Spacecraft

OCO-2 is based on the previously launched Orbiting Carbon Observatory (OCO) satellite and carries a single instrument, consisting of three high-resolution grating spectrometers (instruments that measure properties of light within the electromagnetic spectrum). The Orbital Sciences Corporation LeoStar-2 multi-mission spacecraft bus, used successfully on missions such as SORCE and GALEX missions, serves as the on-orbit service platform for the OCO-2 (Orbiting Carbon Observatory-2) three-channel grating spectrometer instrument.

The spacecraft bus is made primarily of aluminum honeycomb panels that are both lightweight and strong and assembled to form a hexagonal structure approximately 1 meter (3.3 feet) in diameter and 2 meters (6.6 feet) tall

The structure contains most of the spacecraft bus components and much of the instrument. The solar array wings, each approximately 3 meters (10 feet) in length, attach to either side of the spacecraft bus through movable motors. The mass of the entire observatory, spacecraft bus and instrument, is approximately 450 kg (or 990 lbs).

An on-board computer, which was designed to operate in the harsh space radiation environment, controls both the spacecraft bus and the instrument. The flight software running on the computer allows it to respond to commands stored in memory or those issued by ground controllers. The telecommunications system provides a link to the ground through a set of electronics and antennas that operate in the S-band, a set of frequencies that include those typically used for wireless connections in our homes and businesses. Science data from the observatory on the other hand is transmitted in X-band. The higher frequencies in this region allow for the higher rates needed to accommodate the quantity of data to be acquired by the instrument. The solar array panels provide electrical power when the observatory is operating in sunlight and a rechargeable battery provides power when the observatory is operating in the umbra (i.e., shadow of the Earth). A star tracker, inertial measurement unit, and GPS (Global Positioning system) provide for attitude determination (i.e., assists the observatory in determining its orientation with respect to inertial space) and a set of momentum wheels allows the instrument telescope to be trained in the proper direction, e.g., “directly downwards” in Nadir Mode and near the sun’s reflection on the ocean in Glint Mode.