Analysis of Operational-mode Impacts on Future Earth Radiation Budget Satellite Observations

The Earth’s Radiation Budget (ERB), composed of incoming solar irradiance, reflected solar irradiance, and emitted terrestrial irradiance, is a fundamental property of Earth's climate system. As such, observations of ERB are essential for quantifying climate change trends and variability. Libera, the first NASA Earth Venture Continuity Mission, has an overarching goal of providing continuity to the ERB climate data currently acquired from the Clouds and the Earth's Radiant Energy System (CERES) mission. To quantify the ERB it is necessary to convert measured radiance into energy budget relevant irradiance through the application of Angular Distribution Models (ADMs). ADMs are empirically derived, scene- and wavelength-dependent collections of anisotropic factors that account for the degree from which scene reflection in a given direction deviates from an equivalent isotropic case. Libera will include a new “split-shortwave” spectral channel for which ADMs do not yet exist. To create a set of anisotropic factors for the Libera split-shortwave channel, a large number of observations spanning a wide range of viewing geometry and scene combinations is required for statistical sufficiency. The primary Libera operational modes are cross-track scanning, which enables consistent global coverage of outgoing radiance observations, and rotational azimuth plane scanning (RAPS), which accelerates the rate of data collection across the range of viewing geometry combinations. Unlike CERES that had dedicated RAPS mode instruments on Terra and Aqua, the single Libera instrument used for cross-track scanning and RAPS operations modes requires optimizing time spent in the two operational scan modes.

This analysis investigates how the cadence of RAPS mode operations and the azimuthal scan rate impacts the scene and viewing geometry space that is observed over time. The impact of the RAPS cadence was investigated using data from the Suomi National Polar-orbiting Partnership satellite for two independent rates. The first is a full day of RAPS observations on the first day of each month for a year, and the second is a full day of RAPS observations every 10 days for a year. The results of this analysis show that for simple scene types, for example, those defined for Earth Radiation Budget experiment (ERBE), the less frequent, first day of every month, cadence is sufficient, whereas more stratified scene types, such as those used for the CERES instrument on the Tropical Rainfall Measuring Mission (TRMM) satellite, require more frequent RAPS mode operations to fill the scene and viewing geometry space. The investigation of the azimuth rotation rate was performed using RAPS data from Suomi-NPP RAPS mode, which has a nominal azimuthal scan rate of six degrees per second, and Aqua, which operates its RAP mode at a nominal half a degree per second. This presentation will reveal how the coverage of scene and viewing geometry space compares between the slow and fast azimuthal rotation rates, and determine whether the slower azimuthal scan rate will be sufficient for Libera.