May 13, 2016

The A-train

The A-Train

A-Train configuration mid 2014 (after OCO-2 launch)
(video size about 9.4 Mb)

News of the A-Train constellation:

  • OCO-2 (Orbiting Carbon Observatory-2) satellite was successfully launched on July 2, 2014 and joined the A-Train on August 4, 2014 flying ahead the constellation before GCOM-W1, Aqua, CALIPSO, CloudSat and Aura.
  • On December 18, 2013 Parasol has been decommissioned after a series of orbit lowering maneuvers (and propellant depletion) begun 20 days earlier. After nearly 9 years of good services, i.e. 6 times its initially estimated lifetime, Parasol paylod was stopped on October 11, 2013. Parasol had officially left the "705" orbit of the A-Train a little about 3 years ago while maintaining its mission on a lower orbit.
  • On May 17, 2012, successful launch of the Japanese satellite CGOM-W1 also called 'SHIZUKU'.
  • On May 15, 2012, Cloudsat joined again the A-Train constellation but with a mission limited to the day-light part of the orbit and with a new position. Now Cloudsat is behind CALIPSO satellite and the formation flying with CALIPSO is not possible anymore.
  • On April 18, 2011, Cloudsat satellite had a battery anomaly and had to leave the A-Train constellation on June 18, 2011.
  • On March 4 2011, launch failure of the NASA's scientific satellite GLORY due to a fairing failure.
  • On December 2, 2009 at 12:48 UT, the micro satellite PARASOL left its position in the constellation and now continue its mission on a lower orbit.
  • On February 24, 2009, launch failure of the NASA's scientific satellite OCO due to a fairing failure.

The French-U.S.-Japanese A-Train observatory is a world first. Until now, passive instruments measuring solar radiation reflected by the Earth's surface have operated independently. And the use of active lidar and radar instruments to measure clouds and aerosols is relatively recent. The A-Train will offer a unique opportunity to obtain reflectance data from Aqua in regions observed by Calipso and CloudSat across a very wide spectrum, combined with data on the polarization of reflected light from Parasol.

The satellites cross the equator one at a time, a few minutes apart, at around 1.30 pm local time, hence the nickname "afternoon constellation". However, the A-Train's railway metaphor is not strictly accurate, as the satellites do not follow each other in single file like carriages or trucks. Rather, each one flies, collects data and carries out its mission independently of the others.

In orbit since 4 May 2002, AQUA is considered the lead satellite in the A-Train, as it is the first to cross the equator each day (at 1.30 pm local time on ascending passes) and night (at 1.30 am local time on descending passes), and because it is also the largest. Its mission is centred on the Earth's water cycle. Aqua carries an imaging spectroradiometer (MODIS), a radiometer for measuring the Earth's radiation budget (CERES), a microwave scanning radiometer (AMSR-E) and various infrared and microwave sounders to establish temperature and humidity profiles of the atmosphere.

The third satellite in NASA's Earth Observing System after AQUA and TERRA, AURA is dedicating to studying air quality, stratospheric ozone and climate change. In orbit since 15 July 2004, this satellite is helping scientists to monitor phenomena governing the transport of atmospheric pollution on a global scale and to identify the main local and regional sources of this pollution. The HiRDLS instrument (High Resolution Dynamics Limb Sounder) is gathering data on the global distribution of temperatures and chemical species in the stratosphere and upper troposphere by scanning the cloud layer. The MLS instrument (Microwave Limb Sounder) is measuring the concentrations of chemical species contributing to ozone depletion.

Conducted jointly by NASA and the Canadian Space Agency, the CLOUDSAT mission is dedicated to studying clouds. The satellite, in orbit since April 28, 2006, carries a 94-GHz radar that provides data on the structure of sufficiently thick ice and water clouds.

CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation), launched on April 28, 2006, alongside CloudSat on a Delta 2 rocket, is based on a partnership between NASA and CNES, in association with the French national scientific research centre CNRS (Institut Pierre-Simon Laplace). Built around a Proteus bus, this minisatellite provides vertical profiles of the atmosphere using a backscatter lidar at 532 and 1,060 nm, with polarization measurement. The vertical distribution of the properties of aerosols and thin clouds provides vital new information for studying aerosol-cloud-radiation interactions.

PARASOL (Polarization and Anisotropy of Reflectances for Atmospheric Science coupled with Observations from a Lidar) is the second microsatellite in the Myriade series, developed by CNES. It carried the POLDER widefield radiometer (Polarization and Directionality of the Earth's Reflectances), designed with the support of the LOA atmospheric optics laboratory (CNRS-USTL). In orbit since December 18, 2004, the satellite measured the directional characteristics and polarization of light reflected by the Earth and atmosphere to further our understanding of the radiative and microphysical properties of clouds and aerosols. For example, this innovative observing technique allowed scientists to distinguish natural and manmade aerosols more accurately.

CGOM-W1 (Global Change Observation Mission 1st-Water) also called SHIZUKU is a japanese satellite in orbit since May 17, 2012. Positioned in front of AQUA, its mission is centred on the Earth's water cycle. It carries a microwave scanning radiometer (AMSR2) with six frequency ranges between 7 GHz and 89 GHz.

OCO2 (Orbiting Carbon Observatory) is the successor of the NASA's satellite OCO. The launch took place on July 2,2014 and it now flies ahead of GCOM-W1 in front of the A-Train. OCO2, latest satellite in the constellation, will focus on the concentration of carbon dioxide in the atmosphere. These data will then be compared and combined with measurements by ground based and airborne instruments.