National Aeronautics andSpace AdministrationGoddard Space Flight CenterGreenbelt, Maryland 20771http://www.gsfc.nasa.govFormation Flying: The Afternoon“A-Train” Satellite ConstellationThe Earth Science Enterprise SeriesThese articles discuss Earth’s many dynamic processes and their interactions.NASA’s Earth Science Enterprise: http://earth.nasa.govNASA’s Earth Observing System Project Science Office: http://eos.nasa.govNASA’s Earth Observatory: http://earthobservatory.nasa.govMarch 2003FS-2003-1-053-GSFCIntroductionThe changing Earth environment isincreasingly a focus of many agricultural,industrial, and societal concerns. Soundpolicy decisions are needed to addressthese issues. NASA’s Earth ScienceEnterprise (ESE) provides timely EarthScience data and information to decisionmakers to help them craft sound andequitable environmental policy. EarthObserving System (EOS) missions, andcomplementary smaller, more-focusedEarth System Science Pathfinder (ESSP)missions, are a primary source of EarthScience information for the ESE. The “A-Train” satellite formation consists of two ofthe major EOS missions, three ESSPmissions, and a French Centre Nationald’Etudes Spatiales (CNES) mission flyingin close proximity (see Table 1). Thiscarefully planned formation allows forsynergy—meaning that more informationabout the condition of the Earth is ob-tained from the combined observationsthan would be possible from the sum ofthe observations taken independently.TABLE 1: Summary of A-Train Missions [See Figure 1 for Illustration]• 2 •Take the “A-Train” to Scientific DiscoveryOver the next five years, NASA’s ESE plans to launchfour satellite missions that will fly in formation witheach other and with the Aqua satellite, which has beenin orbit since May 2002. A sixth satellite called Polar-ization and Anisotropy of Reflectances for AtmosphericScience coupled with Observations from a Lidar(PARASOL), launched by the French Space Agency,will join the four already launched NASA missions in2005. All six satellites will cross the equator within afew minutes of one another at around 1:30 p.m. localtime. Since these missions will all fly in tandem, the setof satellites is referred to as a constellation, or alterna-tively, as a formation. Furthermore, since this constel-lation is composed of missions with equator crossingsin the early afternoon (and also in the middle of thenight, at about 1:30 a.m.), it is referred to as theAfternoon Constellation - to distinguish it from theMorning Constellation consisting of Terra, Landsat-7,SAC-C and the New Millennium Program’s EarthObserving-1 (EO-1), all currently flying. The term “A-Train” comes from an old jazz tune, “Take the A-Train”composed by Billy Strayhorn and made popular byDuke Ellington’s band, and has become a popularnickname for the Afternoon Constellation becauseAqua is the lead member of the formation and Aura isin the rear*. The other missions that will eventuallymake up the A-Train are CloudSat, Cloud-AerosolLidar and Infrared Pathfinder Satellite Observations(CALIPSO), and the Orbiting Carbon Observatory(OCO). Each individual mission has its own objectivesand will improve our understanding of aspects of theEarth’s climate. The real advantage of formation flying,however, as mentioned in the Introduction, is that thedata from the various satellites are synergistic.A Tour of the A-TrainIf we think of a railroad train we realize that it iscomposed of component parts. It has an engine andlead car, and can have as well freight cars, passengercars, sleeping cars, dining cars, and a caboose.Individual trains may not have all these components,but any train needs a combination of some of thesecomponent parts to be most effective. Passengers onboard may need to make use of services availablefrom several of the component parts to satisfy theirneeds. The analogy of a train isn’t perfect for a groupof Earth Observing satellites orbiting in space. Thesesatellites do not follow each other in a straight line likethe cars of a train along a track. They also aren’tdependent on the lead satellite in any way as a train isdependent on its engine for power. Each satellite isable to function and collect data completely indepen-dently of all the others and has an independentmission to fulfill. Nevertheless, the name “A-Train” hasstuck, and it can be useful to think of a satellite forma-tion as a train to help understand the concept offormation flying. Let’s take a look in greater detail ateach planned component of the A-Train, as we would ifwe were touring through the individual cars that makeup a train, and consider what each individual satellitemission will study. Then we’ll consider how the indi-vidual components of the formation can be combinedto enhance our understanding of important issuesrelated to climate change.We begin our tour at the “front” of the satellite forma-tion. Until OCO is launched, Aqua is the first compo-nent (with the earliest equator crossing time) and canthus be thought of as the engine of the A-Train. Aquawas launched on May 4, 2002, and is the second ofthree major EOS missions, following Terra and preced-ing Aura. As implied by its name, the Aqua missionfocuses on studies of water in the Earth/atmospheresystem, although it also deals with additional elementsof the climate as well, and with the global biosphere.Aqua carries a synergistic instrument payload thatmeasures water in its gaseous, liquid, and solid forms,plus atmospheric and surface temperatures, land andocean vegetation, and many other aspects of theglobal climate system. The Atmospheric InfraredSounder (AIRS) is a high spectral resolution infraredsounder designed primarily to obtain temperature andhumidity profiles from the surface to an altitude ofabout 40 km. The Advanced Microwave Sounding Unit(AMSU) aids in retrieving atmospheric temperatureprofiles, especially under cloudy conditions and in theupper atmosphere. The Humidity Sounder for Brazil(HSB) is used to obtain humidity soundings throughthe atmosphere, for determining cloud liquid water,precipitation, and integrated precipitable water, andwas provided by Brazil's National Institute for SpaceResearch. The microwave measurements of the AMSUand HSB are particularly important for enhancing theAIRS measurements in the presence of clouds. Takentogether the three sounders (AIRS/AMSU/HSB) makeup the most advanced sounding system ever to fly inspace. The Advanced