A-train (satellite constellation) explained

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.

Satellites

Active

The train,,^{[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.

Past

• 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. 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] It was then part of the C-train with Cloudsat until it was officially decommissioned on August 1, 2023.
• 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.

Failed

• 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.

External links

• NASA A-Train Portal
• NASA satellite program impacted
• NASA Program Page
• Orbital Sciences Program Page
• Physics Today . Touring the atmosphere aboard the A-Train . T.S. . L'Ecuyer . J.H. . Jiang . 2010 . 36–41 . 63 . 7 . 10.1063/1.3463626 . 2010PhT....63g..36L . dead . https://archive.today/20130224065020/http://ptonline.aip.org/journals/doc/PHTOAD-ft/vol_63/iss_7/36_1.shtml . 2013-02-24 . none.

Notes and References

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. Web site: Smith . Joseph M. . Aqua Turns 20 . 4 May 2022 .
4. Web site: Individual A-Train Missions. June 5, 2012. 2013-11-15.
5. News: CloudSat Exits the "A-Train". Greicius. Tony. 2018-02-23. NASA. 2018-04-01. en.
6. http://www.cnes.fr/web/CNES-en/8230-gp-parasol-moves-off-the-a-train-s-track.php CNES News on Calipso
7. https://www.nasa.gov/feature/langley/sister-satellites-briefly-separated-working-together-again Sister Satellites, Briefly Separated, Working Together Again
8. Web site: OCO homepage . 2008-02-10 . 2018-09-09 . https://web.archive.org/web/20180909072113/https://oco.jpl.nasa.gov/ . dead .
9. http://www.nasa.gov/home/hqnews/2009/feb/HQ_09-038_OCO_failure_briefing.html Media Briefing Scheduled To Discuss Orbiting Carbon Observatory Mission
10. http://glory.gsfc.nasa.gov/ Glory homepage