Analyzing and Correcting the Degradation of the SNPP, NOAA-20, and NOAA-21 OMPS Nadir Profiler Instruments

The Ozone Mapping and Profiler Suite (OMPS) Nadir Profiler (NP) is an ultraviolet and visible spectrometer that measures spectra in the 250nm to 310nm range, from both Earth radiance and solar irradiance. OMPS NP instruments are currently in orbit aboard three different satellites: the Suomi National Polar-orbiting Partnership (SNPP), NOAA-20, and NOAA-21. Each OMPS NP instrument contains two solar diffusers: a working diffuser that takes solar irradiance measurements every two weeks, and a reference diffuser that takes measurements once per year. Repeated exposure to solar irradiance causes these diffusers to degrade slowly over time. Since the Earth radiance measurements do not share this optical component, the radiance / irradiance ratios develop time-dependent biases. Optical throughput degradation beyond the diffuser affects both the solar and Earth measurements. Neither of these calibration changes have been corrected for in the past OMPS NP Sensor Data Record (SDR) processing. The amount of optical throughput degradation experienced by an instrument exclusive of the diffusers can be determined by scaling and combining the measurements from both of the diffusers combined with an assumption that the diffuser degradation rates are the same per exposure. This information can then be applied to the OMPS Earth radiance data to account for the calibration changes. The solar signals, in addition to degradation related to exposure time, are also dependent on solar activity and wavelength shifts. These factors must be modeled for the irradiance data in order to calculate a correct degradation rate. Solar activity can impact solar irradiance measurements greatly, and is also not currently accounted for in the OMPS SDR Solar Flux data product. In order to both model it in the solar time series analysis and to include it in the Solar Flux product, the impact of solar activity had to be determined. This was done by calculating solar activity scale factors, which were based on measurement variations relative to large short-term changes in the Mg-II Index values. Degradation rates were calculated for each of the SNPP and NOAA-20 OMPS NP sensors, while the analysis for NOAA-21 NP is still to be done. SNPP, having been in orbit for more than 12 years, has experienced the largest amount of degradation, ~3% at the shortest wavelengths, with the magnitude reduce for both of the following sensors. Adjustments will be made to SNPP and NOAA-20 OMPS SDR data to account for these degradation rates, leading to more accurate and robust datasets. The calculation of solar activity scale factors allowed for the calculation of more accurate degradation rates and have also led to improvements in the OMPS SDR Solar Flux data product. Accounting and correcting for both the degradation rates and solar activity have improved the overall quality of the OMPS NP SDR data.

Disclaimer: the presentation contents are solely the opinions of the authors and do not constitute a statement of policy, decision, or position on behalf of NOAA or the U. S. Government.

Acknowledgement: Thanks to Dr. Chunhui Pan for her long-term contributions to the OMPS SDR calibration and reprocessing projects. Thanks to Mr. Eric Beach for collecting historical OMPS SDR data sets that are used in the study.