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The Afternoon Constellation

How It Works: Control Boxes - A-Train

How It Works: Control Boxes

The Afternoon Constellation allows for synergy between the missions. However, in order for synergistic measurements to be successfully obtained, the constellation configuration has to be carefully aligned in time and space, with respect to each other and with respect to the ground track on the Earth. This calls for ongoing coordinated maneuvering of the spacecrafts to keep them in a tight configuration.

The heart of constellation flying is the implementation of control boxes. Each satellite is allowed to drift within its respective control box (seen in the diagram above as colored boxes surrounding the satellites) until it approaches the boundary of its box. At that point the satellite must execute maneuvers to adjust its orbit. These maneuvers maintain the observing times and geometries of the instruments, but more importantly, they avoid potential collisions that would threaten the entire constellation by producing a debris field, not to mention the loss of synergistic data.

In the current A-Train configuration, Aura, OCO-2, and GCOM-W1 are maintained inside control boxes of ±43 seconds. Aqua is maintained inside a control box of ±21.5 seconds (about ±158 km at its orbital velocity). In the current C-Train configuration, CALIPSO experiences about twice as much drag as CloudSat, therefore, CloudSat conducts periodic orbit lower maneuvers to maintain a 40-second, along-track offset with respect to CALIPSO. This positioning will be maintained within about ±8.5 seconds, or about 4km of ground track offset, from CALIPSO.

While the orbits of the A-Train and C-Train follow slightly different ground tracks due to differences in altitude, the two constellation tracks intersect each other about every 20 days. This allows simultaneous A-Train/C-Train instrument measurements on a regular basis.

Afternoon Constellation Fact