A-train (satellite constellation)

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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 that is slightly variable for each satellite.[1]

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"[2]) 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,[3][4][5] 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[6]
  • CloudSat, launched with CALIPSO on April 28, 2006 and moved to another (lower) orbit on February 22, 2018.[5] 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.[7] 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,[8] destroyed by a launch vehicle failure on February 24, 2009,[9] and was replaced by OCO-2.
  • Glory,[10] failed during launch on a Taurus XL rocket on March 4, 2011, and would have flown between CALIPSO and Aura.


  1. ^ «A-train Symposium October 2007: Constellation keeps its promises», CNESMAG, January 2008
  2. ^ NASA, Introducing the A-Train, 10.26.10 (accessed April 30 2012)
  3. ^ Smith, Joseph M. (4 May 2022). "Aqua Turns 20".
  4. ^ "Individual A-Train Missions". June 5, 2012. Retrieved 2013-11-15.
  5. ^ a b Greicius, Tony (2018-02-23). "CloudSat Exits the "A-Train"". NASA. Retrieved 2018-04-01.
  6. ^ CNES News on Calipso
  7. ^ Sister Satellites, Briefly Separated, Working Together Again
  8. ^ "OCO homepage". Archived from the original on 2018-09-09. Retrieved 2008-02-10.
  9. ^ Media Briefing Scheduled To Discuss Orbiting Carbon Observatory Mission
  10. ^ Glory homepage Archived 2008-09-16 at the Wayback Machine

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