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The Afternoon or "A-Train" satellite constellation presently consists of five satellites flying in formation around the globe (NASA's Aqua and Aura satellites and CNES' PARASOL satellite). The CALIPSO and CloudSat satellite missions were inserted in orbit behind Aqua in April 2006. Two additional satellites, OCO and Glory, will join the constellation in the next few years.

Each satellite within the A-Train has unique measurement capabilities that greatly complement each other. For the first time, near simultaneous measurements of aerosols, clouds, temperature, relative humidity, and radiative fluxes (the change of radiation in a layer) will be obtained over globe during all seasons. This ensemble of observations will allow one to understand how large scale aerosol and cloud properties change in response to changing environmental conditions. It will further allow one to determine how changing cloud and aerosols distributions influence our climate with greater clarity than possible before.

For much of its life, the A-Train will be maintained in orbit within 15 minutes of the leading and trailing spacecraft while traveling at over 15,000 miles per hour. CloudSat and CALIPSO will be controlled to an even finer requirement, within 15 seconds of each other, so that both instrument suites will view the same cloud area at nearly the same moment. This capability is crucial for studying clouds, which have lifetimes often less than 15 minutes.

The different instruments on each platform are described in Table 1: A-Train Instruments.
+ View Table 1

A-Train Formation.

CALIPSO flies as part of the Aqua satellite constellation (or A-Train), which consists of the Aqua, CloudSat, CALIPSO, PARASOL, and Aura satellite missions. The constellation has a nominal orbital altitude of 705 km and inclination of 98 degrees. Aqua will lead the constellation with an equatorial crossing time of about 1:30 PM. CloudSat and CALIPSO lag Aqua by 1 to 2 minutes and will be separated from each other by 10 to 15 seconds.

The satellites in the A-Train are maintained in orbit to match the World Reference System 2 (WRS-2) reference grid. This reference system was developed to facilitate regular sampling patterns by remote sensors in the Landsat program. Each satellite completes 14.55 orbits per day with a separation of 24.7 degrees longitude between each successive orbit at the equator. The orbit tracks at the equator progresses westward 10.8 degrees on succeeding days, which over a 16-day period, produces a uniform WRS grid over the globe. The WRS grid pattern consists of 233 orbits with separation between orbits at the equator of 172 km. The Aqua satellite will be controlled to the WRS grid to within +/- 10 km. Additional information on the WRS can be obtained from the Landsat 7-WRS Web site. + View LandSat Site


The following table lists each of the satellites in the A-Train constellation, their position within the constellation, a summary of each mission, and a description of each of their corresponding scientific instruments.

Spacecraft Position in the A-Train/
Formation Requirements
Summary of Mission Instruments Carried
Aqua Lead spacecraft in formation until the launch of OCO. Synergistic instrument package studies global climate with an emphasis on water in the Earth/atmosphere system, including its solid, liquid and gaseous forms. AIRS/AMSU-A/HSB
CloudSat Lags Aqua by between 30 seconds and 2 minutes. Must maintain extremely precise positioning relative to both Aqua and CALIPSO to permit synergistic measurements with Aqua and CALIPSO. Cloud Profiling Radar (CPR) will allow for most detailed study of clouds to date and should better characterize the role clouds play in regulating the Earth's climate. CPR
CALIPSO Lags CloudSat by no more than 15 seconds. Must maintain position relative to Aqua to permit synergistic measurements with Aqua. Observations from spaceborne lidar, combined with passive imagery, will lead to improved understanding of the role aerosols and clouds play in regulating the Earth's climate, in particular, how the two interact with one another. CALIOP
PARASOL Lags CALIPSO by about 1 minute. Polarized light measurements will allow better characterization of clouds and aerosols in the Earth's atmosphere,in particular, distinguishing natural and manmade aerosols. POLDER
Aura Lags Aqua by about 15 minutes but crosses equator 8 minutes behind Aqua due to different orbital track to allow for synergy with Aqua. Synergistic payload will study atmospheric chemistry, focusing on the horizontal and vertical distribution of key atmospheric pollutants and greenhouse gases and how these distributions evolve and change with time. HIRDLS
OCO Will precede Aqua by 15 minutes when it is launched. Will make global, space-based observations of the column integrated concentration of carbon dioxide, a critical greenhouse gas. Three grating spectrometers
Glory Will lag CALIPSO by no more than 2 minutes. Will enable a greater understanding of the seasonal variability of aerosol properties and whether climate temperature increases and changes are by-products of natural events or are caused by man-made sources. APS
Cloud Camera Sensor Package

A-Train Updates:
The A-Train web site contains detailed information of all of the satellites flying in the same path can be found at

Last Updated: November 28, 2017
Curator: Charles R. Trepte
NASA Official: Charles R. Trepte

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