The A-Train & C-Train

NASA and its international partners operate several Earth-observing satellites that closely follow one another along the same (or very similar) orbital “track.” A particular example of a coordinated group of satellites are in a sun-synchronous polar orbit, crossing the equator in an ascending (northbound) direction at about 1:30 PM local solar time, within seconds to minutes of each other—hence the name Afternoon Constellation. This allows near-simultaneous observations from a wide variety of instruments that are synergistically used to aid the scientific community in advancing our knowledge of Earth-system science and applying this knowledge for the benefit of society.

The original Afternoon Constellation comprised a single set of coordinated satellites, that began in earnest in 2006 when CloudSat and CALIPSO joined in orbit with the Aqua (launched in 2002), Aura (2004), and PARASOL (2004) satellites. The constellation was nicknamed the “A-Train” because satellite names starting with an “A” led and trailed the constellation at the time. The train was later expanded with the inclusion of GCOM-W1 (2012) and OCO-2 (2014). 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. In February 2018, CloudSat lowered its orbit out of the A-Train after technical issues that potentially affected the satellite’s maneuvering capability. In September 2018, CALIPSO executed a series of maneuvers to join CloudSat’s orbit.

Therefore, as depicted above, the international Afternoon Constellation now includes the A-Train satellites (OCO-2, GCOM-W1, Aqua, and Aura) as well as the C-Train satellites (“C” for CALIPSO and CloudSat). The C-Train flies 16.5 km (~10 mi) below the A-Train and therefore follows a slightly different ground track, though it intersects the A-Train ground track about every 20 days allowing for regular simultaneity between A-Train and C-Train instrument observations.

The instruments on these satellites are used synergistically to improve our knowledge of clouds, aerosols, atmospheric chemistry, and other elements critical for understanding Earth’s environment and changing climate.