Charles G. Wellemeyer

An overview of the nadir sensor and algorithms for the NPOESS ozone mapping and profiler suite (OMPS)

The Ozone Mapping and Profiler Suite (OMPS) nadir sensor and algorithms for the United States National Polar-orbiting Operational Environmental Satellite System (NPOESS) comprise a system to map ozone total column globally in 24 hours and to measure the altitude distribution of ozone in the upper stratosphere (30-50 km). The sensor consists of a wide field (110 degree) telescope and two spectrometers: an imager covering 300 to 380 nm with a 50 km nadir footprint for mapping total column ozone across a 2800 km swath, and a 250 to 310 nm spectrometer with a single 250 km footprint to provide ozone profile data with SBUV/2 heritage. Both spectrometers provide 1 nm resolution (full-width at half-maximum, FWHM) spectra. The sensitivity of the OMPS total column algorithm to sensor random and systematic errors is analyzed, and a preliminary evaluation of the potential for deriving concentrations of other trace gases from the calibrated spectral radiances is provided.

OMPS total column algorithm performance: comparison to TOMS and to the NPOESS requirements

One of the objectives of the National Polar-orbiting Operational Environmental Satellite System (NPOESS) program is to continue the long-term data set of total column ozone measurements from the Total Ozone Mapping Spectromenter (TOMS) systems while providing the increased accuracy and precision required by the NPOESS Integrated Program Office (IPO). In developing an Ozone Mapping and Profiler Suite (OMPS) sensor-algorithm system to meet the NPOESS requirements, we systematically analyzed the performance of the TOMS system and determined that it provided a strong starting point for the design of the OMPS system. In fact, our analysis showed that modern TOMS systems meet the NPOESS accuracy requirements for retrievals below 475 Dobson Units (DU). However, the NPOESS precision requirements are met only for retrievals below 225 DU. In order to meet the NPOESS accuracy and, particularly, precision requirements for all total column ozone amounts, we identified areas where improvements in the heritage design lead to the improved performance needed for the OMPS system. Simulations performed using the OMPS system design confirm that the algorithm enhancements, coupled with improvements contained in the OMPS sensor, provide performance that meets the NPOESS IPO requirements.

Calibration and intercalibration of backscatter ultraviolet (BUV) satellite ozone data

Abstract Eight TOMS and SBUV ozone instruments have been in orbit since 1978. In addition eight Shuttle SBUV flights have been conducted since 1989. The ESA GOME instrument, using the BUV technique, is flying on ERS-2 since April 1995. At least two more TOMS type instruments will fly on US, Japanese, and Russian environmental satellites. NOAA will continue flying SBUV instruments on their polar platforms until about the year 2004. Combined satellite and ground based data have verified that global ozone depletion has occurred over the past decade. Confidence in the satellite detected trends results from NASAs comprehensive studies of the various instruments pre- and post- launch calibrations and algorithm refinements. Pre-launch calibrations of BUV type instruments is now standardized using an integrating sphere whose radiance is derived from a NIST irradiance standard. Pre-launch calibrations among several BUV instruments is now consistent to 1–2%. Several techniques are used to characterize and correct instrument long term sensitivity once in orbit. These techniques include: on board calibration systems, spectral discrimination, in-orbit intercomparisons, and scene stabilization. These techniques have been applied to the SBUV, TOMS, and SBUV/2 observations resulting in a data record with trend uncertainty of 1% per decade.

TOMS profile shape error estimates at high latitude

The Total Ozone Mapping Spectrometer (TOMS) ozone measurement is derived by comparing measured backscattered ultraviolet (BUV) radiances with theoretical radiances which are pre- computed using standard climatological ozone profiles and stored in a look-up table. Profile shape errors occur in this algorithm at high latitudes (or more specifically, high optical path lengths) when the actual vertical ozone distribution differs significantly from the standard profile used in constructing the tables. These errors are estimated using sensitivities derived from radiative transfer calculations and measurements of the actual ozone profile from Solar Backscatter Ultraviolet (SBUV) and Balloonsonde. These estimates include a short term uncertainty with a standard deviation of 10% in total column ozone amount as well as a systematic error in the long-term trend at very high solar zenith angles. At the maximum retrieval solar zenith angle of 88 degrees, these calculations indicate that TOMS long-term ozone depletions are over-estimated by 5%/decade.