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Space-Based Measurements of Ozone and Air Quality in the Ultraviolet and Visible

OMI Frequently Asked Questions

General Questions

OMI Products and Tools:


General Questions


What is OMI? OMI Instrument? OMI Project?

OMI is a nadir-viewing near-UV/Visible CCD spectrometer aboard NASA's Earth Observing System's (EOS) Aura satellite. Aura flies in formation about 15 minutes behind Aqua, both of which orbit the earth in a polar Sun-synchronous pattern. Aura was launched on July 15, 2004, and OMI has collected data since August 9, 2004.

OMI measurements cover a spectral region of 264-504 nm (nanometers) with a spectral resolution between 0.42 nm and 0.63 nm and a nominal ground footprint of 13 x 24 km2 at nadir. Essentially complete global coverage is achieved in one day. The significantly improved spatial resolution of OMI measurements as well as the vastly increased number of wavelengths observed, as compared to TOMS, GOME and SCIAMACHY, sets a new standard for trace gas and air quality monitoring from space. The OMI observations provide the following capabilities and features:

  • A mapping of ozone columns at 13 km x 24 km and profiles at 13 km x 48 km (a continuation of TOMS and GOME ozone column data records and the ozone profile records of SBUV and GOME)
  • A measurement of key air quality components: NO2, SO2, BrO, HCHO, and aerosol (a continuation of GOME measurements)
  • The ability to distinguish between aerosol types, such as smoke, dust and sulfates
  • The ability to measure aerosol absorption capacity in terms of aerosol absorption optical depth or single scattering albedo
  • A measurement of cloud pressure and coverage
  • A mapping of the global distribution and trends in UV-B radiation
  • A combination of processing algorithms including TOMS Version 8, DOAS (Differential Optical Absorption Spectroscopy), Hyperspectral BUV retrievals and forward modeling to extract the various OMI data products
  • Near real-time measurements of ozone and other trace gases

The OMI is a contribution of NIVR (Netherlands Institute for Air and Space Development) of Delft, in collaboration with FMI (Finnish Meteorological Institute), Helsinki, Finland, to the EOS Aura mission. The Dutch industrial efforts focused on the optical bench design and assembly, thermal design and project management. The detector modules and the readout and control electronics were provided by Finnish industrial partners.

For more details on the OMI instrument and project, see the OMI web site, the OMI Project Guide, and the OMI instrument overview. For details on OMI results, see the OMI publications web site.

OMI multimedia information


Where can I find information about the OMI instrument?

See the OMI web site, and also the article by Dobber, Marcel J. et al. "Ozone Monitoring Instrument Calibration ", IEEE. Trans. Geosci. Rem. Sens. 44 (5), 1209-1238 (May 2006).

OMI Instrument Description
Parameter Value
Wavelength Range UV-1: 264-311 nm
UV-2: 307-383 nm
VIS: 349-504 nm
Spectral Resolution
(FWHM)		 UV-1: 0.63 nm
UV-2: 0.42 nm
VIS: 0.63 nm
Spectral Sampling
(FWHM):		 UV-1: 1.9 px
UV-2: 3.0 px
VIS: 3.0 px
Telescope FOV: 115° (2600 km on ground)
IFOV: 12 km x 6 km (flight direction x crossflight direction)
Detector: CCD: 780 x 576 (spectral x spatial) pixels
Mass: 65 kg
Duty Cycle: 60 minutes on daylight side
10-30 minutes on eclipse side (calibration)
Power: 66 watts
Data Rate: 0.8 Mbps (average)

What is the OMI swath?

The instantaneous swath of any imaging instrument, including OMI, is the width of the region that is actually observed across the track of the instrument at any time during any particular overflight.

The global measurement mode is the default mode, sampling the complete swath of 2600 km for the complete wavelength range. The ground pixel size at nadir position in the global mode is 13 x 24 km2 (along-track x cross-track) for the UV-2 and VIS channels, and 13 x 48 km2 for the UV-1 channel.

The spatial zoom-in mode has a nadir ground pixel size of 13 x 12 km2, but the swath width has a minimum of 725 km. The spatial zoom-in mode is used one day each 32 days, always above the same geo-locations. In the spectral range of 264-311 nm, the pixel size in the cross-track direction is twice as large (that is, a nadir ground pixel size of 13 x 24 km2). The swath is symmetric with respect to the sub-satellite track. The spatial zoomin mode results in two products:

  • A Zoom Radiance product consisting of all the zoom data.
  • A Global Radiance product in which the zoom data are effectively degraded to match the resolution of images produced in the global mode but only cover half the normal mode radiance.

The spectral zoom-in mode has a nadir ground pixel size of 13 x 12 km2 and a full swath of 2600 km. It has a limited spectral coverage of 307-432 nm to cover the most important scientific products. This mode was tested during the pre-launch period and run a few times between early August and early October 2004, during Launch and Early Operations (LEO). Because this mode has not been used since that time, it is not addressed in this document.


What is a OMI orbit?
The Aura satellite orbits at an altitude of 705 km in a sun-synchronous polar orbit with an exact 16-day repeat cycle and with a local equator crossing time of 13.45 (1:45 P.M.) on the ascending node. The orbital inclination is 98.1 degrees, providing latitudinal coverage from 82° N to 82° S.

What is a OMI path?
Every 233 orbits, the EOS-Aura orbital repeat cycle, the spacecraft covers the exact same ground track. By describing the ground coverage for each orbit in the orbital repeat cycle, the ground coverage of a science data product can be described in terms of a reference to one of these pre-defined paths, rather than using bounding boxes or polygons. This method for describing the ground coverage of a science data product in metadata is called NOSE, Nominal Orbit Spatial Extent.

The NOSE metadata can be used for geo-spatial searches. To refine these searches, each ground track is split up in blocks. Each science data product then only needs to refer to the paths and blocks covered by the product in order to allow for geo-spatial searches. The definition of the NOSE paths and blocks needs to be included in the search system.

For OMI, 466 NOSE paths are defined. Paths 1 .. 233 specify the ground coverage of nominal global measurements for each orbit in the orbital repeat cycle, paths 234 .. 466 specify the ground coverage of spatial zoom-in measurements. The paths are split up in blocks: each block covers about two minutes of measurement time.


What is the data coverage for OMI?
The OMI instrument has daily global coverage of data. More information on data coverage is available in the summary page.

Where can I find spectral response information for the OMI bands?
OMI is a non-scanning wide angle nadir pointing hyper-spectral push broom imaging spectrograph, to observe solar backscatter radiation in the visible and ultraviolet spectral region. The Earth will be viewed in 1560 wavelength bands in the spectral region 270 to 500 nm with a spectral resolution of approximately 0.5 nm. The spectral samplings are grouped into two optical channels, UV & visible (VIS). The UV channel is split into two sub-channels: UV-1 ranging from 270 to 310 nm, and the UV-2 ranging from 310 to 365 nm for stray-light considerations. The VIS-channel ranges from 365 to 500 nm. Two 2-dimensional CCD detectors are used, one for UV-1 & 2 and other for the VIS channel. The nominal integration time of the CCD image is 0.4 sec. To decrease the data rate and increase the signal-noise ratio, onboard binning and co-adding of subsequent CCD images are performed. The number of exposures or images that are averaged is specified by the "binning factor", which is set by the instrument operations team.

Where can I find detailed information about the contents of OMI data products?
See the OMI Data User Guide and OMI Data Products and Data Access Information. The theory behind the algorithms used to process OMI data is in the Algorithm Theoretical Basis Documents (ATBD).

What is meant by Collection?

As OMI continues to reprocess the data products, Collections of scientifically consistent sets of data product versions become available. Collections may contain multiple format, production, and ESDT Versions. The current collection version is collection 3.


What is OMI's File Naming Convention?
The file names used for OMI products all contain two date/time stamps: the data date and the production date. The data date is the start time for the OMI granule or orbit measurement data in the file. The processing date/time indicates when the file for the given product was created. Here is an example of an aerosol product Level 2 file name.

OMI-Aura_L2-OMAERO_2008m0830t1828-o21953_v003-2008m0831t021052.he5

OMI = Instrument Name
Aura = Satellite Name
L2 = Level
OMBRO = Product Name
2008m0830t1828 = Data Date
o21953 = Orbit Number
v003 = Collection Number
2008m0831t021052 = Production Date
he5 = Extension

OMI data set file naming convention description can be found in OMI Level 2 Aerosol Data Product Specification.


What is the quality of the OMI data products?
Intensive assessment of OMI product data quality is an ongoing activity. Users can refer to the Quality Assessment Document for more information.

Where can I obtain OMI imagery?
OMI SO2 imagery can be obtained from SO2 group's web site, which includes daily images. OMI O3, NO2, UV Index and other images are available from Tropospheric Emission Monitoring Internet Service (TEMIS).

OMI Products and Tools

How do I obtain OMI information and data?
OMI data is availabe from Goddard Earth Sciences Data and Information Center (DISC) site, Follow the steps below to order the data you need.
  • Go to Goddard Earth Sciences Data and Information Center (DISC) site
  • You will see all the data products listed (Level 1, Level 2 and Level 3). Click on the product you need. For example you need OMAERUV Level 2 data. Then click on OMAERUV
  • You will arrive in Data Access Page. Click on "WHOM - search & download OMAERUV data"
  • Then select the "Year" you want the data product for. You will also have other selection options. For example: you want data for 2007. Then click on 2007.
  • Then you can choose the months and days by selecting them
  • You will then get instructions on what to do to get the data files. For example:
    1. Download the "FTP script for full size granules" provided after the table of granules below.
    2. On SGI or Linux machines, run: ftp -p -n aurapar2u.ecs.nasa.gov < script
    3. Or on SunOS, Windows/DOS, or Mac platforms, run: ftp -n aurapar2u.ecs.nasa.gov < script
  • You will also have options to select granules, select parameters and do spatial subsetting. Choose your selections and the click on "Submit SUBSET Request".
  • You will then get instructions for single and multiple file download. For example for multiple download you will get instructions such as:
    1. Download the FTP_script: order_xxx....txt
    2. On SGI or Linux machine, run: ftp -p -n aurapar2u.ecs.nasa.gov < order_16498.txt
    3. On DOS, SunOS or Windows/Mac platforms, run: ftp -n aurapar2u.ecs.nasa.gov < order_16498.txt
  • Follow the instructions and you get your data.

How are OMI science data products generated?
OMI data products are grouped into three processing levels. Level 1 processing provides corrected (or calibrated) instrument data. These data are processed and calibrated to remove many of the instrument effects. The resulting products thus contain minimal instrument or spacecraft artifacts and are most suitable for subsequent scientific derivations. Level 2 processing provides retrieval of derived scientific quantities, such as atmospheric aerosol and cloud measurements. Level 3 processing produces global grids of various parameter elements from the Level 2 products. These global grids are produced daily.

What OMI products are currently available? What is meant by Level 1, 2, and 3 data products?
Public release information for OMI Level 1, and 2 products is available at KNMI website. Information about Level 3 globally gridded products is available at Goddard Giovanni site.

How do I decide which product files to order?
Chapter 3, OMI Data Products, of the OMI Data User's Guide may be helpful in determining which data products to order.

What Level 1 products are available?
See the DISC web site OMI Level 1 Products for a complete list of data products The main products are OMI geolocated earth radiances and OMI solar irradiances.

What Level 2 Products are available?
Level-2 products can be divided into products in 3 different categories. They are:
  • Ozone products
  • Clouds, Aerosols and Surface UV Irradiance products
  • Trace Gases Products
A complete list of Level 2 products is available at DISC web site.

What types of Level 3 products are available?
OMI Level-3 products are produced for some Level-2 standard products. Each Level-2 product file contains data from a single orbit. For each Level-2 product there will be 14 files per day. OMI Level-3 daily global products are produced by averaging data over small equal angle grids (0.25 deg x 0.25 deg), (0.5 deg x 0.5 deg) or (1 deg x 1 deg) covering the whole globe. Each grid also contains the corresponding statistical parameters (number of pixels, minimum, maximum, and standard deviation).

What tools are available to work with OMI data products?
The OMI data products are in HDF-EOS format. Aura Tools site lists available tools for reading and working with OMI data. Tools are also available from KNMI site.

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