MiniCAS

The acquisition of the laser link is the most critical step in the commissioning phase of an inter-satellite laser interferometer, such as the Laser Ranging Interferometer (LRI) on board the GRACE Follow-On Mission. This mission involves two satellites in low Earth orbit operating an interferometric laser link over a distance of approximately 220km. This measurement technology can be used to determine changes in the Earth's gravitational field, which can be used, for example, to better understand climate change.

The transmitter unit of the MiniCAS system consists of a light source with a sufficiently large beam divergence to cover the entire uncertainty cone without any spatial scans (see drawing (a) in the figure). Such a signal is often referred to as a beacon beam. On the receiving satellite, the light enters the MiniCAS system through an aperture and is focused on the sensor by a lens system. Over the distance, the beam emitted by the transmitter resembles a spherical wave front. If the transmitting spacecraft tilts, the beam also tilts. However, the section of the beam that enters the aperture shows no change in angle (see (b)). However, when the receiving spacecraft tilts, the beam passes through the aperture at an angle (see (c)). The lens system converts this inclination into a beam offset on the sensor.