A world first, the A-Train is a constellation of six Franco-American satellites flying in formation at a few minute interval in a heliosynchronous orbit passing over the equator at 13:30 local time. This space rendezvous is designed to make almost simultaneous use of all the observation techniques currently available to scrutinise the Earth's atmosphere. They are independent but complementary to each other; the different A-Train missions are all concerned with the climate and the study of the interactions between radiation, clouds, aerosols and the water cycle. The matrix of measurements from their fifteen instruments will provide the scientific community with an unprecedented wealth of data which will be used to test and improve digital forecasting models both for weather and climate pollution.
Aqua (NASA), Aura (NASA), Calipso (NASA/CNES), Cloudsat (NASA/ASC), Parasol (CNES) and OCO (NASA) will make an exceptional space observatory combining all the active and passive measurement techniques to gain a better understanding of how the climate machine works.
The A-Train especially identifies the various types of aerosol and helps to understand their direct and indirect effects on the climate; the role of stratospheric polar clouds on the hole in the ozone layer is also being studied.
AQUA, which has been in orbit since 4 May 2002, is considered to be the "leader" of the constellation because it is the first to cross the equator every day (at 13h30 local time for ascending orbits) and night (at 1h30 for descending orbits), but also because it is the largest. Its mission is focussed on the water cycle.