What is the A-Train?

The "A-Train" satellite constellation will consist of seven satellites flying in close proximity. Each individual mission has its own science objectives; all will improve our understanding of aspects of the Earth's climate. The synergism that is expected to be gained by flying in close proximity to each other should enable the overall science results of the Afternoon Constellation to be greater than the sum of the science of each individual mission.

The A-Train formation will help answer these important questions.

• What are the aerosol types and how do observations match global emission and transport models?
• How do aerosols contribute to the Earth Radiation Budget (ERB)/climate forcing?
• How does cloud layering affect the Earth Radiation Budget?
• What is the vertical distribution of cloud water/ice in cloud systems?
• What is the role of Polar Stratospheric Clouds in ozone loss and denitrification of the Arctic vortex?

History of the A-Train

NASA launched the Aqua satellite on May 4, 2002 and it is currently performing nominally on orbit. On July 15, 2004, the Aura satellite was launched and phased with Aqua such that one of the Aura instruments, the Microwave Limb Sounder (MLS), is able to view the same air mass that Aqua observed eight minutes earlier. In 2008, Aura was moved forward to eliminate this eight minute delay. The joint NASA/CNES CALIPSO, and the CloudSat missions were launched on April 28, 2006. Both launched on the same expendable launch vehicle and CALIPSO flies from 30 to 120 seconds behind Aqua with CloudSat currently leading CALIPSO by 17.5�2.5 seconds. This tight formation enables synergistic measurements with Aqua, which is a key science requirement for the Afternoon Constellation. A French mission, PARASOL, was launched on December 18, 2004 by the French Space Agency/CNES and flies 58 seconds behind the CALIPSO control box.

Video : "The A-Train"	+ Small (48 mb) + Large (170 mb)

Whats next?

The OCO mission is currently scheduled for launch in 2009. OCO will maintain its MLT of ascending node to be at least 3 minutes earlier than the Aqua's MLT but no earlier than 13:25. The NASA Glory mission, also scheduled for launch in 2009, will fly 15 to 101 seconds behind the trailing edge of the PARASOL box.

All seven satellites will cross the equator within a few minutes of one another at approximately 13:30 MLT. Since these missions are all at the same mean equatorial altitude, 705 kilometers (km), they are referred to as a constellation (note that the term "formation" has also been used although it applies only to CloudSat with respect to CALIPSO). The set of missions is referred to as the Afternoon Constellation due to the early afternoon equator crossing of each mission.

A Closer Look at Each A-Train Satellite

	Aqua is designed to acquire precise atmospheric and oceanic measurements to provide a greater understanding of their role in the Earth's climate and its variations. The satellite's instruments provide regional to global land cover, land cover change, and atmospheric constituents.
	Aura's mission is designed to observe the atmosphere to answer the following three high-priority environmental questions: Is the Earth's ozone layer recovering? Is air quality getting worse? How is the Earth's climate changing? Aura's new objective over previous atmospheric research missions is also to probe the Earth's troposphere.
	CloudSAT, a cooperative mission with Canada, will use advanced radar to "slice" through clouds to see their vertical structure, providing a completely new observational capability from space. CloudSAT will look at the structure, composition, and effects of clouds and will be one of the first satellites to study clouds on a global basis.
	CALIPSO will provide key measurements of aerosol and cloud properties needed to improve climate predictions. CALIPSO will fly a 3-channel LIDAR with a suite of passive instruments in formation with Aqua to obtain coincident observations of radiative fluxes and atmospheric conditions. CloudSAT will also fly in formation with CALIPSO to provide a comprehensive characterization of the structure and composition of clouds and their effects on climate under all weather conditions.
	PARASOL (Polarization and Anisotropy of Réflectances for Atmospheric Sciences coupled with Observations from a Lidar) is a French's CNES microsatellite project. Its main purpose is to improve the characterization of the clouds and aerosols microphysical and radiative properties, needed to understand and model the radiative impact of clouds and aerosols. (Credit: CNES)
	The OCO provides space-based observations of atmospheric carbon dioxide (CO2), the principal anthropogenic driver of climate change. This mission uses mature technologies to address NASA's carbon cycle measurement requirement. OCO generates the knowledge needed to improve projections of future atmospheric CO2.
	The Glory satellite will help scientists fill in a gap in their understanding of global warming: the role of clouds and airborne particles. It will collect data on the properties of aerosols and black carbon in the Earth's atmosphere and climate system.