This image shows the amount of time each spacecraft is separated along the group’s orbit track. At present, the A-Train consists of four NASA missions, one French mission, and one Japanese mission: Aqua (launched in 2002); Aura and the French satellite mission, Polarization and Anisotropy of Reflectances for Atmospheric Science coupled with Observations from a Lidar (PARASOL) (both launched in 2004); the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and CloudSat missions (launched together in 2006); and the Japanese satellite mission, the Global Change Observation Mission-Water (GCOM-W1) (launched in 2012). On November 16, 2011, PARASOL was lowered to 9.5 km under the A-Train and continues its nominal mission observing clouds and aerosols. PARASOL will exit the A-Train fully in the fall of 2013. One more NASA mission—the second Orbiting Carbon Observatory (OCO-2)—will join the A-Train in 2014.
A satellite constellation like the A-Train is composed of a number of satellites following one another along a “track” in space. While they aren’t literally connected like railroad cars, precise engineering and planning—called constellation flying—allows for them to function as if they were “connected.”
Constellation flying allows the instruments on all of the A-Train satellites to function as if they were on a large platform together. This means that scientists can use instruments on several different satellites in the constellation to study a particular atmospheric phenomenon of interest—e.g., clouds, aerosols, greenhouse gases—and learn more than they could have with any one satellite by itself.
Combining data from these satellites enables scientists to gain a better understanding of a variety of Earth-system processes, including those relevant to climate. Data collected synchronously gives more-complete answers to important scientific questions than would be possible with satellite data collected at different times.