A-train in 2013. As of 2020, the A-Train consists of four satellites. CloudSat and CALIPSO are no longer officially part of the constellation.
The A-train (from Afternoon Train) is a satellite constellation of four Earth observation satellites of varied nationality in sun-synchronous orbit at an altitude of slightly variable for each satellite.
The orbit, at an inclination of 98.14°, crosses the equator each day at around 1:30 pm solar time, giving the constellation its name (the "A" stands for "afternoon") and crosses the equator again on the night side of the Earth, at around 1:30 am.
They are spaced a few minutes apart from each other so their collective observations may be used to build high-definition three-dimensional images of Earth's atmosphere and surface.
A-train and C-train in 2019
The train, as of January 2022 consists of three active satellites:
- OCO-2, lead spacecraft in formation, replaces the failed OCO and was launched for NASA on July 2, 2014.
- GCOM-W1 "SHIZUKU", follows OCO-2 by 11 minutes, launched by JAXA on May 18, 2012.
- Aura, a multi-national satellite, lags OCO-2 by 19 minutes, launched for NASA on July 15, 2004.
- PARASOL, launched by CNES on December 18, 2004 and moved to another (lower) orbit on December 2, 2009. PARASOL was deactivated in 2013
- CloudSat, launched with CALIPSO on April 28, 2006 and moved to another (lower) orbit on February 22, 2018. Now part of the C-train.
- CALIPSO, launched on April 28, 2006, is a joint effort of CNES and NASA. It follows CloudSat by no more than 8.5 seconds. CALIPSO was moved to CloudSat's new orbit in September 2018. Now part of the C-train.
- Aqua, used to run 4 minutes behind GCOM-W1, launched for NASA on May 4, 2002. In January 2022, it descended from the A-Train to save fuel and now is in a free-drift mode, wherein its equatorial crossing time is slowly drifting to later times.
- OCO, destroyed by a launch vehicle failure on February 24, 2009, and was replaced by OCO-2.
- Glory, failed during launch on a Taurus XL rocket on March 4, 2011, and would have flown between CALIPSO and Aura.