrum derived by R. L. Kurucz as well as with solar irradiances measured by the UARS instruments SOLSTICE and SUSIM.

Carbon Monoxide Vertical Column Density Retrieval from SCIAMACHY Infrared Nadir Observations

Nadir observations in the shortwave infrared channels of SCIAMACHY onboard the ENVISAT satellite can be used to derive information on CO, CH4, N2O, CO2, and H2O. BIRRA (Beer InfraRed Retriaval Algorithm) is a nonlinear least squares fit of the measured radiance: Trace gas vertical profiles are scaled to fit the observed data, further auxiliary parameters are code dependent. Here we discuss some features of the code and present results of carbon monoxide vertical column densities retrieved from SCIAMACHY infrared nadir observations.

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.

Validation of Carbon Monoxide Total Column Retrievals from SCIAMACHY Observations with NDACC/TCCON Ground-Based Measurements

The objective was to validate the carbon monoxide (CO) total column product inferred from Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) full-mission (2003 - 2011) short-wave infrared (SWIR) nadir observations using the Beer InfraRed Retrieval Algorithm (BIRRA). Globally distributed Network for the Detection of Atmospheric Composition Change (NDACC) and Total Carbon Column Observing Network (TCCON) ground-based (g-b) measurements were used as a true reference. Weighted averages of SCIAMACHY CO observations within a circle around the g-b observing system were utilized to minimize effects due to spatial mismatch of space-based (s-b) and g-b observations, i.e., disagreements due to representation errors rather than instrument and/or algorithm deficiencies. In addition, temporal weighted averages were examined and then the unweighted (classical) approach was compared to the weighted (non-classical) method. The delivered distance-based filtered SCIAMACHY data were in better agreement with respect to CO averages as compared to square-shaped sampling areas throughout the year. Errors in individual SCIAMACHY retrievals have increased substantially since 2005. The global bias was determined to be in the order of -10 parts per billion in volume (ppbv) depending on the reference network and validation strategy used. The largest negative bias was found to occur in the northern mid-latitudes in Europe and North America, and was partly caused by insufficient a priori estimates of CO and cloud shielding. Furthermore, no significant trend was identified in the global bias throughout the mission. The global analysis of the CO columns retrieved by the BIRRA shows results that are largely consistent with similar investigations in previous works.

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.

In-Flight Retrieval of SCIAMACHY Instrument Spectral Response Function

The instrument Spectral Response Function (ISRF) has a strong impact on spectral calibration and the atmospheric trace gases retrievals. An accurate knowledge or a fine characterization of the ISRF shape and its FWHM (Full width at half maximum) as well as its temporal behavior is therefore crucial. Designing a strategy for the characterization of the ISRF both on ground and in-flight is critical for future missions, such as the spectral imagers in the Copernicus program. We developed an algorithm to retrieve the instrument ISRF in-flight. Our method uses solar measurements taken in-flight by the instrument to fit a parameterized ISRF from on ground based calibration, and then retrieves the shape and FWHM of the actual in-flight ISRF. With such a strategy, one would be able to derive and monitor the ISRF during the commissioning and operation of spectrometer imager missions. We applied our method to retrieve the SCIAMACHY instrument ISRF in its different channels. We compared the retrieved ones with the on ground estimated ones. Besides some peculiarities found in SCIAMACHY channel 8, the ISRF results in other channels were relatively consistent and stable over time in most cases.

Addendum: Hochstaffl, P. et al. Validation of Carbon Monoxide Total Columns from SCIAMACHY with NDACC/TCCON Ground-Based Measurements. Remote Sens. 2018, 10, 223

It was brought to our attention that, due to a recent change of the Total Carbon Column Observing Network (TCCON) Data Use Policy, citation of the individual TCCON dataset references used in the publication published in Remote Sensing [1] is now mandatory.[...]

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.

Calibration and Monitoring

Spaceborne spectral measurements over long time periods require calibration and monitoring of the instrument as a crucial prerequisite for successful retrieval of atmospheric parameters. Calibration applies a sequence of steps to the measurement data while monitoring assesses the optical performance thus permitting degradation corrections. The parameters characterising the instrument were obtained in a sequence of on-ground calibration runs under different environmental conditions. They are stored as Key Data and serve as input when deriving calibrated spectra. Relevant calibration steps include the memory effect and non-linearity, wavelength calibration and both spectral and spatial stray light corrections. Since SCIAMACHY is sensitive to the polarisation state of the incoming light, polarisation needs to be thoroughly taken into account. The final step performs the radiometric calibration. Once in orbit, the optical performance monitoring establishes information concerning the channel and wavelength dependent degradation. From the combination of the results for the different light paths it is even possible to learn how individual optical components degrade.