## 3 Historical Review to State-of-the-Art
### Erbe  
The Earth Radiation Budget Experiment (ERBE), launched in 1984, was the first multi-satellite constellation system for observing the ERB. From two sun-synchronous satellites, NOAA-9 (850 km, ascending 14:30 equator crossing) and NOAA-10 (810 km, descending 07:30 equator crossing), and a dedicated precessing satellite, the Earth Radiation Budget Satellite (ERBS; 610 km, 57\({}^{\circ}\) inclination), ERBE provided global coverage and sampled diurnal variability. Shortwave ADMs were never generated with ERBE observations. Instead, ERBE shortwave ADMs were generated from 205 days (1 April 1979-22 June 1980) of Nimbus VII ERB scanner observations [21] that are summarized in this section. The ERBE ADMs included 12 scene types combining various cloud fractions over different surfaces (Table 2). The distinct anisotropy of these scene types is shown in Figure 4. For example, at low sun the anisotropy over land and ocean in cloud-free conditions (Figure 4c,d, right half of plots) is stronger than cloud-free over snow and overcast conditions (Figure 4a,b, right half of plots) due to multiple scattering within clouds and within the snow-pack, which tends to smooth the angular distribution of reflected solar radiation. To assign each of the Nimbus VII ERB scanner radiances to a scene type for the generation of ERBE ADMs, a fixed surface type was used and a cloud detection algorithm was applied that used co-located observations from the Temperature and Humidity Infrared Radiometer (THIR) and the Total Ozone Mapping Spectrometer (TOMS), both of which were on the Nimbus VII spacecraft [34]. The Nimbus VII ERB scanner had an approximate nadir resolution of 90 \(\times\) 90 km extending to more than 250 km near the limb, and therefore scene identification was unavoidably coarse. Only 9 non-mixed scene types were defined for the Nimbus VII observations; the land-ocean mixed scene types in ERBE ADMs are weighted averages of observations from each of the non-mixed scene types. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Mission** & **Analysis Period** & **Scene Types (\# Scenes)** & **Notes** & **Reference(s)** \\ \hline TIROS IV & 1962 & Low latitudes (1) & First to account for changes in reflection with solar zenith angle & [28] \\ \hline TIROS VII & 1963–1964 & N/A: isotropic & Isotropic assumption resulted in underestimate of albedo in polar regions & [30] \\ \hline TIROS VII & 1963–1964 & Near-global (1) & Similar approach to TIROS IV but improved statistics & [29] \\ \hline Nimbus II & 1966 & Global (1) & First global anisotropic function & [32] \\ \hline Nimbus III & 1969–1970 & Ocean, snow, land-cloud combination (3) & Multiple scene types, “gross-empirical” sources including aircraft, balloons, and early satellite data & [6] \\ \hline Nimbus VII & 1978–1979 & Ocean, land, snow-ice combination, cloud (4) & First attempt at dynamic cloud identification & [3,33] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of shortwave ADMs applied to pre-ERBE satellite observations. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Scene ID Number** & **Cloud Fraction** & **Surface Type** \\ \hline
1 & Cloud-free (0–5\%) & Ocean \\
2 & Cloud-free (0–5\%) & Land \\
3 & Cloud-free (0–5\%) & Snow \\
4 & Cloud-free (0–5\%) & Desert \\
5 & Cloud-free (0–5\%) & Land-ocean mix \\
6 & Partly cloudy (5–50\%) & Ocean \\
7 & Partly cloudy (5–50\%) & Land or desert \\
8 & Partly cloudy (5–50\%) & Land-ocean mix \\
9 & Mostly cloudy (50–95\%) & Ocean \\
10 & Mostly cloudy (50–95\%) & Land or desert \\
11 & Mostly cloudy (50–95\%) & Land-ocean mix \\
12 & Overcast & All \\ \hline \hline \end{tabular}
\end{table} 
Table 2: ERBE shortwave ADM scene types. Reproduced and adapted from [PERSON] et al. [21]. Figure 4: Shortwave anisotropic factors for four ERBE scene types: (**a**) cloud-free over snow, (**b**) overcast, (**c**) cloud-free over land, and (**d**) cloud-free over ocean. Each polar plot contains data for high sun (\(\theta_{S}\): 0.0–25.8) on the left and low sun (\(\theta_{S}\): 66.4–72.5) on the right.