This image shows the order and relative distance between satellites that make up the A-Train. At present, the A-Train consists of five NASA missions and one Japanese mission: Aqua (launched in 2002); Aura (launched in 2004); the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and CloudSat missions (launched together in 2006); the second Orbiting Carbon Observatory (OCO-2, launched in 2014); 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 on December 18, 2013, PARASOL ceased operation, fully exiting the A-Train.
The A-Train is a series of satellites following each other in orbit. In order for multiple instruments to effectively observe the same Earth scene—and thereby obtain more data than could be provided by any one instrument— the instruments must view the scene within a short time period to prevent changes in observing conditions (especially true for clouds). Different satellites that carry these instruments must, therefore, fly closely together in a carefully planned and executed manner.
Science from the MODIS instrument on Aqua can be compromised by sun glint. Therefore, CALIPSO and CloudSat are positioned so that coincident MODIS views avoid this contaminating phenomenon. These two satellites are positioned on the side of Aqua’s ground track that is opposite the sun and they must never precede Aqua. Aura is positioned substantially behind the others, so its Microwave Limb Sounder (MLS) can view horizontally under the same portion of the atmosphere that Aqua is viewing from above.
There is a remarkable advantage to this precise constellation: At the expense of a slight temporal separation, constellation flying of multiple satellites simulates a single satellite that is hundreds of kilometers in size!