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


Originally, the data obtained from Earth Probe (EP) TOMS were intended to complement data obtained from ADEOS TOMS, which gave complete equatorial coverage due to its higher orbit. EP-TOMS was launched into a 500 kilometer orbit rather than the originally planned 950 kilometer orbit. The lower orbit decreased the size of the "footprint" of each measurement, which increased the resolution and also increased the ability to make measurements over cloudless scenes. This orbit was chosen to improve the ability of the TOMS instrument to make measurements of UV-absorbing aerosols in the troposphere. Tropospheric aerosols play a major role in the Earth's climate and the capability to measure them from a TOMS instrument has recently been developed using data from Nimbus-7 and Meteor-3 TOMS.

The increased probability of making measurements over cloud-free areas enhanced the capability of converting the TOMS aerosol measurements into geophysical quantities such as optical depth. Although the lower orbit precluded full global coverage in the equatorial region, the scanning range of the TOMS instrument still provided full coverage over the poles. EP-TOMS would have still been able to completely track and map the development of the Antarctic ozone hole and the springtime decrease of ozone in the northern hemisphere.

However, ADEOS failed in June 1997. The orbit of EP has been boosted to 740 km and circularized to provide coverage that is as complete as possible. As with the pre-boosted measurements, the EP-TOMS data still has missing elements at the equator, but these do not present a significant problem in analysis.

Current TOMS and OMI data were processed with the Version 8 algorithm that has been developed by NASA Goddard's Ozone Processing Team to address errors associated with extreme viewing conditions. The basic algorithm used just 2 wavelengths (317.5 and 331.2 nm under most conditions, and 331.2 and 360 nm for high ozone and high solar zenith angle conditions). The longer of the two wavelengths is used to derive the surface reflectivity (or cloud fraction). Once the surface reflectivity has been established, the shorter wavelength, which is heavily absorbed by ozone, may be used to derive total ozone. The algorithm also calculates the "aerosol index" (AI) from the difference in surface reflectivity derived from the 331.2 and 360 nm measurements. The AI primarily provides a measure of absorption of UV radiation by smoke and desert dust. This algorithm is described in detail in the TOMS algorithm theoretical basis document (ATBD). Interested viewers may also wish to read about the Version 8 algorithm.

Orbital characteristics (as of 12/13/97):

  • Altitude : 740 km
  • Inclination : 98.385 deg
  • Period : 99.65 min
  • EP-TOMS FOV at nadir : 39 km latitude x 39 km longitude

Original orbital characteristics (before 12/5/97):

  • Apogee Altitude : 515.2 km
  • Perigee Altitude : 490.5 km
  • Orbit Inclination : 97.432 deg.
  • Period : 94.6 min
  • EP-TOMS FOV at nadir : 26 km latitude x 26 km longitude

There are no EP TOMS data between (12/05/1997 - 12/10/1997)* and (12/13/1998 - 01/02/1999)*,

[Earth Probe Mission Patch]Here's the official Earth Probe TOMS mission patch (98 Kbytes)

Note: A 200 dpi version of the mission patch is also available (437k)

Check out EP-TOMS data and images.

* Date in mm/dd/yyyy format

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