The A-train (from Afternoon Train) is a satellite constellation of six Earth observation satellites of varied nationality in sun-synchronous orbit at an altitude of 705 km (438 mi) above the Earth.
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 the Earth's atmosphere and surface.
A-train (satellite constellation) Wikipedia
The train, as of July 2014, consists of six 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.
Aqua, runs 4 minutes behind GCOM-W1, launched for NASA on May 4, 2002.
CloudSat, a cooperative effort between NASA and the Canadian Space Agency, runs 2 minutes and 30 seconds behind Aqua, launched with CALIPSO on April 28, 2006.
CALIPSO, a joint effort of CNES and NASA, follows CloudSat by no more than 15 seconds, launched on April 28, 2006.
Aura, a multi-national satellite, lags Aqua by 15 minutes, crossing the equator 8 minutes behind due to different orbital track to allow for synergy with Aqua, 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.
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.