The A-Train is a carefully planned constellation that allows for synergy between the missions. Synergy means that more information about the condition of Earth is obtained from combined observations than would be possible from the sum of the observations taken independently. However, in order for synergistic measurements to be successfully obtained, the constellation configuration has to be precisely aligned in time and space, with respect to each other and with respect to the planet below. 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, GCOM-W1 is maintained inside a control box of ±43 seconds. Aqua is maintained inside a control box of ±21.5 seconds (about ±158 km at its orbital velocity). CALIPSO, followed by CloudSat, also fly inside control boxes of ±21.5 seconds. CALIPSO is never closer than 30 seconds (about 225 km) to Aqua. Finally, Aura flies about 640 seconds behind GCOM-W1. Later, OCO-2 will join the A-Train and also be maintained in a ±43-second control box that is at least 15 seconds in front of GCOM-W1’s control box. Each satellite makes precisely 233 complete orbits in 16 days.
