Figure 1.

Nadir and azimuth viewing angle conventions illustrated for a reference system centered on the water surface (black dot). (a) Viewing nadir angle, $\theta$, is measured from the downward vertical axis: upward radiances are viewed at $\theta <\pi /2$, downward radiances (from sky and Sun) are viewed at $\theta >\pi /2$. (b) Azimuth viewing angle, $\varphi$, and relative azimuth viewing angle, $\text{Δ}\varphi$, are measured for viewing directions clockwise from the north and Sun respectively: radiance viewed by a radiometer pointing toward north has an azimuth viewing angle of 0, and radiance viewed by a radiometer pointing toward and away from the Sun have relative azimuth viewing angles of 0 and $\pi$, respectively.