Fraction of the satellites in range and illuminated by the Sun as a function of the Sun’s elevation above the horizon. The satellites’ orbits have an altitude of 1000 km in this example. The region shaded in dark red corresponds to day time; in red (C) to civil twilight, when observations are not possible or are not affected by bright sources; in orange (N) to nautical twilight, when most observations are not possible; in yellow (A) to astronomical twilight, when observations in the IR or short observations in the visible are possible.