Large phase angle observations of GEO satellites

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Observations of satellites in geosynchronous orbit (GEO) are routinely performed by optical sensors at night when the phase angle—the angle between the satellite’s lines of sight to earth and to sun—is less than about 85 degrees. Daytime optical observations of satellites in the GEO belt will be performed at larger phase angles. In afternoon and morning hours, small phase angles are available, but at mid-day the phase angle is constrained to be greater than a minimum value that depends upon the latitude of the sensor and time of year. On Maui at summer solstice, for example, the minimum phase angle is 110 degrees at mid-day for observations of the GEO belt. In order to predict their visibility during the daytime, observations of GEO satellites were conducted at night with a small-aperture telescope at the large phase angles available soon after dusk and before dawn. Analysis of the satellite images reveals a flattening in the light curve for phase angles greater than 100 degrees, and the data provide an empirical model of the expected satellite signal at all phase angles.