## 3 Historical Review to State-of-the-Art
### Application to Other Instruments  
Empirical ADMs can be, and have been, adapted and applied to convert radiance to irradiance for satellite instruments other than those for which they were designed. Examples include other low-Earth orbiting satellites such as the Scanner for Radiation Budget (ScaRaB) instrument of the Megha-Tropiques mission [54; 55], and the Broadband Radiometer (BBR) of the planned EarthCARE mission [56; 57]. Here, we highlight two applications from different orbits: the Geostationary Earth Radiation Budget (GERB) instruments on board the Meteosat Second Generation (MSG) satellites, and the National Institute of Standards and Technology Advanced Radiometer (NISTAR) and Earth Polychromatic Imaging Camera (EPIC) on the Deep Space Climate Observatory (DSCOVR). The GERB instruments [58] provide a unique vantage point from geostationary orbit, where the MSG satellites orbit the Earth at the same rate as Earth's rotation and therefore continuously sample the same locations, well suited for observing and understanding the diurnal evolution of the ERB [59]. Since each location within the GERB field-of-view is always sampled at the same viewing geometry, accurate ADMs are crucial for the conversion of GERB radiance to irradiance. This conversion is performed for GERB shortwave observations using the CERES-TRMM ADMs. The scene type is determined based on a fixed surface-type map and the results of a real-time cloud detection, phase, and optical depth retrieval algorithm [60; 61] applied to narrowband observations from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) instrument on the same MSG satellite [62]. The SEVIRI observations are angularly matched with GERB and have a  
Figure 10: Variations in anisotropy as sea ice albedo increases from (**a**) 0.42 to (**b**) 0.54 to (**c**) 0.60 for solar zenith angle bin 65\({}^{\circ}\)–70\({}^{\circ}\), as captured in CERES-Terra/Aqua shortwave ADMs. Note that data are azimuthally symmetric. Reproduced and adapted from [PERSON] et al. [24]. compatible temporal (~15 min) resolution but a higher spatial resolution (~3 km at nadir). To lessen the impact of the resolution differences between GERB and the observations used to generate the CERES-TRMM ADMs, the ADMs are not assigned at the GERB pixel scale (~50 km at nadir), but rather on sub-regions within each GERB footprint corresponding to 3 \(\times\) 3 SEVIRI pixels (~9 km at nadir). The fixed view geometry of the GERB instrument means that view angle specific errors will introduce regional biases that do not cancel over time. While empirical ADMs alleviate this problem, issues can remain for certain scenes due to limited sampling in the ADMs, observational conditions specific to the region, or geometry-based biases in scene identification. For GERB, the prevalence of Saharan dust within the field-of-view has been shown to significantly influence the conversion from shortwave radiance to irradiance, introducing a mean bias of 2.5 W m\({}^{-2}\) and much larger instantaneous, but compensating, differences [63]. Other shortwave radiance to irradiance conversion biases of several percent are also known to occur in the GERB products at low aerosol loadings over cloud-free ocean, for viewing geometries that are particularly sensitive to the effects of ocean wind speed or are close to the ocean glint angle at which specular reflection occurs [64]. The DSCOVR provides another unique view of ERB from the Lagrange-1 point where it orbits the Sun at the same rate as the Earth, and therefore provides continuous monitoring of the sunlit side of the Earth. On board, the NISTAR observes the entire Earth-integrated radiance in total, shortwave, and near-infrared channels while the EPIC simultaneously provides imagery in 10 narrow bands. Although the Earth rotates through the field-of-view of these instruments, each location within the field-of-view is continuously sampled at almost the same solar-viewing geometry in a very narrow back-scatter direction, so ADMs are arguably even more critical than for GERB.