The Ozone Monitoring Instrument (OMI) is an ultravioletvisible imaging spectrograph that uses two-dimensional CCD detectors to register both the spectrum and the swath perpendicular to the flight direction. This allows having a 114 degrees wide swath combined with an unprecedented small ground pixel (nominally 13 x 24 km<sup>2</sup>), which in turn enables global daily ground coverage with high spatial resolution. The OMI instrument is part of NASA’s EOSAURA satellite, which will be launched in the second half of 2004. The on-ground calibration of the instrument was performed in 2002. This paper presents and discusses results for a number of selected topics from the on-ground calibration: the radiometric calibration, the spectral calibration and spectral slit function calibration. A new method for accurately calibrating spectral slit functions, based on an echelle grating optical stimulus, is discussed. The in-flight calibration and trend monitoring approach and facilities are discussed.
The OMI instrument is an ultraviolet-visible imaging spectrograph that uses two-dimensional CCD detectors to register both the spectrum and the swath perpendicular to the flight direction with a 115° wide swath, which enables global daily ground coverage with high spatial resolution. This paper presents a number of examples of scientific results from the first two years in orbit, as well as a selection of in-flight radiometric, spectral and CCD detector performance and calibration results. The scientific results will show the OMI capability of measuring atmospheric phenomena with high spatial and temporal resolution. It will be shown that the OMI radiometric and spectral calibration are accurately understood. Radiation damage effects on the CCD detectors will be discussed in detail and it will be shown that it is possible to correct for the consequences to a large extent in order to minimise the impact on the scientific level-1 and level-2 data products.
Proc. SPIE. 6749, Remote Sensing for Environmental Monitoring, GIS Applications, and Geology VII
KEYWORDS: Data modeling, Satellites, Geographic information systems, Meteorology, Data centers, Earth's atmosphere, Atmospheric monitoring, Atmospheric particles, Atmospheric modeling, Standards development
Historically the atmospheric and meteorological communities are separate worlds with their own data formats and tools
for data handling making sharing of data difficult and cumbersome. On the other hand, these information sources are
becoming increasingly of interest outside these communities because of the continuously improving spatial and temporal
resolution of e.g. model and satellite data and the interest in historical datasets. New user communities that use
geographically based datasets in a cross-domain manner are emerging. This development is supported by the progress
made in Geographical Information System (GIS) software. The current GIS software is not yet ready for the wealth of
atmospheric data, although the faint outlines of new generation software are already visible: support of HDF, NetCDF
and an increasing understanding of temporal issues are only a few of the hints.
TROPOMI (Tropospheric Ozone-Monitoring Instrument) is a five-channel UV-VIS-NIR-SWIR non-scanning nadir
viewing imaging spectrometer that combines a wide swath (114°) with high spatial resolution (10 × 10 km<sup>2</sup> ). The
instrument heritage consists of GOME on ERS-2, SCIAMACHY on Envisat and, especially, OMI on EOS-Aura.
TROPOMI has even smaller ground pixels than OMI-Aura but still exceeds OMI's signal-to-noise performance. These
improvements optimize the possibility to retrieve tropospheric trace gases. In addition, the SWIR capabilities of
TROPOMI are far better than SCIAMACHY's both in terms of spatial resolution and signal to noise performance.
TROPOMI is part of the TRAQ payload, a mission proposed in response to ESA's EOEP call. The TRAQ mission will
fly in a non-sun synchronous drifting orbit at about 720 km altitude providing nearly global coverage. TROPOMI
measures in the UV-visible wavelength region (270-490 nm), in a near-infrared channel (NIR) in the 710-775 nm range
and has a shortwave infrared channel (SWIR) near 2.3 μm. The wide swath angle, in combination with the drifting orbit,
allows measuring a location up to 5 times a day at 1.5-hour intervals. The spectral resolution is about 0.45 nm for UVVIS-
NIR and 0.25 nm for SWIR. Radiometric calibration will be maintained via solar irradiance measurements using
various diffusers. The instrument will carry on-board calibration sources like LEDs and a white light source. Innovative
aspects include the use of improved detectors in order to improve the radiation hardness and the spatial sampling
capabilities. Column densities of trace gases (NO<sub>2</sub>, O<sub>3</sub>, SO<sub>2</sub> and HCHO) will be derived using primarily the Differential
Optical Absorption Spectroscopy (DOAS) method. The NIR channel serves to obtain information on clouds and the
aerosol height distribution that is needed for tropospheric retrievals. A trade-off study will be conducted whether the
SWIR channel, included to determine column densities of CO and CH<sub>4</sub>, will be incorporated in TROPOMI or in the
Fourier Transform Spectrometer SIFTI on TRAQ.
The TROPI instrument is similar to the complete TROPOMI instrument (UV-VIS-NIR-SWIR) and is proposed for the
CAMEO initiative, as described for the U.S. NRC Decadal Study on Earth Science and Applications from Space.
CAMEO also uses a non-synchronous drifting orbit, but at a higher altitude (around 1500 km). The TROPI instrument
design is a modification of the TROPOMI design to achieve identical coverage and ground pixel sizes from a higher
altitude. In this paper capabilities of TROPOMI and TROPI are discussed with emphasis on the UV-VIS-NIR channels
as the TROPOMI SWIR channel is described in a separate contribution .
The OMI instrument that flies on the EOS Aura mission was launched in July 2004. OMI is a UV-VIS imaging
spectrometer that measures in the 270 - 500 nm wavelength range. OMI provides daily global coverage with high
spatial resolution. Every orbit of 100 minutes OMI generates about 0.5 GB of Level 0 data and 1.2 GB of Level 1 data.
About half of the Level 1 data consists of in-flight calibration measurements. These data rates make it necessary to
automate the process of in-flight calibration. For that purpose two facilities have been developed at KNMI in the
Netherlands: the OMI Dutch Processing System (ODPS) and the Trend Monitoring and In-flight Calibration Facility
(TMCF). A description of these systems is provided with emphasis on the use for radiometric, spectral and detector
calibration and characterization.
With the advance of detector technology and the need for higher spatial resolution, data rates will become even higher
for future missions. To make effective use of automated systems like the TMCF, it is of paramount importance to
integrate the instrument operations concept, the information contained in the Level 1 (meta-)data products and the inflight
calibration software and system databases. In this way a robust but also flexible end-to-end system can be
developed that serves the needs of the calibration staff, the scientific data users and the processing staff. The way this
has been implemented for OMI may serve as an example of a cost-effective and user friendly solution for future
missions. The basic system requirements for in-flight calibration are discussed and examples are given how these
requirements have been implemented for OMI. Special attention is paid to the aspect of supporting the Level 0 - 1 processing with timely and accurate calibration constants.
In-flight performance and calibration results of the Ozone Monitoring Instrument OMI, successfully launched on 15 July
2004 on the EOS-AURA satellite, are presented and discussed. The radiometric calibration in comparison to the high-resolution
solar irradiance spectrum from the literature convolved with the measured spectral slit function is presented. A
correction algorithm for spectral shifts originating from inhomogeneous ground scenes (e.g. clouds) is discussed.
Radiometric features originating from the on-board reflection diffusers are discussed, as well as the accuracy of the
calibration of the instrument's viewing properties. It is shown that the in-flight performance of both CCD detectors shows
evidence of particle hits by trapped high-energetic protons, which results in increased dark currents and increase in the
Random Telegraph Signal (RTS) behaviour.
In preparation for future atmospheric space missions a consortium of Dutch organizations is performing design studies on a nadir viewing grating-based imaging spectrometer using OMI and SCIAMACHY heritage. The spectrometer measures selected species (O<sub>3</sub>, NO<sub>2</sub>, HCHO, H<sub>2</sub>O, SO<sub>2</sub>, aerosols (optical depth, type and absorption index), CO and CH4) with sensitivity down to the Earth's surface, thus addressing science issues on air quality and climate. It includes 3 UV-VIS channels continuously covering the 270-490 nm range, a NIR-channel covering the 710-775 nm range, and a SWIR-channel covering the 2305-2385 nm range. This instrument concept is, named TROPOMI, part of the TRAQ-mission proposal to ESA in response to the Call for Earth Explorer Ideas 2005, and, named TROPI, part of the CAMEO-proposal prepared for the US NRC decadal study-call on Earth science and applications from space. The SWIR-channel is optional in the TROPOMI/TRAQ instrument and included as baseline in the TROPI/CAMEO instrument.
This paper focuses on derivation of the instrument requirements of the SWIR-channel by presenting the results of retrieval studies. Synthetic detector spectra are generated by the combination of a forward model and an instrument simulator that includes the properties of state-of-the-art detector technology. The synthetic spectra are input to the CO and CH<sub>4</sub> IMLM retrieval algorithm originally developed for SCIAMACHY. The required accuracy of the Level-2 SWIR data products defines the main instrument parameters like spectral resolution and sampling, telescope aperture, detector temperature, and optical bench temperature. The impact of selected calibration and retrieval errors on the Level-2 products has been characterized. The current status of the SWIR-channel optical design with its demanding requirements on ground-pixel size, spectral resolution, and signal-to-noise ratio will be presented.
Several organizations in the Netherlands are cooperating to develop user requirements and instrument concepts in the line of SCIAMACHY and OMI but with an increased focus on measuring tropospheric constituents from space. The concepts use passive spectroscopy in dedicated wavelength sections in the range of 300 to 2400 nm and wide angle, non-scanning, swath viewing.
To be able to penetrate into the troposphere small ground pixels are used to obtain a fair fraction of cloud-free pixels and to allow precise detection of the sources of polluting gases.
The trace gas products aimed for are O<sub>3</sub>, NO<sub>2</sub>, HCHO, H<sub>2</sub>O, SO<sub>2</sub>, Aerosol (optical depth, type and absorption index), CO and CH<sub>4</sub>, covering science issues on air quality and climate.
The main challenge in the instrument design is to obtain a good signal-to-noise for cloud free pixels and for low ground albedo and light levels. Also the retrieval of separated tropospheric and stratospheric column amounts from a nadir looking instrument is challenging.
The paper discusses the user requirements and compares alternative measurement strategies. It explains the selection of passive UV-Visible-NIR spectroscopy and comes with an instrument concept which provides the current best realisation of the user requirements.
The Ozone Monitoring Instrument (OMI) was launched on 15 July 2004 on NASA's EOS AURA satellite. The OMI instrument is an ultraviolet-visible imaging spectrograph that uses two-dimensional CCD detectors to register both the spectrum and the swath perpendicular to the flight direction with a 115 degrees wide swath, which enables global daily ground coverage with high spatial resolution. This paper presents a number of in-flight radiometric and spectral instrument performance and calibration results.
Launched on 15 July 2004 aboard the EOS AURA satellite, the Ozone Monitoring Instrument (OMI) is intended as the successor to the Total Ozone Mapping Spectrometer (TOMS). OMI's improved horizontal spatial resolution and extended wavelength range (264-504nm) will provide total column ozone, surface reflectance, aerosol index, and ultraviolet (UV) surface flux as well as ozone profiles and tropospheric column ozone, trace gases, and cloud fraction and height. We present results from a variety of calibration techniques that have been developed over the years to assess the calibration accuracy of backscatter UV sensors. Among these are comparisons of OMI solar measurements with external solar reference spectra and radiances measured over Antarctica and Greenland. OMI UV measured irradiances show wavelength dependencies and spectral features on order of 5% when compared to external solar spectra while all channels exhibit a nearly wavelength independent 1% seasonal goniometric error. No instrument throughput degradation has been identified beyond this level and has been confirmed through ice radiance comparisons. A 3% OMI radiance cross-track swath dependence is seen when comparing radiances over ice fields to radiative transfer results. Reflectances derived at low latitudes show the same cross-track swath dependence with an additional 5% offset.
The Ozone Monitoring Instrument is an UV-Visible imaging spectrograph using two-dimensional CCD detectors to register both the spectrum and the swath perpendicular to the flight direction. This allows having a wide swath (114 degrees) combined with a small ground pixel (nominally 13 x 24 km<sup>2</sup>). The instrument is planned for launch on NASA’s EOS-AURA satellite in January 2004. The on-ground calibration measurement campaign of the instrument was performed May-October 2002, data is still being analyzed to produce the calibration key data set. The paper highlights selected topics from the calibration campaign, the radiometric calibration, spectral calibration including a new method to accurately calibrate the spectral slitfunction and results from the zenith sky measurements and gas cell measurements that were performed with the instrument.
With the Dutch-Finnish Ozone Monitoring Instrument (OMI) hardware mounted on NASA's EOS-AURA spacecraft and the AURA planned for launch in 2004, we are working to prepare for flight. An important step in this preparation is the science validation of the software converting the instrument bit stream into (ir-) radiances, the 0-1b processor. The paper contains a description of the main elements of the 0-1b processor and it discusses the methods we have chosen for the validation process. Next it we discuss the outcomes of the various tests and thereby reveal the criticality of each of the algorithms. The algorithms we are dealing with are CCD detector corrections, algorithms to implement radiometric sensitivity of the instrument, stray light correction and the Fraunhofer lines based wavelength calibration algorithm. Because of the CCD, the stray light correction algorithm is two dimensional and the wavelength calibration algorithm is complex due to the fact that we aim at an extreme accuracy of 1/100 pixel or 2.10-3 nm. The validation partly makes use of the OMI Instrument Response Simulator and partly of on-ground performance and calibration measurement data.
Recently the performance verification phase of the Ozone Monitoring Instrument (OMI) was successfully completed and the calibration has started. The OMI is a next generation imaging spectrograph suitable for trace gas retrieval using the UV-Visible wavelength range. The instrument combines a wide field-of-view (114 degrees) with high spatial resolution enabling trace gas retrieval in the troposphere and providing continuous monitoring. The paper summarises the important performance aspects for the OMI such as the spectral, radiometric, polarisation, viewing and stray light properties of the instrument. It focuses on some aspects that we consider of particular importance such as polarisation scrambling and diffuser features. These features can potentially mix with trace gas absorption features and thereby form error sources. Historically an important issue is the spectral stray light at the steep gradient in the Earth shine radiance around 300 nm. In this paper we show that OMI has a very good stray light performance at these wavelengths. The OMI will be launched on NASA's EOS-AURA satellite early 2004.
The Ozone Monitoring Instrument (OMI) is an UV-Visible imaging spectrograph using two dimensional CCD detectors to register both the spectrum and the swath perpendicular to the flight direction. This allows having a wide swath (114 degrees) combined with a small ground pixel (nominally 13 x 24 km). The instrument is planned for launch on NASA's EOS-AURA satellite in June 2003. Currently the OMI Flight Model is being build. This shortly follows the Instrument Development Model (DM) which was built to, next to engineering purposes, verify the instrument performance. The paper presents measured results from this DM for optical parameters such as distortion, optical efficiency, stray light and polarization sensitivity. Distortion in the spatial direction is shown to be on sub-pixel level and the stray light levels are very low and almost free from ghost peaks. The polarization sensitivity is presently demonstrated to be below 10<SUP>-3</SUP> but we aim to lower the detection limit by an order of magnitude to make sure that spectral residuals do not mix with trace gas absorption spectra. Critical detector parameters are presented such as the very high UV quantum efficiency (60 % at 270 nm), dark current behavior and the sensitivity to radiation.
The Ozone Monitoring Instrument (OMI) is a nadir viewing wide field imaging spectrometer for ozone monitoring. The instrument is the Dutch/Finnish contribution to the NASA EOS-AURA mission. OMI observes earth's back scattered radiation in two spectral channels: the UV channel (270 nm - 350 nm) and the VIS channel (350 nm - 500 nm). Each channel employs a CCD detector (576 X 780 px). The extreme wide field of view of 114 degrees, equal to a swath wide of 2600 km, is obtained by an all reflective telecentric telescope and enables global ozone coverage in one day. Other key features are the spectral range (270 nm - 500 nm) and resolution (spectral sampling distance 0.15 - 0.32 nm/px), the application of a polarization scrambler and its compact design (400 X 300 X 500 mm). Excellent stray light performance in the UV channel is obtained by an elegant opto-mechanical design of the UV optics where the UV wavelength range is split in two parts with separate optical paths and the separate spectra are imaged on one CCD. Onboard calibration includes a white light source, LEDs, and multi-surface solar-calibration diffuser. The OMI-EOS project follows a Proto-Flight approach, supported by breadboards and engineering qualification models on parts and sub-system level. In order to increase confidence in the design, the instrument development model was built. During intensive testing critical performance parameters were checked , e.g. UV stray light behavior, polarization sensitivity, distortion, spatial and spectral ranges and resolutions.
The Ozone Monitoring Instrument (OMI) is a UV/VISible spectrograph (270 - 500 nm). It employs two-dimensional arrays of CCD detectors for simultaneous registration of numerous spectra from ground pixels in the swath perpendicular to the flight direction. As a result, OMI provides (almost) daily global coverage in combination with small ground pixel sizes (nominally 13 X 24 km<SUP>2</SUP> at nadir, minimum 13 X 12 km<SUP>2</SUP> at nadir). The small ground pixels allow retrieval of tropospheric constituents. The OMI Flight Model is currently being integrated and will be launched on the Aura satellite in2003 as part of NASA's Earth Observing System. This paper discusses relationships between and the details of the on-ground calibration approach of OMI, the data processing of level 0 data (raw data) to level 1b data (geophysical data) and the foreseen activities for in-flight calibration.
The Ozone Monitoring Instrument (OMI) is a UV/VIS spectrograph (270-500 nm) in the line of GOME3 and SCIAMACHY4. It employs two-dimensional CCD detectors for simultaneous registration of numerous spectra from ground pixels in the swath perpendicular to the flight direction. The OMI field of view is 13 x 2600 km2 per two seconds nominal exposure time providing (almost) daily global coverage in combination with small ground pixel sizes (nominally 13 x 24 km2, minimum 13 x 12 km2). The small ground pixels will allow retrieval of tropospheric constituents. The OMI contains various new and innovative design elements such as a polarisation scrambler and programmable CCD read-out modes. This paper discusses the overall design of the OMI together with the instrumental capabilities.