A ground-based laser can be reflected off the satellite, and light's return time measured to determine the round-trip distance.

EGP's orbital altitude is high enough that atmospheric drag has no significant effect on the spacecraft trajectory.

There is a flash produced when the sun's reflection from one of the satellite's mirrors crosses an observer's position on the earth.

The corner reflectors are used to reflect laser beams and are invisible to amateur observers, but the mirrors are spectacular.

They are designed to reflect sunlight so the satellite can be photographed by ground stations for precise geodetic surveying measurements.

The glints are probably in the third magnitude range but are visible to the naked eye only in very dark skies under good conditions.