The "A-Train" satellite formation will consist of six satellites flying in close proximity. The first one, Aqua, was launched in 2002. The second one, Aura, will be launched in June 2004, while CloudSAT, CALIPSO and PARABOL will start their missions in October 2004. The last one, OCO, will join them in 2006. The individual missions and the A-Train formation are described in this paper, "Formation Flying: The Afternoon 'A-Train' Satellite Constellation" (PDF format, 6 pages, 263 KB). The satellites will cross the equator within a few minutes of one another at around 1:30 p.m. local time. By combining the different sets of observations, scientists will be able to gain a better understanding of important parameters related to climate change.

By combining the components, scientists are able to gain a better understanding of important parameters related to climate change. The A-Train formation will allow for coordinated measurements.  Data from several different satellites can be used together to obtain comprehensive information about atmospheric components or processes. Combining the information from several sources gives a more complete answer to many questions than would be possible from any satellite taken by itself.

Click here: A-Train Animation

"This graphic (not to scale) depicts the satellites that make up the Afternoon Constellation -- "The A-Train." Listed under each satellite’s name is its equator crossing time. Note that though Aura crosses the equator eight minutes behind Aqua, in terms of local time, because it is along a different orbit track, it actually lags Aqua by fifteen minutes. Note also that CALIPSO trails CloudSat by only 15 seconds to allow for synergy between Aqua, CloudSat, and CALIPSO. Credit: Alex McClung.

	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. Click for Aura Animation
	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. Click here for an animation about CALIPSO's LIDAR
	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 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?

It will be tough to get these satellites to work harmoniously together, because of the great variety of instruments and resolutions. And the formation will need to be precisely aligned, which means a coordinated maneuvering of the different satellites.

Let's hope the scientists solve all these challenges.

Sources: Various websites from NASA and CNES