This document details the sources and integration approach used to derive sensor-specific, band-averaged atmospheric and ocean optical properties that are utilized by NASA's Ocean Biology Processing Group (OBPG) in various geophysical retrieval algorithms and radiative transfer simulations. Hyperspectral data sources that characterize these optical properties are integrated with the satellite sensor relative spectral response functions (RSR), as described in the equation below.
X = integral{ x(L) ∗ RSR(L) ∗ W(L) ∗ dL } / integral{ RSR(L) ∗ W(L) ∗ dL }
Here, the band-averaged quantity X is derived by integrating the spectral values x(L) at each wavelength L over the full wavelength range of the RSR for each sensor band, with W(L) is an optional weighting value (typically 1 or the band-averaged solar irradiance, F0).
An example integration using Python is provided here.The RSRs for each sensor are provided in a standardized netcdf format on the Sensor Spectral Characterization page. The RSRs are interpolated to a standardized sampling of 0.1-deg or 1-deg, with the sampling frequency determined based on the resolution of the source data collected in prelaunch sensor characterization. The RSRs are provided over the full spectral range of prelaunch measurements, including the out-of-band response when available.
There are currently two reference solar irradiance spectra in use within the OBPG: 1) Thullier et al. 2004, and 2) TSIS-1 Version 2, as described in Coddington et al. 2023. The reference solar irradiance spectrum for the PACE mission is TSIS-1 Version 2. All other missions currently use the Thullier 2004 solar irradiance spectrum, but the OBPG plans to adopt TSIS-1 for the next reprocessing of these heritage missions, as well as any future missions.
The spectral model for Rayleigh optical thickness is derived from the work of Bodhaine et al. 1999, assuming standard pressure of 1013.25mb, temperature of 288.15K, and CO2 concentration of 360ppm. In computing the band averages, the Rayleigh optical thickness model was integrated with the sensor RSRs with weighting by the solar spectrum. Band-averaged depolarization factors were also computed for use in the radiative transfer computations.
There are currently two ozone absorption spectra in use within the OBPG: 1) Anderson (1990, 1991, 1992, 1993) combined with Burkholder and Talukdar, 1994, and 2) Serdyuchenko et al. 2014. The Anderson ozone absorption crossections were computed at a temperature of 220.15K, while Serdyuchenko assumed a temperature of 229.15K. The reference ozone absorption spectrum for the PACE mission is Serdyuchenko. All other missions currently use the Anderson ozone absorption spectrum, but the OBPG plans to adopt Serdyuchenko for the next reprocessing of these heritage missions, as well as any future missions. In computing the band averages, the ozone attenuation coefficients were integrated with the sensor RSRs with weighting by the solar spectrum.
The NO2 absorption cross-sections were taken from Bogumil et al. 2003 and Schneider et al. 1987. In computing the band averages, the NO2 absorption cross-sections were integrated with the sensor RSRs with weighting by the solar spectrum.
To cover the range from ultra-violet to short-wave infrared, the pure seawater water absorption spectrum was derived by combination of Smith and Baker 1981, Pope and Fry 1997, and Kou et al. 1993. The pure seawater scattering spectrum was taken from Smith and Baker 1981. In computing the band averages, the water absorption and scattering coefficients were integrated with the sensor RSRs with no additional weighting. The scattering coefficients were converted to backscattering coefficients by applying a multiplicative factor of 0.5.
Results of the band-pass integrations are presented for each sensor on the Sensor Spectral Characterization page, as are the RSRs for each sensor and the interpolated source spectra described above. The sensor nominal center wavelengths (Wave center) presented in the tables are computed as the bisection of the RSR width at half maximum response. Wave average is the integrated band wavelength, and wave peak is the location of maximum response.
