The Afternoon Constellation - A-Train

What is the A-Train?

Introduction

From their vantage point high above the Earth’s surface, NASA’s Earth-observing satellite missions are uniquely positioned to obtain comprehensive global observations of our home planet. NASA’s Earth Science Division freely and openly shares these Earth Science data with various federal, state, local, and international partners, who use the data for scientific research as well as a variety of practical societal applications that policy makers can use to help craft environmental policy.

NASA satellite constellations are groups of satellites that fly in close proximity to each other in a carefully planned coordination that allows for measurement synergy between the missions—synergy means that more information about the Earth environment is obtained from the combined observations than would be possible from the sum of the observations taken independently.

The Afternoon Constellation is one such satellite constellation whose members all have equator crossings in a northerly direction in the early afternoon at about 1:30 PM local solar time (as well as a southerly nighttime equatorial crossing at about 1:30 AM). At the present time, the international Afternoon Constellation now includes the A-Train satellites (OCO-2, GCOM-W1, Aqua, and Aura) as well as the C-Train satellites (CALIPSO and CloudSat). PARASOL fully exited the A-Train and ceased operation on December 18, 2013. See Table 1 for a complete list of current and past missions.

The Afternoon Constellation and Scientific Discovery

Five NASA missions are presently part of the Afternoon Constellation. This includes Aqua (launched in 2002), Aura (launched in 2004), the second Orbiting Carbon Observatory (OCO-2, launched in July 2014), and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and CloudSat (both launched together in 2006). In addition, there is a Japanese mission (launched in 2012) named the Global Change Observation Mission-Water (GCOM-W1). The French mission named the Polarization and Anisotropy of Reflectances for Atmospheric Science coupled with Observations from a Lidar (PARASOL) was lowered to 9.5 km under the A-Train on November 16, 2011, and ceased operation, fully exiting the A-Train on December 18, 2013.

Each one of these missions has its own objectives and makes its own unique contribution to improving our understanding of aspects of the Earth’s environment and climate. The real advantage of constellation flying, however, is that the data from the various satellites are synergistic and can be combined together to allow for even more comprehensive studies.

> Take a tour of the A-Train

Satellite	Summary of	Instruments	Launch Date	Responsible Organization
Aqua	A synergistic instrument package measuring at visible, infrared, and microwave frequencies allows comprehensive studies of water in the Earth/atmosphere system, including its solid, liquid and gaseous forms, plus studies of vegetation, both on the land and in the ocean.	AMSR-E, AIRS, AMSU-A, HSB, CERES, MODIS	May 4, 2002	NASA/GSFC
Aura	Observations from limb sounding and nadir imaging allow studies of the horizontal and vertical distribution of key atmospheric pollutants and greenhouse gases and how these distributions evolve and change with time.	HDRLS, MLS, TES, OMI	July 15, 2004	NASA/GSFC
CALIPSO	Observations from space-borne lidar, combined with passive imagery, will lead to improved understanding of the role aerosols and clouds play in regulating the Earth’s climate.	CALIOP, IIR, WFC	April 28, 2006	NASA/GSFC, NASA/LaRC, CNES
CloudSat	Cloud Profiling Radar will allow for the most detailed study of clouds to date and should better characterize the role clouds play in regulating the Earth’s climate.	CPR	April 28, 2006	NASA/GSFC, NASA/JPL
GCOM-W1	Observations of water circulation changes. Specifically it will observe precipitation, vapor amounts, wind velocity above the ocean, sea water temperature, water levels on land areas, and snow depths.	AMSR2	May 18, 2012	JAXA
OCO-2	Three grating spectrometers will make global, space-based observations of the column-integrated concentration of CO2, a critical greenhouse gas.	Three grating spectrometers	July 2, 2014	NASA/GSFC NASA/JPL
PARASOL	Polarized light measurements will allow better characterization of clouds and aerosols in the Earth’s atmosphere. PARASOL fully exited the A-Train and ceased operation on December 18, 2013.	POLDER	December 18, 2004	CNES

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¹ This is in contrast to the Morning Constellation, whose members all have equator crossings in the late morning around 10:30 a.m. (and late evening, at about 10:30 p.m.)  The morning constellation presently consists of Terra, Landsat-7, SAC-C and the New Millennium Program’s Earth Observing-1 (EO-1).

² The term “A-Train” comes from an old jazz tune, Take the A-Train, composed by Billy Strayhorn and made popular by Duke Ellington’s band, and has become a popular nickname for the Afternoon Constellation, particularly since EOS Aqua and Aura are both part of the formation.